Soil moisture effects on the activity of three entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) isolated from Meghalaya, India

Associational resistance through intercropping reduces yield losses to soil-borne pests and diseases

Associational resistance to herbivore and pathogen attack is a well documented ecological phenomenon and, if applied to agriculture, may reduce impact of pests and diseases on crop yields without recourse to pesticides. The value of associational resistance through intercropping, planting multiple crops alongside each other, as a sustainable control method remains unclear, due to variable outcomes reported in the published literature. We performed a meta-analysis to provide a quantitative assessment of benefits of intercropping for target plant resistance to plant-parasitic nematodes and soil-borne diseases. We found that intercropping reduced damage to focal crops from nematodes by 40% and disease incidence by 55%. Intercropping efficacy varied with biological variables, such as field fertilisation status and intercrop family, and methodology, including whether study samples were potted or in fields. Nematode control using intercropping was sufficient to offset reductions in focal crop yield from intercrop presence, making intercropping a viable agricultural tool. We identify key drivers for underpinning the success of intercropping and indicate areas for future research to improve efficacy. This study also highlights the potential benefits of harnessing ecological knowledge on plant-enemy interactions for improving agricultural and landscape sustainability.

Ecological Characterisation of Native Isolates of Heterorhabditis indica from Viti Levu, Fiji Islands

Entomopathogenic nematodes (EPNs) in the families Steinernematidae and Heterorhabditidae are obligate parasites of soil inhibiting insects. EPNs are being widely researched as promising biocontrol agents for a wide range of agricultural pests. It is known that strains of EPNs isolated from different geographical regions differ in their attributes, such as host-finding ability, host range, infectivity, reproduction, and environmental stress tolerance. A precise knowledge of these factors is therefore an essential pre-requisite for devising successful strategies to use these nematodes in biological control programmes. Thus, ecological characterisation of the EPN Heterorhabditis indica (Rhabditida: Heterorhabditidae) newly isolated and representing the only species of EPN reported from the island of Viti Levu, Fiji was carried out using Galleria mellonella larvae (L) (Pyralidae: Galleriinae) as hosts to allow comparisons between bioassays conducted in different laboratories around the world. Temperature data showed that native isolates of H. indica are warm-adapted nematodes with thermal range for infectivity between 15˚C and 35˚C and can reproduce between 20˚C and 30˚C. They are highly virulent with LC50 values against G. mellonella ranging from 2.8 IJ to 3.8 IJ/larva. However, they showed poor desiccation tolerance and fail to infect hosts in soil with moisture levels below 8%. They showed a moderate level of hypoxic tolerance and can be stored at 15˚C for 4 months. Results also showed great variability within the selected native isolates of H. indica. Beneficial traits for selected isolates were added up to identify a superior candidate. The current study also suggested that the thermal niche breadth for infection can differ among conspecific strains of an EPN species. The results of this experimental study on ecological aspects of these native isolates of H. indica should form a basis for their potential use in biological control of insect pests in Fiji.

Quantification of pH tolerance levels among entomopathogenic nematodes

Soil pH affects the availability of nutrients, which impacts plant growth and development. Similarly, soil pH may also influence microorganisms in the soil, either beneficial or nonbeneficial. One such group of beneficial microorganisms is entomopathogenic nematodes (EPN), parasites of soil-inhabiting insects. Entomopathogenic nematodes have a number of attributes that make them good alternatives to chemical insecticides. The objