Isolation and identification of entomopathogenic nematodes from citrus orchards in South Africa and their biocontrol potential against false codling moth

PI3K/Akt/CncC signaling pathway mediates the response to EPN-Bt infection in Holotrichia parallela larvae

BACKGROUND

Combining the entomopathogenic nematode (EPN), Heterorhabditis beicherriana LF strain, and Bacillus thuringiensis (Bt) HBF-18 strain is a practical strategy to manage the larvae of Holotrichia parallela Motschulsky (white grubs). However, the mechanisms underlying the larval defense response to this combined biocontrol strategy are unknown.

RESULTS

The activities of some antioxidant enzymes (SOD, POD, CAT) and some detoxifying enzymes (AChE, P-450, CarE, GST) in grubs showed an activation-inhibition trend throughout the EPN-Bt exposure time course. Eight potentially key antioxidant and detoxifying enzyme genes in response to EPN-Bt infection were identified from the midgut of grubs through RNA sequencing. After silencing CAT, CarE18, and GSTs1, the enzyme activities were significantly decreased by 30.29%, 68.80%, and 34.63%, respectively. Meanwhile, the mortality of grubs was increased by 18.40%, 46.30%, and 42.59% after exposure to EPN-Bt for 1 day. Interestingly, the PI3K/Akt signaling pathway was significantly enriched in KEGG enrichment analysis, and the expression levels of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), cap 'n' collar isoform-C (CncC), kelch-like ECH-associated protein 1 (Keap1), and CarE18 were all up-regulated when exposed to EPN-Bt for 1 day. Furthermore, RNAi-mediated PI3K silencing showed a similar down-regulated trend between PI3K/Akt/CncC and CarE18. Moreover, silencing PI3K rendered grubs more susceptible to EPN-Bt and accelerated symbiotic bacteria multiplication in grubs.

CONCLUSION

These results suggest that the PI3K/Akt/CncC pathway mediates the expression of CarE18 and participates in the defense response of H. parallela larvae against EPN-Bt infection. Our data provide valuable insights into the design of appropriate management strategies for this well-known agricultural pest. © 2022 Society of Chemical Industry.

Nematophilic bacteria associated with entomopathogenic nematodes and drug development of their biomolecules

The importance of Xenorhabdus and Photorhabdus symbionts to their respective Steinernema and Heterorhabditis nematode hosts is that they not only contribute to their entomopathogenicity but also to their fecundity through the production of small molecules. Thus, this mini-review gives a brief introductory overview of these nematophilic bacteria. Specifically, their type species, nematode hosts, and geographic region of isolations are tabulated. The use of nucleotide sequence-based techniques for their species delineation and how pangenomes can improve this are highlighted. Using the Steinernema–Xenorhabdus association as an example, the bacterium-nematode lifecycle is visualized with an emphasis on the role of bacterial biomolecules. Those currently in drug development are discussed, and two potential antimalarial lead compounds are highlighted. Thus, this mini-review tabulates forty-eight significant nematophilic bacteria and visualizes the ecological importance of their biomolecules. It further discusses three of these biomolecules that are currently in drug development. Through it, one is introduced to Xenorhabdus and Photorhabdus bacteria, their natural production of biomolecules in the nematode-bacterium lifecycle, and how these molecules are useful in developing novel therapies.

Development of cost-effective media for the in vitro liquid culture of entomopathogenic nematodes

Entomopathogenic nematodes (EPN) are successful biological control agents of a variety of soilborne insect pests. They have the potential to be mass-produced, using in vitro liquid culture technology, and can be formulated and sold as a biopesticide. To commercialise an EPN-based biopesticide successfully, the method of liquid mass production requires in-depth optimisation to reduce the cost of production and to increase yields, to make it affordable to the farming community. This study attempted to optimise the liquid culture protocol for the South African isolates, Steinernema jeffreyense and S. yirgalemense, by investigating the impact of cheaper medium ingredients on the recovery and yield of the liquid culture process. Studies were conducted by investigating alternative protein, lipid and nitrogen/yeast sources, compared to the more expensive laboratory-grade ingredients currently used. The results showed that egg yolk has no impact on the yield in the case of S. jeffreyense. However, for S. yirgalemense, egg yolk was shown to be a superior protein source to soy and insect-based protein in terms of nematode yield. Moreover, neither canola oil nor olive oil showed a significant difference in the yield of S. yirgalemense, with yeast extract being found to be the optimal nitrogen/yeast source. When comparing the yields with those in other liquid culture research on S. yirgalemense, yields have been successfully increased by 300%, with the cost of the nematode nutrient medium having decreased by 77%. Thus, it is imperative that, prior to a scale up to large bioreactors, the nutrient medium should be optimised to reduce the cost of production.

Entomopathogenic nematodes associated with organic honeybush (Cyclopia spp.) cultivation in South Africa

Entomopathogenic nematodes (EPNs) of the genera Steinernema and Heterorhabditis have demonstrated great potential as useful bio-control agents in the management of certain important soil-inhabiting insect pests of agricultural crops. In a survey of nematodes associated with organic honeybush cultivation, soil samples were obtained from nine organic honeybush plots, which are located in the Bredasdorp area of the Western Cape province of South Africa. The EPNs were isolated from soil by baiting with larvae of Galleria mellonella (greater wax moth) and identified by the amplification of the internal transcribed spacer region using the primer set TW81 and AB28. EPNs were abundant in the honeybush orchards, accounting for about 50% of the sampled fields and five EPN species were identified including Heterorhabditis bacteriophora, Heterorhabditis safricana, Steinernema khoisanae,. Steinernema nguyeni, Oscheius sp. and an unknown EPN, therefore suggesting a substantial diversity of EPNs in the sampled fields. A 100% mortality of infected G. mellonella larvae was recorded within 48 h of exposure to the nematodes. However, although these EPNs have been previously reported in South Africa, it is the first time they are found in such diversity on a conservative tillage management system in organic honeybush cultivation.

Exploring the Use of Entomopathogenic Nematodes and the Natural Products Derived from Their Symbiotic Bacteria to Control the Grapevine Moth, Lobesia botrana (Lepidoptera: Tortricidae)

Simple Summary

The European grapevine moth (EGVM) Lobesia botrana (Lepidoptera: Tortricidae) attacks vineyards in Europe, the Middle East, and North and South America. Global movement toward sustainable agriculture urges the development of environmentally friendly tools that can replace traditional pesticides. Entomopathogenic nematodes (EPNs) are well-known biological control agents against various arthropod pests. The EPNs act together with symbiotic bacteria that produce natural products with insecticidal potential. Novel formulations and application technology allow their application against aerial pests, including those associated with vineyards. This study investigated the viability of four EPN species and their corresponding bacteria derivates (unfiltered ferment, UF, or cell-free supernatant, CFS) against EGVM (larval and pupa instars). The results revealed that all EPN species killed various EGVM larval stages. Killing pupae required a higher number of IJs than controlling larvae. Steinernema carpocapsae registered the most promising results, killing ~50% L1 and >75% L3/L5 in 2 days. The use of the bacterial bioactive compounds achieved similar results, with UF registering higher activity than CFS. Overall, we demonstrated that both EPN and bacterial bioproducts have a great potential to control EGVM in sustainable viticulture. Further research in co-formulation with adjuvants is required to ensure their survival in the aboveground grapevine areas.

Abstract

The European grapevine moth (EGVM) Lobesia botrana (Lepidoptera: Tortricidae) is a relevant pest in the Palearctic region vineyards and is present in the Americas. Their management using biological control agents and environmentally friendly biotechnical tools would reduce intensive pesticide use. The entomopathogenic nematodes (EPNs) in the families Steinernematidae and Heterorhabditidae are well-known virulent agents against arthropod pests thanks to symbiotic bacteria in the genera Xenorhabdus and Photorhabdus (respectively) that produce natural products with insecticidal potential. Novel technological advances allow field applications of EPNs and those bioactive compounds as powerful bio-tools against aerial insect pests. This study aimed to determine the viability of four EPN species (Steinernema feltiae, S. carpocapsae, S. riojaense, and Heterorhabditis bacteriophora) as biological control agents against EGVM larval instars (L1, L3, and L5) and pupae. Additionally, the bioactive compounds from their four symbiotic bacteria (Xenorhabdus bovienii, X. nematophila, X. kozodoii, and Photorhabdus laumondii subsp. laumondii, respectively) were tested as unfiltered ferment (UF) and cell-free supernatant (CFS) against the EGVM larval instars L1 and L3. All of the EPN species showed the capability of killing EGVM during the larval and pupal stages, particularly S. carpocapsae (mortalities of ~50% for L1 and >75% for L3 and L5 in only two days), followed by efficacy by S. feltiae. Similarly, the bacterial bioactive compounds produced higher larval mortality at three days against L1 (>90%) than L3 (~50%), making the application of UF more virulent than the application of CFS. Our findings indicate that both steinernematid species and their symbiotic bacterial bioactive compounds could be considered for a novel agro-technological approach to control L. botrana in vineyards. Further research into co-formulation with adjuvants is required to expand their viability when implemented for aboveground grapevine application.

Dieback and Leaf Spot in Box Elder (Acer negundo) Caused by Exserohilum rostratum

Leaf spot and dieback were observed on box elder (Acer negundo) grown in a nursery in Tai'an city, Shandong Province, China, in 2019, with a disease incidence of 86%. The incidences of Exserohilum rostratum isolation were 75% from the shoots and 66.6% from the leaves of field-infected plants. Isolates were identified at the species level on the basis of morphological characteristics and through phylogenetic analysis of concatenated partial sequences of the internal transcribed spacer (ITS) region and cam, gapdh, tef1, rpb2, tub2, and his genes from the Exserohilum isolates. The effects of temperature on the mycelial growth of the Exserohilum rostratum isolates were also characterized. In greenhouse tests, seedlings inoculated with the pathogen exhibited systemic symptoms similar to those observed in the field. In pathogenicity experiments on shoots, wounded seedlings were observed to be blighted, suggesting that leaf spot and dieback may develop into more severe blight or dieback when high winds, sudden temperature decreases, or insect infestations occur. To our knowledge, this is the first report of dieback and leaf spot caused by E. rostratum on a species of A. negundo.

Potential of in vivo- and in vitro-cultured entomopathogenic nematodes to infect Lobesia vanillana (Lepidoptera: Tortricidae) under laboratory conditions

Entomopathogenic nematodes (EPNs) have been successfully applied as biological control agents against above ground and soil stages of insect pests. However, for commercial application, it is crucial to mass culture these nematodes using in vitro liquid culture technology, as it is not attainable when using susceptible insects as hosts. Lobesia vanillana (Lepidoptera: Tortricidae) is regarded a sporadic pest of wine grapes in South Africa. The in vivo- and in vitro-cultured South African EPNs, Steinernema yirgalemense and Steinernema jeffreyense (Rhabditida: Steinernematidae), were evaluated against larvae and pupae of L. vanillana in laboratory bioassays. For larvae, high mortality was observed for all treatments: In vitro-cultured S. yirgalemense (98%) performed better than S. jeffreyense (73%), while within in vivo cultures, there was no difference between nematode species (both 83%). No significant difference was detected between in vivo- and in vitro cultures of the same nematode species. The LD50 of the in vitro-cultured S. yirgalemense, was 7.33 nematodes per larva. Mortality by infection was established by dissecting L. vanillana cadavers and confirming the presence of nematodes, which was > 90% for all treatments. Within in vitro cultures, both S. yirgalemense and S. jeffreyense were able to produce a new cohort of infective juveniles from L. vanillana larvae. Pupae, however, were found to be considerably less susceptible to EPN infection. This is the first study on the use of EPNs to control L. vanillana. The relative success of in vitro-cultured EPN species in laboratory assays, without any loss in pathogenicity, is encouraging for further research and development of this technology.

A study on Xenorhabdus and Photorhabdus isolates from Northeastern Thailand: Identification, antibacterial activity, and association with entomopathogenic nematode hosts

Xenorhabdus and Photorhabdus are gram negative bacteria that can produce several secondary metabolites, including antimicrobial compounds. They have a symbiotic association with entomopathogenic nematodes (EPNs). The aim of this study was to isolate and identify Xenorhabdus and Photorhabdus species and their associated nematode symbionts from Northeastern region of Thailand. We also evaluated the antibacterial activity of these symbiotic bacteria. The recovery rate of EPNs was 7.82% (113/1445). A total of 62 Xenorhabdus and 51 Photorhabdus strains were isolated from the EPNs. Based on recA sequencing and phylogeny, Xenorhabdus isolates were identified as X. stockiae (n = 60), X. indica (n = 1) and X. eapokensis (n = 1). Photorhabdus isolates were identified as P. luminescens subsp. akhurstii (n = 29), P. luminescens subsp. hainanensis (n = 18), P. luminescens subsp. laumondii (n = 2), and P. asymbiotica subsp. australis (n = 2). The EPNs based on 28S rDNA and internal transcribed spacer (ITS) analysis were identified as Steinernema surkhetense (n = 35), S. sangi (n = 1), unidentified Steinernema (n = 1), Heterorhabditis indica (n = 39), H. baujardi (n = 1), and Heterorhabditis sp. SGmg3 (n = 3). Antibacterial activity showed that X. stockiae (bMSK7.5_TH) extract inhibited several antibiotic-resistant bacterial strains. To the best of our knowledge, this is the first report on mutualistic association between P. luminescens subsp. laumondii and Heterorhabditis sp. SGmg3. This study could act as a platform for future studies focusing on the discovery of novel antimicrobial compounds from these bacterial isolates.

Immune-related genes of the larval Holotrichia parallela in response to entomopathogenic nematodes Heterorhabditis beicherriana LF

Background

Entomopathogenic nematodes (EPNs) emerge as compatible alternatives to conventional insecticides in controlling Holotrichia parallela larvae (Coleoptera: Scarabaeidae). However, the immune responses of H. parallela against EPNs infection remain unclear.

Results

In present research, RNA-Seq was firstly performed. A total of 89,427 and 85,741 unigenes were achieved from the midgut of H. parallela larvae treated with Heterorhabditis beicherriana LF for 24 and 72 h, respectively; 2545 and 3156 unigenes were differentially regulated, respectively. Among those differentially expressed genes (DEGs), 74 were identified potentially related to the immune response. Notably, some immune-related genes, such as peptidoglycan recognition protein SC1 (PGRP-SC1), pro-phenoloxidase activating enzyme-I (PPAE-I) and glutathione s-transferase (GST), were induced at both treatment points. Bioinformatics analysis showed that PGRP-SC1, PPAE-I and GST were all involved in anti-parasitic immune process. Quantitative real-time PCR (qRT-PCR) results showed that the three immune-related genes were expressed in all developmental stages; PGRP-SC1 and PPAE-I had higher expressions in midgut and fat body, respectively, while GST exhibited high expression in both of them. Moreover, in vivo silencing of them resulted in increased susceptibility of H. parallela larvae to H. beicherriana LF.

Conclusion

These results suggest that H. parallela PGRP-SC1, PPAE-I and GST are involved in the immune responses to resist H. beicherriana LF infection. This study provides the first comprehensive transcriptome resource of H. parallela exposure to nematode challenge that will help to support further comparative studies on host-EPN interactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07506-4.

Biological Control of a Phytosanitary Pest (Thaumatotibia leucotreta): A Case Study

Thaumatotibia leucotreta, known as the false codling moth, is a pest of citrus and other crops in sub-Saharan Africa. As it is endemic to this region and as South Africa exports most of its citrus around the world, T. leucotreta has phytosanitary status for most markets. This means that there is zero tolerance for any infestation with live larvae in the market. Consequently, control measures prior to exporting must be exemplary. Certain markets require a standalone postharvest disinfestation treatment for T. leucotreta. However, the European Union accepts a systems approach, consisting of three measures and numerous components within these measures. Although effective preharvest control measures are important under all circumstances, they are most critical where a standalone postharvest disinfestation treatment is not applied, such as within a systems approach. Conventional wisdom may lead a belief that effective chemical control tools are imperative to achieve this end. However, we demonstrate that it is possible to effectively control T. leucotreta to a level acceptable for a phytosanitary market, using only biological control tools. This includes parasitoids, predators, microbial control, semiochemicals, and sterile insects. Simultaneously, on-farm and environmental safety is improved and compliance with the increasing stringency of chemical residue requirements imposed by markets is achieved.

Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) on the "Offensive" in Africa: Prospects for Integrated Management Initiatives

The South American tomato pinworm Tuta absoluta (Meyrick) has aggressively invaded the African continent. Since its first detection in North Africa in Morocco and Tunisia in 2008, it has successfully invaded the entire southern, eastern and western Africa, where it has been on the offensive, causing significant damage to Solanaceous food crops. While control of this prolific invader is primarily based on conventional synthetic pesticides, this form of control is consistently losing societal approval owing to (1) pesticide resistance development and consequential loss of field efficacy; (2) growing public health concerns; (3) environmental contamination and loss of biological diversity and its associated ecological services; and (4) unsustainable costs, particularly for resource-poor African farmers. As such, more ecologically sound pest management strategies, e.g., the use of natural substances (NSs), may offer a more sustainable approach to tackling this offensive. A systematic literature search through digital libraries and online databases (JSTOR, PubMed, Web of Science, SCOPUS and Google Scholar) was conducted using predetermined keywords on T. absoluta, e.g., South American tomato pinworm. We use this to explain the invasion of T. absoluta in Africa, citing mechanisms facilitating African invasion and exploring the potential of its control using diverse biological control agents, natural and low-risk substances. Specifically, we explore how botanicals, entomopathogens, semiochemicals, predators, parasitoids, host plant resistance, sterile insect technique and others have been spatially employed to control T. absoluta and discuss the potential of these control agents in African landscapes using more integrated approaches. We discuss the use of NSs as assets to general insect pest control, some potential associated liabilities and explain the potential use and barriers to adoption in African systems from a legislative, economic, ecological and social standpoint.

A survey of entomopathogenic nematodes and their symbiotic bacteria in agricultural areas of northern Thailand

Entomopathogenic nematodes (EPNs) Steinernema and Heterorhabditis and their symbiotic bacteria, Xenorhabdus and Photorhabdus, have been successfully used for the control of insect pests. The objectives of this study were to survey the EPNs and symbiotic bacteria in the agricultural areas of the Phitsanulok province, Thailand, and to study the association between the soil parameters and presence of EPNs. We collected 200 soil samples from 40 soil sites in agricultural areas (field crops, horticulture crops and forest). The prevalence of EPNs was 8.0% (16/200). Fifteen of the EPN isolates were molecularly identified (based on 28S ribosomal DNA and internal transcribed spacer regions) as Steinernema siamkayai. Seven isolates of Xenorhabdus stockiae were identified using recombinase A sequencing. Phylogenetic analysis revealed that all the Steinernema and Xenorhabdus isolates were closely related to S. siamkayai (Indian strain) and X. stockiae (Thai strain), respectively. Significantly more EPNs were recovered from loam than from clay. Although the association between soil parameters (pH, temperature and moisture) and the presence of EPNs was not statistically significant, the elevation levels of the soil sites with and without EPNs were found to be different. Moreover, statistical comparisons between the agricultural areas revealed no significant differences. Therefore, we concluded that S. siamkayai is associated with X. stockiae in agricultural areas and that there is no association between the soil parameters of agricultural areas and presence of EPNs, except for soil texture and the elevation. Steinernema siamkayai may be applied as a biocontrol agent in agricultural areas.

NK. Control of the tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) larvae in laboratory using entomopathogenic nematodes from subtropical environment

Tomato (Solanum esculentum) is one of the vegetable crops grown by both smallholder and commercial farmers in the Kingdom of Eswatini. Tomato leaf miner, Tuta absoluta M. is a major insect pest of tomatoes resulting in reduced tomato yields throughout the country. The study investigated the virulence of two sub-tropical EPN species on T. absoluta larvae. Steinernema yirgalemense and S. jeffreyense at different concentrations (0, 20, 40, 60 IJs/insect) were screened for efficacy (i.e. mortality rate) against larvae of T. absoluta in laboratory bioassays. Results obtained showed that S. yirgalemense and S. jeffreyense were able to kill T. absoluta larvae without significant differences between the two EPN species in 24-well bioassay trays. Significantly higher (p < 0.05) mortality was observed when 60 IJs/insect was used. The combination of S. yirgalemense at 60 IJs/insect (100%) and S. jeffreyense at 60 IJs/insect (98.3%) resulted in significantly higher (p < 0.05) mortality, compared with the other four combinations of EPN and concentration tested. In the leaf bioassays, S. yirgalemense (58.8%) resulted in significantly higher (p < 0.05) mean mortality compared to S. jeffreyense (46.4%). Steinernema yirgalemense at 60 IJs/insect resulted in significantly higher mean mortality compared to the other EPN and concentration combinations in the leaf bioassay. The results indicated that both EPNs tested were effective against T. absoluta larvae. Steinernema yirgalemense at 60 IJs/insect can effectively find T. absoluta larvae inside leaf mines, but large-scale field trials are recommended to demonstrate the potential use of the biocontrol agent within integrated pest management programs.

Tomato (Solanum esculentum) is one of the vegetable crops grown by both smallholder and commercial farmers in the Kingdom of Eswatini. Tomato leaf miner, Tuta absoluta M. is a major insect pest of tomatoes resulting in reduced tomato yields throughout the country. The study investigated the virulence of two sub-tropical EPN species on T. absoluta larvae. Steinernema yirgalemense and S. jeffreyense at different concentrations (0, 20, 40, 60 IJs/insect) were screened for efficacy (i.e. mortality rate) against larvae of T. absoluta in laboratory bioassays. Results obtained showed that S. yirgalemense and S. jeffreyense were able to kill T. absoluta larvae without significant differences between the two EPN species in 24-well bioassay trays. Significantly higher (p < 0.05) mortality was observed when 60 IJs/insect was used. The combination of S. yirgalemense at 60 IJs/insect (100%) and S. jeffreyense at 60 IJs/insect (98.3%) resulted in significantly higher (p < 0.05) mortality, compared with the other four combinations of EPN and concentration tested. In the leaf bioassays, S. yirgalemense (58.8%) resulted in significantly higher (p < 0.05) mean mortality compared to S. jeffreyense (46.4%). Steinernema yirgalemense at 60 IJs/insect resulted in significantly higher mean mortality compared to the other EPN and concentration combinations in the leaf bioassay. The results indicated that both EPNs tested were effective against T. absoluta larvae. Steinernema yirgalemense at 60 IJs/insect can effectively find T. absoluta larvae inside leaf mines, but large-scale field trials are recommended to demonstrate the potential use of the biocontrol agent within integrated pest management programs.

Xenorhabdus khoisanae SB10 produces Lys-rich PAX lipopeptides and a Xenocoumacin in its antimicrobial complex

BACKGROUND

Xenorhabdus spp. live in close symbiosis with nematodes of the Steinernema genus. Steinernema nematodes infect an insect larva and release their symbionts into the haemocoel of the insect. Once released into the haemocoel, the bacteria produce bioactive compounds to create a semi-exclusive environment by inhibiting the growth of bacteria, yeasts and molds. The antimicrobial compounds thus far identified are xenocoumacins, xenortides, xenorhabdins, indole derivatives, xenoamicins, bicornutin and a number of antimicrobial peptides. The latter may be linear peptides such as the bacteriocins xenocin and xenorhabdicin, rhabdopeptides and cabanillasin, or cyclic, such as PAX lipopeptides, taxlllaids, xenobactin and szentiamide. Thus far, production of antimicrobial compounds have been reported for Xenorhabdus nematophila, Xenorhabdus budapestensis, Xenorhabdus cabanillasii, Xenorhabdus kozodoii, Xenorhabdus szentirmaii, Xenorhabdus doucetiae, Xenorhabdus mauleonii, Xenorhabdus indica and Xenorhabdus bovienii. Here we describe, for the first time, PAX lipopeptides and xenocoumacin 2 produced by Xenorhabdus khoisanae. These compounds were identified using ultraperformance liquid chromatography, linked to high resolution electrospray ionisation mass spectrometry and tandem mass spectrometry.

RESULTS

Cell-free supernatants of X. khoisanae SB10 were heat stable and active against Bacillus subtilis subsp. subtilis, Escherichia coli and Candida albicans. Five lysine-rich lipopeptides from the PAX group were identified in HPLC fractions, with PAX1' and PAX7 present in the highest concentrations. Three novel PAX7 peptides with putative enoyl modifications and two linear analogues of PAX1' were also detected. A small antibiotic compound, yellow in colour and λ_max of 314 nm, was recovered from the HPLC fractions and identified as xenocoumacin 2. The PAX lipopeptides and xenocoumacin 2 correlated with the genes and gene clusters in the genome of X. khoisanae SB10.

CONCLUSION

With UPLC-MS and MS^e analyses of compounds in the antimicrobial complex of X. khoisanae SB10, a number of PAX peptides and a xenocoumacin were identified. The combination of pure PAX1' peptide with xenocoumacin 2 resulted in high antimicrobial activity. Many of the fractions did, however, contain labile compounds and some fractions were difficult to resolve. It is thus possible that strain SB10 may produce more antimicrobial compounds than reported here, as suggested by the APE Ec biosynthetic complex. Further research is required to develop these broad-spectrum antimicrobial compounds into drugs that may be used in the fight against microbial infections.

Development of reduced cost diets for the mass rearing of false codling moth (Thaumatotibia leucotreta)

BACKGROUND

Four trials were conducted; the first established optimal cooking time for false codling moth diets using a microwave oven while the other three trials aimed at determining nutrient requirements, testing these, and evaluating alternative cheaper raw materials.

RESULTS

Microwave preparation for 8 min, using a 1000 W microwave oven was found to be the optimal cooking time for all treatment diets at 200 g kg^-1 moisture. The minimum specification (MS) diet yielded more larvae per jar (mean ± standard deviation) (759 ± 29) than the control (CON) (652 ± 32) diet and the ideal amino acid profile (IAAP) diet (596 ± 46). The third trial subsequently utilized diets formulated according to the MS diet nutrient specifications but using novel and cheaper raw materials (D1 and D2). Production characteristics of diets D1 and D2 was compared to that of CON. Diets D1 and D2 yielded more larvae per jar (D1 = 911 ± 40; D2 = 830 ± 40) than that of CON (428 ± 40) with the added advantage that the newly formulated diets were cheaper.

CONCLUSION

Reduced cost feeds were formulated, mixed and tested which resulted in substantial saving in the mass rearing of the species for integrated pest management. This method shows potential for diet development of other insect species as well. © 2019 Society of Chemical Industry.

First Screening of Entomopathogenic Nematodes and Fungus as Biocontrol Agents against an Emerging Pest of Sugarcane, Cacosceles newmannii (Coleoptera: Cerambycidae)

Cacosceles newmannii (Coleoptera: Cerambycidae) is an emerging pest of sugarcane in South Africa. The larvae of this cerambycid beetle live within the sugarcane stalk and drill galleries that considerably reduce sugar production. To provide an alternative to chemical control, entomopathogenic nematodes and fungus were investigated as potential biological control agents to be used in an integrated pest management system. The nematodes Steinernema yirgalemense, S. jeffreyense, Heterorhabditis indica, and different concentrations of the fungus Metarhiziumpinghaense were screened for efficacy (i.e., mortality rate) against larvae of C. newmannii. The different biocontrol agents used, revealed a low level of pathogenicity to C. newmannii larvae, when compared to control treatments.

Field and Laboratory Performance of False Codling Moth, Thaumatotibia Leucotreta (Lepidoptera: Troticidae) on Orange and Selected Vegetables

False codling moth (FCM), Thaumatotibia leucotreta is a key pest of citrus orange and other plants causing fruit loss through larval feeding. Although this pest is native to sub-Saharan Africa little is known on its performance on orange and vegetables in Kenya and Tanzania. Our objective was to assess the incidence, oviposition preference and offspring performance of FCM on orange and vegetables, namely, okra, African eggplant, chili and sweet peppers. A higher percentage of orange with FCM damage symptoms was recorded from the ground than from the tree sampled fruit. However, FCM larval incidence was higher for the latter (tree sampled fruit). The highest FCM larval incidence amongst the vegetables was recorded on African eggplant (12%) while the lowest was on okra (3%). Orange was the most while African eggplant was the least preferred for oviposition by FCM. Among the vegetables tested, strong oviposition preference was found for sweet pepper; however, larval survival was lowest (62%) on this crop. Highest larval survival (77%) was recorded on orange. Most demographic parameters (i.e., intrinsic rate of increase, doubling time) were comparable among the studied host plants. The results are discussed in line of FCM management.

Bacteria of the Genus Xenorhabdus, a Novel Source of Bioactive Compounds

The genus Xenorhabdus of the family Enterobacteriaceae, are mutualistically associated with entomopathogenic nematodes of the genus Steinernema. Although most of the associations are species-specific, a specific Xenorhabdus sp. may infect more than one Steinernema sp. During the Xenorhabdus-Steinernema life cycle, insect larvae are infected and killed, while both mutualists produce bioactive compounds. These compounds act synergistically to ensure reproduction and proliferation of the nematodes and bacteria. A single strain of Xenorhabdus may produce a variety of antibacterial and antifungal compounds, some of which are also active against insects, nematodes, protozoa, and cancer cells. Antimicrobial compounds produced by Xenorhabdus spp. have not been researched to the same extent as other soil bacteria and they may hold the answer to novel antibacterial and antifungal compounds. This review summarizes the bioactive secondary metabolites produced by Xenorhabdus spp. and their application in disease control. Gene regulation and increasing the production of a few of these antimicrobial compounds are discussed. Aspects limiting future development of these novel bioactive compounds are also pointed out.

Shoot Blight on Chinese Fir (Cunninghamia lanceolata) is Caused by Bipolaris oryzae

Chinese fir (Cunninghamia lanceolata) is a significant timber species that has been broadly cultivated in southern China. A shoot blight disease on Chinese fir seedlings was discovered in Fujian, China and a fungus was then consistently associated with the symptoms. This fungus was determined to be causing this disease, among others by fulfilling Koch's postulates. Based on morphological characteristics and multilocus phylogenetic analyses with the sequences of the internal transcribed spacer, partial glyceraldehyde-3-phosphate dehydrogenase gene, partial translation elongation factor 1-α gene, and partial 28S large subunit ribosomal RNA gene, the fungus was identified as Bipolaris oryzae. These characteristics and phylogenetic analyses clearly support that this pathogen is different from B. sacchari, which was, until now, considered to be the causal agent of a similar blight on Chinese fir in Guangdong, China. The fungus was also shown to be strongly pathogenic to rice, one of the most susceptible hosts to B. oryzae. Crop rotation involving rice is often carried out with Chinese fir in southern China, a practice that most likely increases the risk of shoot blight on C. lanceolata. To our knowledge, shoot blight caused by B. oryzae is reported for the first time in a gymnosperm species.

Microbial control of phytophagous invertebrate pests in South Africa: Current status and future prospects

Invertebrate pests pose a significant threat to food security on the African continent. In response, South Africa has become one of the largest importers of chemical pesticides in sub-Saharan Africa, with several hundred active ingredients registered. To address the over-reliance on such chemicals, the South African Department of Agriculture, Forestry and Fisheries (DAFF) has eliminated or restricted several pesticides since the late 1970s. The recent launch of the South African National Bio-Economy Strategy and establishment of the South African Bioproducts Organisation (SABO), together with new guidelines for registration of biopesticides in 2015, also support this endeavour. Concurrently, entomopathogen-related research and bioproduct development has increased over the past decade. Currently, 31 products (seven manufactured locally) are registered under the Fertilizers, Farm Feeds, Agricultural Remedies and Stock Remedies Act 36 of 1947. Commercially important microbes include Beauveria bassiana (Cordycipitaceae), Metarhizium anisopliae (Clavicipitaceae), Cydia pomonella granulovirus, Cryptophlebia leucotreta granulovirus, Helicoverpa armigera nucleopolyhedrovirus (Baculoviridae) and Bacillus thuringiensis subsp. kurstaki and B. thuringiensis subsp. aizawai (Bacillaceae). Both parasitic and entomopathogenic nematodes (EPNs) show potential for development as bioinsecticides with one commercial EPN product, based on Heterorhabditis bacteriophora (Heterorhabditidae), registered under the Act. Rapid scientific progression, supported by a favourable legislative environment, should facilitate further advances in microbial control of phytophagous invertebrate pests in South Africa.

Biological characterization of the entomopathogenic nematode, Steinernema innovationi: a South African isolate

Entomopathogenic nematode species perform differently under different environmental conditions; therefore, the authors investigated the biological and environmental characteristics that could optimize performance of Steinernema innovationi. The authors studied the effect of temperature on infectivity and reproduction, the foraging behavior and host range. Thermal activity was optimal between 22 and 25°C. Highest infective juvenile (IJ) yields in last instar Galleria mellonella were observed at 22°C (333,014 IJs/g) and 25°C (354,165 IJs/g). An average of 26% of the IJs infected G. mellonella larvae at depths of 15 cm within 24 hr. Steinernema innovationi IJs raised greater than 95% of their body off the substrate and moved in a circular pattern, but did not jump. The hosts, Acheta domesticus, Chilo partellus, and Plutella xylostella showed the least susceptibility. All other hosts, [Eldana saccharina, Sesamia calamistis, Tenebrio molitor, G. mellonella, Cydia pomonella] suffered 100% larval mortality. Pupal mortality ranged from 47 to 68%. An LC₅₀ and LC₇₀ of 3 and 31 IJs/larva, respectively, were calculated for Agrotis ipsilon. The results indicate that S. innovationi is adapted to moderate temperatures. Furthermore, to the author's knowledge this is the first report of a nematode (particularly from 'glaseri group') that shows cruiser-type behavior in movement, but also nictates.

Entomopathogenic nematode species perform differently under different environmental conditions; therefore, the authors investigated the biological and environmental characteristics that could optimize performance of Steinernema innovationi. The authors studied the effect of temperature on infectivity and reproduction, the foraging behavior and host range. Thermal activity was optimal between 22 and 25°C. Highest infective juvenile (IJ) yields in last instar Galleria mellonella were observed at 22°C (333,014 IJs/g) and 25°C (354,165 IJs/g). An average of 26% of the IJs infected G. mellonella larvae at depths of 15 cm within 24 hr. Steinernema innovationi IJs raised greater than 95% of their body off the substrate and moved in a circular pattern, but did not jump. The hosts, Acheta domesticus, Chilo partellus, and Plutella xylostella showed the least susceptibility. All other hosts, [Eldana saccharina, Sesamia calamistis, Tenebrio molitor, G. mellonella, Cydia pomonella] suffered 100% larval mortality. Pupal mortality ranged from 47 to 68%. An LC₅₀ and LC₇₀ of 3 and 31 IJs/larva, respectively, were calculated for Agrotis ipsilon. The results indicate that S. innovationi is adapted to moderate temperatures. Furthermore, to the author’s knowledge this is the first report of a nematode (particularly from ‘glaseri group’) that shows cruiser-type behavior in movement, but also nictates.

Steinernema borjomiense n. sp. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from Georgia

A new species of entomopathogenic nematode, Steinernema borjomiense n. sp., was isolated from the body of the host insect, Oryctes nasicornis (Coleoptera: Scarabaeidae), in Georgia, in the territory of Borjomi-Kharagauli. Morphological characters indicate that the new species is closely related to species of the feltiae-group. The infective juveniles are characterised by the following morphological characters: body length of 879 (777-989) μm, distance between the head and excretory pore = 72 (62-80) μm, pharynx length = 132 (122-142) μm, tail length = 70 (60-80) μm, ratio a = 26.3 (23.0-29.3), H% = 45 (40-51), D% = 54 (47-59), E% = 102 (95-115), and lateral fields consisting of seven ridges (eight incisures) at mid-body. Steinernema borjomiense n. sp. was molecularly characterised by sequencing three ribosomal regions (the ITS, the D2-D3 expansion domains and the 18S rRNA gene) and the mitochondrial COI gene. Phylogenetic analyses revealed that S. borjomiense n. sp. differs from all other known species of Steinernema and is a member of the monticolum-group.

Diversity of entomopathogenic nematodes and their symbiotic bacteria in south African plantations and indigenous forests

The occurrence and diversity of entomopathogenic nematodes (EPN) and their symbiotic bacteria was evaluated in commercial forestry plantations (Eucalyptus spp., Pinus spp. and Acacia mearnsii) and indigenous forests in South Africa. EPN were most prevalent in A. mearnsii plantations, accounting for 60.7% of the isolates, while indigenous forests, plantations of Pinus spp. and Eucalyptus spp. accounted for 35.7, 3.6 and 0% of the isolates, respectively. DNA sequences of the internal transcribed spacer (ITS) and D2-D3 28S rDNA regions were used to identify the nematode species. Four Steinernema spp. were identified, including S. citrae, S. sacchari, two undescribed species, as well as Heterorhabditis bacteriophora and H. baujardi. Heterorhabditis baujardi is reported from South Africa for the first time. Analysis of 16S rRNA of the bacteria confirmed the presence of at least three Xenorhabdus species from Steinernema isolates and two subspecies of Photorhabdus luminescens from Heterorhabditis species.

Colletotrichum gloeosporioides sensu stricto Is a Pathogen of Leaf Anthracnose on Evergreen Spindle Tree (Euonymus japonicus)

The genus Colletotrichum is considered the eighth most important group of plant-pathogenic fungi in the world due to its scientific and economic importance. Colletotrichum spp. cause anthracnose disease in a wide range of economically important plants. Euonymus japonicus Thunb. (Celastraceae) is a broad-leaved evergreen tree that is widely planted in the parks and landscapes of China. An anthracnose occurs on E. japonicus in China but there has been a disagreement on the identity of the fungal pathogen. In this study, the fungal isolate HYCG2-3 was determined by Koch's postulates to be a pathogen on E. japonicus. Based on the morphological and molecular methods, isolate HYCG2-3 was identified as Colletotrichum gloeosporioides sensu stricto within the C. gloeosporioides species complex.

Steinernema pwaniensis n. sp., a new entomopathogenic nematode (Nematoda: Steinernematidae) from Tanzania

A new species of entomopathogenic nematode, Steinernema pwaniensis n. sp., belonging to the glaseri group, is described from Tanzania. The infective juveniles of S. pwaniensis n. sp. are characterized by a body length of 978 μm (808-1131), distance from anterior end to excretory pore of 86 μm (80-95) and a tail length of 87 μm (75-95). The ratios a, c and E% of S. pwaniensis n. sp. are substantially lower than those of all other African 'glaseri' group members. The first-generation males of S. pwaniensis n. sp. can be distinguished by a large spicule length of 92 μm (80-97) and by the absence of the caudal mucron, while second-generation males possess a short spine-like mucron. First-generation females have a peg-like tail tip bearing three spine-like projections. Second-generation females can be recognized by a slightly protruding vulva and well-developed post-anal swelling. The new species is further characterized by sequences of the internal transcribed spacer (ITS) and partial 28S regions of the ribosomal DNA. Phylogenetic analyses show that S. pwaniensis n. sp. forms a strongly supported monophyletic clade with two other East African species, S. ethiopiense and S. karii.

Steinernema biddulphi n. sp., a New Entomopathogenic Nematode (Nematoda: Steinernematidae) from South Africa

A new species of entomopathogenic nematode (EPN), Steinernema biddulphi n. sp., was isolated from a maize field in Senekal, Free State Province of South Africa. Morphological and molecular studies indicated the distinctness of S. biddulphi n. sp. from other Steinernema species. Steinernema biddulphi n. sp. is characterized IJs with average body length of 663 μm (606-778 μm), lateral fields with six ridges in mid-body region forming the formula 2,6,2. Excretory pore located anterior to mid-pharynx (D% = 46). Hyaline layer occupies approximately half of tail length. Male spicules slightly to moderately curved, with a sharp tip and golden brown in color. The first generation of males lacking a mucron on the tail tip while the second generation males with a short filamentous mucron. Genital papillae with 11 pairs and one unpaired preanal papilla. The new species is further characterized by sequences of the internal transcribed spacer (ITS) and partial 28S regions (D2-D3) of the ribosomal DNA (rDNA). Phylogenetic data show that S. biddulphi n. sp. belongs to the "bicornutum" clade within the Steinernematidae family.

Steinernema nguyeni n. sp. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from South Africa

Steinernema nguyenin. sp. was recovered by baiting from beneath anOlea africanatree in South Africa. The combination of morphological and molecular features suggests thatS. nguyenin. sp. is a member of thefeltiae-kraussei-oregonensegroup, clustering with members of this group in Clade III. The new species is morphologically characterised by the infective juvenile body length of 737 (673-796) μm and the number of ridges in the infective juvenile lateral field is 2, 8, 2. The male of the first generation can be recognised by the spicule length of 66 (58-75) μm and a gubernaculum length of 43 (30-55) μm. The first generation female can be recognised by the vulval lips only slightly protruding and the presence of low, double-flapped epiptygmata. Analysis of the ITS and D2-D3 regions of the ribosomal DNA confirms thatS. nguyenin. sp. differs from all other knownSteinernemaspecies.

Steinernema beitlechemi n. sp., a new entomopathogenic nematode (Nematoda: Steinernematidae) from South Africa

Steinernema beitlechemi n. sp. is a new species of entomopathogenic nematode isolated from soil samples collected in Bethlehem, Free State province, South Africa. The infective juveniles of S. beitlechemi n. sp. are characterised by a body length of 696 (606-768) μm, position of nerve ring of 96 (86-105) μm from the head and lateral fields with six ridges (i.e., seven lines) in mid-body. First-generation males of S. beitlechemi n. sp. can be distinguished by a spicule length of 93 (88-100) μm and 12 pairs of genital papillae. First-generation females have a moderately protruding vulva and a rounded tail tip bearing one spine-like projection. The new species is further characterised by sequences of the ITS and partial 28S regions of the ribosomal DNA. Phylogenetic analyses show S. beitlechemi n. sp. as a member of a strongly supported monophyletic clade with three other African species: S. sacchari, S. cameroonense and S. nyetense.

Molecular characterisation of novel isolates of entomopathogenic nematodes

Entomopathogenic nematodes belong to the families of Steinernematidae and Heterorhabditidae. They are obligate and lethal parasites of insects that can provide effective control of some important pests of commercial crops. A total of 53 isolates of EPN were molecularly characterised (ITS region-based) in the present study. Most of the studied isolates belong to theSteinernemagenus and only few isolates belong to theHeterorhabditisgenus. The phylogenetic relations ofSteinernemaandHeterorhabditisspecies were analysed by utilising the maximum likelihood method. In theSteinernemaphylogenetic tree, 99 isolates formed five major, moderately or highly reinforced clades: clade I:affine-intermediumgroup; clade II:carpocapsae-siamkayai-tami-scapterisci; clade III:bicornutum-riobrave-thermophilum; clade IV:glaseri-arenarium-karii-longicaudum; and clade V:feltiae-schliemanni-kushidai-kraussei-oregonense. The BLAST analysis of the ITS region of the rDNA of the steinernematid isolate PAL10 showed a rather low similarity of 93% withS. vulcanicum(accession number: GU929442), supporting the possible designation of a new species. In theHeterorhabditisphylogenetic tree, 25 isolates formed three main clades: clade I:bacteriophora-argentinensis-hepialius; clade II:baujardi-sonorensis-amazonensis; and clade III:indica-brevicaudis-hawaiiensis. All five studied isolates ofHeterorhabditiswere identified asH. indicaandH. bacteriophora. In both phylogenetic trees, the intra-specific variability level was different among clades for some species. The description of the new species (PAL10 isolate) would need further morphometric characterisation, morphologically identification and cross-breeding studies.

Steinernema fabii n. sp. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from South Africa

A new species of entomopathogenic nematode, Steinernema fabii n. sp., was isolated by trapping with wax moth (Galleria mellonella) larvae from soil in an Acacia mearnsii plantation in the Mpumalanga province of South Africa. The new species is morphologically characterised by the length of the infective juvenile (IJ) of 641 (590-697) μm, by a tail length of 58 (52-64) μm, ratio a = 24 (21-41), H% = 53 (37-61) and E% = 93 (83-105). The pattern of the lateral field of the IJ of the new species is 2, 5, 2 ridges (3, 6, 3 incisures). The male of the first generation can be recognised by the long spicule of 90 (79-106) μm and gubernaculum of 66 (56-77) μm; D% = 64 (52-75) and GS% = 73 (63-86). The first generation female can be recognised by a protruding vulva with a short, double-flapped epiptygmata, and the lack of a postanal swelling, while the second generation differs in having a postanal swelling and a conical, sharply pointed tail. Analysis of DNA sequences for the ITS and D2-D3 gene regions showed S. fabii n. sp. to differ from all other Steinernema species and to belong to a new monophyletic group, the ‘Cameroonian’ clade, consisting of S. cameroonense, S. nyetense, S. sacchari and S. fabii n. sp., all from the African continent. This group is closely related to species in the feltiae-kraussei-oregonense Clade III.

Life history traits, liquid culture production and storage temperatures of Steinernema yirgalemense

Using the hanging drop technique with nematode growth gelrite medium, life history traits of Steinernema yirgalemense (strain Sy 157-C) were investigated at a bacterial density of 10 × 109 cells ml−1 of Xenorhabdus indica at 25°C. With the same technique, the exit of dauer juveniles (DJ) from the arrested stage (recovery) was assessed at 5 × 109, 10 × 109 and 20 × 109 cells ml−1 of X. indica. Additionally, S. yirgalemense was incubated in nematode liquid medium at 25, 27 and 30°C. At each culture temperature, DJ recovery, sex ratio at 3 days post DJ inoculation and DJ yield and DJ as a percentage of non-DJ stages at 15 days post DJ inoculation were assessed. DJ survival in Ringer’s solution stored at 4, 15 and 25°C was assessed for 66 days. Steinernema yirgalemense has a total fertility rate and net reproductive rate of 487 and 314 offspring per female, respectively. The intrinsic rate of natural increase was 0.98 day−1, population doubling time PDT = 0.71 days and mean generation time days. The average lifespan of S. yirgalemense females starting from first-stage juveniles was 6.55 days. In liquid culture, DJ recovery ranged from 63-75% at 72 h post DJ inoculation and was not significantly different between the incubation temperatures. Parental male to female ratio was not influenced by incubation temperature and usually was at a ratio of 1:2. The percentage of females that entered into endotokia matricida at 72 h post DJ inoculation was 61% at 25°C, whereas at 27 and 30°C it was 24% and 0.5%, respectively. The highest DJ yield was recorded at 25°C (284 114 DJ ml−1) followed by 27°C (176 932 DJ ml−1) and the lowest at 30°C with 26 298 DJ ml−1. At a storage temperature of 4°C, DJ survival did not exceed 42 days, whereas at 15 and 25°C more than 95% of the DJ survived 66 days. Although S. yirgalemense DJ survived for long periods at both 15 and 25°C in liquid storage, their survival in formulated product and virulence after storage needs further investigation.

Entomopathogenic nematodes for the control of the codling moth (Cydia pomonellaL.) in field and laboratory trials

Three commercially available entomopathogenic nematode (EPN) strains (Steinernema feltiaeandHeterorhabditis bacteriophoraHb1 and Hb2) and two local species (S.jeffreyenseandS. yirgalemense) were evaluated for the control of the codling moth (Cydia pomonella). In field spray trials, the use ofS. jeffreyenseresulted in the most effective control (67%), followed byH. bacteriophora(Hb1) (42%) andS. yirgalemense(41%). Laboratory bioassays using spray application in simulated field conditions indicateS. feltiaeto be the most virulent (67%), followed byS. yirgalemense(58%). A laboratory comparison of the infection and penetration rate of the different strains showed that, at 14°C, all EPN strains resulted in slower codling moth mortality than they did at 25°C. After 48 h, < 15% mortality was recorded for all species, whereas, at the warmer temperature, >98% mortality was recorded for all species involved. However, the washed codling moth larvae, cool-treated (at 14°C) withS. feltiaeorS. yirgalemense, resulted in 100% mortality 24 h later at room temperature, whereas codling moth larvae treated with the twoH. bacteriophorastrains resulted in 68% and 54% control, respectively. At 14°C,S. feltiaehad the highest average penetration rate of 20 IJs/larva, followed byS. yirgalemense, with 14 IJs/larva. At 25°C,S. yirgalemensehad the highest penetration rate, with 39 IJs/larva, followed byS. feltiae, with 9 IJs/larva. This study highlights the biocontrol potential ofS. jeffreyense, as well as confirming thatS. feltiaeis a cold-active nematode, whereas the other three EPN isolates tested prefer warmer temperatures.

Adjuvants to improve aerial control of the citrus mealybug Planococcus citri (Hemiptera: Pseudococcidae) using entomopathogenic nematodes

The citrus mealybug, Planococcus citri, is a highly destructive pest of citrus, occurring only in the aerial parts of plants. Humidity will be one of the key factors to consider when using entomopathogenic nematodes (EPN) as biological control agents. Different adjuvants can be added to suspensions of EPNs, to improve control as a foliar application. An aqueous suspension containing Heterorhabditis zealandica and 0.3% Zeba® significantly increased P. citri mortality by 22% at 80% relative humidity (RH) with a temperature cycle starting at 22°C for 14 h and 11°C for 11 h. The same polymer formulation was tested for Steinernema yirgalemense and mortality of P. citri increased by 21% at 60% RH and by 27% at 80% RH. The addition of Nu-Film-P® and Zeba® to H. zealandica suspensions did not significantly retard application run-off on citrus leaves. The combination of Nu-Film-P® and Zeba®, however, was able to significantly retard sedimentation, increasing the average number of nematodes deposited on 2-cm2 leaf discs by 10 nematodes. In an aqueous suspension, nematodes settle rapidly to the bottom, resulting in an uneven distribution of nematodes. Xanthan gum, at a concentration of 0.2%, was highly effective at retarding sedimentation, with 72% of the initial nematode number still in suspension after 1 h. Zeba®, at a concentration of 0.3%, despite not being as effective as Xanthan gum, nevertheless still retarded sedimentation significantly. This is the first report of the potential of Nu-Film-P® and Zeba® to improve EPN performance against P. citri when used above ground in citrus orchards.

Development and population dynamics of Steinernema yirgalemense (Rhabditida: Steinernematidae) and growth characteristics of its associated Xenorhabdus indica symbiont in liquid culture

Entomopathogenic nematodes have become a valuable addition to the range of biological control agents available for insect control. An endemic nematode, Steinernema yirgalemense, has been found to be effective against a wide range of key insect pests. The next step would be the mass production this nematode for commercial application. This requires the establishment of monoxenic cultures of both the nematode and the symbiotic bacterium Xenorhabdus indica. First-stage juveniles of S. yirgalemense were obtained from eggs, while X. indica was isolated from nematode-infected wax moth larvae. The population density of the various life stages of S. yirgalemense during the developmental phase in liquid culture was determined. The recovery of infective juveniles (IJs) to the third-stage feeding juveniles, was 67 ± 10%, reaching a maximum population density of 75,000 IJs ml- 1 on day 13 after inoculation. Adult density increased after 8 days, with the maximum female density being 4600 ml- 1 on day 15, whereas the maximum male density was 4300 ml- 1 on day 12. Growth curves for X. indica showed that the exponential phase was reached 15 h after inoculation to the liquid medium. The stationary phase was reached after 42 h, with an average of 51 × 107 colony-forming units ml- 1. Virulence tests showed a significant difference in insect mortality between in vitro- and in vivo-produced nematodes. The success obtained with the production of S. yirgalemense in liquid culture can serve as the first step in the optimizing and upscaling of the commercial production of nematodes in fermenters.

Divergent thermal specialisation of two South African entomopathogenic nematodes

Thermal physiology of entomopathogenic nematodes (EPN) is a critical aspect of field performance and fitness. Thermal limits for survival and activity, and the ability of these limits to adjust (i.e., show phenotypic flexibility) depending on recent thermal history, are generally poorly established, especially for non-model nematode species. Here we report the acute thermal limits for survival, and the thermal acclimation-related plasticity thereof for two key endemic South African EPN species, Steinernema yirgalemense and Heterorhabditis zealandica. Results including LT50 indicate S. yirgalemense (LT50 = 40.8 ± 0.3 °C) has greater high temperature tolerance than H. zealandica (LT50 = 36.7 ± 0.2 °C), but S. yirgalemense (LT50 = -2.4 ± 0 °C) has poorer low temperature tolerance in comparison to H. zealandica (LT50 = -9.7 ± 0.3 °C), suggesting these two EPN species occupy divergent thermal niches to one another. Acclimation had both negative and positive effects on temperature stress survival of both species, although the overall variation meant that many of these effects were non-significant. There was no indication of a consistent loss of plasticity with improved basal thermal tolerance for either species at upper lethal temperatures. At lower temperatures measured for H. zealandica, the 5 °C acclimation lowered survival until below -12.5 °C, where after it increased survival. Such results indicate that the thermal niche breadth of EPN species can differ significantly depending on recent thermal conditions, and should be characterized across a broad range of species to understand the evolution of thermal limits to performance and survival in this group.

Steinernema jeffreyense n. sp. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from South Africa

During a non-targeted survey for entomopathogenic nematodes in South Africa, a new species of Steinernema was isolated from a soil sample collected from underneath a guava tree, close to the shore at Jeffrey's Bay. The nematode was isolated by means of the insect-baiting technique using last-instar larvae of Galleria mellonella. It is described herein as Steinernema jeffreyense n. sp. The nematode can be separated from other described, closely related species in terms of the morphological and morphometric characteristics of the different life stages, and in terms of the characterization and phylogeny of DNA sequences of the internal transcribed spacer (ITS) rDNA of the 18S gene, and of the D2D3 region of the 28S rDNA gene. The new species is placed molecularly in the arenarium-glaseri-karii-longicaudatum group characterized by the following morphological characters: infective third-stage juvenile with a body length of 926 (784-1043) μm, distance from head to excretory pore of 87 (78-107) μm, tail length of 81 (50-96) μm, with an E% of 109 (86-169), and eight evenly spaced ridges (i.e. nine lines) in the middle of the body. First-generation males have a spicule length of 88 (79-95) μm and gubernaculum length of 57 (51-61) μm. Male mucron is absent in both generations. First-generation females have an asymmetrical protuberance and a short, double-flapped epiptygmata, with both flaps directed to the front. The tail of the first-generation female is shorter than the anal body width, with a mucron on the dorsal tail tip, with D% = 78 (59-99). Cross-hybridization with S. khoisanae, S. tophus and S. innovationi showed the new species to isolate reproductively from the others. The analyses of ITS rDNA and D2D3 sequence of the 18S and 28S rDNA genes support the studied nematode isolate to be a valid new species belonging to the 'glaseri' group (Clade V).

Research on plant-parasitic and entomopathogenic nematodes in Ethiopia: a review of current state and future direction

Food self-sufficiency is Ethiopia’s national priority goal. Given that pest management seriously impacts agriculture, research on crop diseases is of paramount significance to the national goal. Here we provide a comprehensive account of research on plant-parasitic and entomopathogenic nematodes in Ethiopia. We show that the limited information available indicates that plant-parasitic nematodes impact crop production. There exists a serious gap in knowledge with regard to the effects of plant-parasitic nematodes on almost all major crops. This gap includes surveys with appropriate levels of identification, distribution, and yield loss and damage threshold studies on target crops. The current state of knowledge hinders the nation’s ability to design and implement appropriate control strategies for plant-parasitic nematodes. We propose a strategic assessment of plant-parasitic nematodes of all major crops, the need for systematic manpower training and continued search for entomopathogenic nematodes in the major agro-ecological zones of the nation and further research on those entomopathogenic nematodes already identified.

First report of the symbiotic bacterium Xenorhabdus indica associated with the entomopathogenic nematode Steinernema yirgalemense

The entomopathogenic nematode Steinernema yirgalemense is considered a promising agent in the biocontrol of insects. However, little is known about the bacteria living in symbiosis with the nematode. In this study, we have identified the only available bacterial strain (157-C) isolated from S. yirgalemense, as a member of the species Xenorhabdus indica. Identification was based on 16S rDNA, recA, dnaN, gltX, gyrB and infB gene sequence analyses. The relatedness of strain 157-C to the type strain of X. indica (DSM 17 382) was confirmed with DNA–DNA hybridization. The phenotypic characteristics of strain 157-C are similar to those described for the type strain of X. indica. This is the first report associating X. indica with S. yirgalemense.

Steinernema innovationi n. sp. (Panagrolaimomorpha: Steinernematidae), a new entomopathogenic nematode species from South Africa

Morphological and molecular sequence data were combined with cross-hybridization studies and used to identify a new Steinernema sp. from Free State, South Africa. Molecular and morphological data indicate that the new species belongs to the 'glaseri-group' of Steinernema spp. Key morphological diagnostic characters for S. innovationi n. sp. include the morphometric features of the third-stage infective juveniles: total body length = 1054 (1000-1103) μm, tail length = 108 (97-117) μm, location of the excretory pore = 88 (82-91) μm, and D% = 58 (54-63), E% = 115 (104-137) and H% = 43 (37-46). Additionally, the morphology of the spicules and gubernaculum of the first-generation males are considered key diagnostic traits. Steinernema innovationi n. sp. was also characterized by analysis of both rDNA and mitochondrial gene sequence data, which further indicate the uniqueness of this Steinernema species.

Photorhabdus heterorhabditis sp. nov., a symbiont of the entomopathogenic nematode Heterorhabditis zealandica

The bacterial symbionts SF41T and SF783 were isolated from populations of the insect pathogenic nematode Heterorhabditis zealandica collected in South Africa. Both strains were closely related to strain Q614 isolated from a population of Heterorhabditis sp. collected from soil in Australia in the 1980s. Sequence analysis based on a multigene approach, DNA-DNA hybridization data and phenotypic traits showed that strains SF41T, SF783 and Q614 belong to the same species of the genus Photorhabdus with Photorhabdus temperata subsp. cinerea as the most closely related taxon (DNA-DNA hybridization value of 68%). Moreover, the phylogenetic position of Photorhabdus temperata subsp. cinerea DSM 19724T initially determined using the gyrB sequences, was reconsidered in the light of the data obtained by our multigene approach and DNA-DNA hybridization experiments. Strains SF41T, SF783 and Q614 represent a novel species of the genus Photorhabdus, for which the name Photorhabdus heterorhabditis sp. nov. is proposed (type strain SF41T=ATCC BAA-2479T=DSM 25263T).

Steinernema sacchari n. sp. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from South Africa

A new species of entomopathogenic nematode, Steinernema sacchari n. sp., was isolated by trapping with the sugar cane borer, Eldana saccharina, from soil of a sugar cane field in the KwaZulu-Natal province of South Africa. The new species is morphologically characterised by the length of the infective juvenile (IJ) of 680 (630-722) μm, tail length of 64 (51-74) μm, ratio a = 19 (14-23), H% = 49 (43-57) and E% = 82 (70-109). The pattern of the lateral field of the IJ of the new species is 2, 5, 2 ridges (3, 6, 3 lines or incisures). The male of the first generation can be recognised by the long spicule of 83 (73-89) μm, gubernaculum of 61 (50-68) μm, D% = 67 (54-88) and GS% = 73 (66-81). The first generation male lacks a mucron, while the second generation male always has one. The first generation female can be recognised by the vulval lips not being raised, the possession of long double-flapped epiptygmata and the lack of a postanal swelling. Analysis of the ITS and D2D3 regions showed S. sacchari n. sp. to differ from all other Steinernema species and to belong to a new monophyletic group, the ‘Cameroonian’ clade, consisting of S. cameroonense, S. nyetense and S. sacchari n. sp. This group is closely related to the feltiae-kraussei-oregonense Clade III.

Photorhabdus luminescens subsp. noenieputensis subsp. nov., a symbiotic bacterium associated with a novel Heterorhabditis species related to Heterorhabditis indica

The bacterial symbiont AM7T, isolated from a novel entomopathogenic nematode species of the genus Heterorhabditis, displays the main phenotypic traits of the genus Photorhabdus and is highly pathogenic to Galleria mellonella. Phylogenetic analysis based on a multigene approach (16S rRNA, recA, gyrB, dnaN, gltX and infB) confirmed the classification of isolate AM7T within the species Photorhabdus luminescens and revealed its close relatedness to Photorhabdus luminescens subsp. caribbeanensis , P. luminescens subsp. akhurstii and P. luminescens subsp. hainanensis . The five concatenated protein-encoding sequences (4197 nt) of strain AM7T revealed 95.8, 95.4 and 94.9 % nucleotide identity to sequences of P. luminescens subsp. caribbeanensis HG29T, P. luminescens subsp. akhurstii FRG04T and P. luminescens subsp. hainanensis C8404T, respectively. These identity values are less than the threshold of 97 % proposed for classification within one of the existing subspecies of P. luminescens . Unlike other strains described for P. luminescens , strain AM7T produces acid from adonitol, sorbitol and xylitol, assimilates xylitol and has no lipase activity on medium containing Tween 20 or 60. Strain AM7T is differentiated from P. luminescens subsp. caribbeanensis by the assimilation of N-acetylglucosamine and the absence of haemolytic activity. Unlike P. luminescens subsp. akhurstii , strain AM7T does not assimilate mannitol, and it is distinguished from P. luminescens subsp. hainanensis by the assimilation of trehalose and citrate, the inability to produce indole from tryptophan and the presence of acetoin production and urease activity. Strain AM7T ( = ATCC BAA-2407T  = DSM 25462T) belongs to a novel subspecies, and is proposed as the type strain of Photorhabdus luminescens subsp. noenieputensis sp. nov.

Potential of entomopathogenic nematodes for the control of the banded fruit weevil,Phlyctinus callosus(Schönherr) (Coleoptera: Curculionidae)

Entomopathogenic nematodes (EPN) were evaluated for their potential use as biological control agents againstPhlyctinus callosus, the banded fruit weevil (BFW). The susceptibility of larvae and adults to EPN was evaluated using 400 infective juveniles (IJ) per insect after 4 days in 24-well bioassay trays. The nematode isolates used were all able to infect BFW, although the larvae were found to be more susceptible than were the adults. The percentage mortality for BFW larvae ranged from 41 to 73% and for BFW adults from 13 to 45%. The most effective isolate, SF41 ofHeterorhabditis zealandica, was used to investigate the effect of vertical movement of nematodes in sand and sandy loam soil, at specified concentration and temperature. A higher (82.2 ± 0.084%) percentage mortality rate was obtained with the sandy loam soil, than with the use of sand (67.5 ± 0.12%). The LD50and LD90values after 4 days of incubation were 96 and 278 IJ/50 μl, respectively. Nematodes were inactive below 15°C, with the highest mortality of 74 ± 0.081% for BFW larvae recorded at 25°C.Heterorhabditis zealandicawas able to complete its life cycle successfully in sixth-instar BFW larvae after a period of 22 days. The study showed BFW larvae not to be as susceptible to nematode infection as they need a high concentration (400 IJ/larva) and 4 days to give effective control.

Description of Xenorhabdus khoisanae sp. nov., the symbiont of the entomopathogenic nematode Steinernema khoisanae

Bacterial strain SF87(T), and additional strains SF80, SF362 and 106-C, isolated from the nematode Steinernema khoisanae, are non-bioluminescent Gram-reaction-negative bacteria that share many of the carbohydrate fermentation reactions recorded for the type strains of recognized Xenorhabdus species. Based on 16S rRNA gene sequence data, strain SF87(T) is shown to be closely related (98% similarity) to Xenorhabdus hominickii DSM 17903(T). Nucleotide sequences of strain SF87 obtained from the recA, dnaN, gltX, gyrB and infB genes showed 96-97% similarity with Xenorhabdus miraniensis DSM 17902(T). However, strain SF87 shares only 52.7% DNA-DNA relatedness with the type strain of X. miraniensis, confirming that it belongs to a different species. Strains SF87(T), SF80, SF362 and 106-C are phenotypically similar to X. miraniensis and X. beddingii, except that they do not produce acid from aesculin. These strains are thus considered to represent a novel species of the genus Xenorhabdus, for which the name Xenorhabdus khoisanae sp. nov. is proposed. The type strain is SF87(T) ( =DSM 25463(T) =ATCC BAA-2406(T)).

Heterorhabditis noenieputensis n. sp. (Rhabditida: Heterorhabditidae), a new entomopathogenic nematode from South Africa

A new entomopathogenic nematode in the genus Heterorhabditis is described from South Africa, from two singular isolates found 1000 km from each other, from beneath a fig tree and in a citrus orchard, respectively. Morphological and molecular studies indicate both isolates to be the same and a new undescribed Heterorhabditis species. Comparison of sequences of the internal transcribed spacer (ITS) rDNA and the D2D3 region of the 28S rDNA gene with available sequences of other described species within the genus, indicate the two isolates as a new species. Phylogenetic analysis of the sequence data concerned placed the new species, H. noenieputensis n. sp., closest to H. indica and H. gerrardi in the indica-group. The new species, H. noenieputensis n. sp., is distinguished from other species in the genus by a combination of several morphological traits of the males and the infective juveniles (IJs). The new species differs from all other species previously described, as regards the body length of the IJs, except for H. indica and H. taysearae, in which the IJ is smaller. The IJ also differs from that of H. indica in the length of the oesophagus, the body diameter, the length of the tail and the E%. In addition, males of H. noenieputensis n. sp. differ from their closest relative, H. indica, in the position of the excretory pore, SW% and D%; and from H. gerrardi in the length of the oesophagus and SW%. The seventh pair of genital papillae of H. noenieputensis n. sp. are normally developed, while for H. indica they are often branched or swollen at the base, while 8 and 9 are usually absent in both species.

Potential of South African entomopathogenic nematodes (Heterorhabditidae and Steinernematidae) for control of the citrus mealybug, Planococcus citri (Pseudococcidae)

Planococcus citri, the citrus mealybug, is the most important species of mealybug known to infest citrus in South Africa. Various laboratory bioassays were conducted to determine the potential of entomopathogenic nematodes to control P. citri. Adult female P. citri were screened for susceptibility to six indigenous nematode species. P. citri was found to be most susceptible to Steinernema yirgalemense and Heterorhabditis zealandica, causing 97% and 91% mortality, respectively. The development of nematodes after infecting adult female P. citri showed both H. zealandica and S. yirgalemense were able to complete their life cycles inside the host. Further bioassays illustrated a linear relationship between mealybug mortality and the concentration of nematodes applied, with the highest level of control using a concentration of 80 infective juveniles (IJs)/insect. As nematodes would be used as an above-ground application to control P. citri in citrus orchards, available water is a major limiting factor. Insecticidal activity proved to be dependent on the available surface moisture after nematode application. The water activity (a(w)) bioassay indicated that S. yirgalemense to be two times more tolerant to lower levels of free water, with a(w50)=0.96 and a(w90)=0.99, compared to H. zealandica with a(w50)=0.98 and a(w)90=1.0. After application, nematodes have a limited time frame in which to locate and infect hosts, as the level of available free water gradually decreases, as trees dry out. S. yirgalemense proved able to locate and infect P. citri quicker than H. zealandica. Nematode activity was not significantly affected when exposed to 15°C, 20°C and 25°C. IJs were able to infect P. citri at an exposure time as short as half an hour. Results also showed that the first 2-4h post application is the most decisive time for establishing successful infection of mealybugs. This is the first report on the potential use of nematodes for the control of P. citri.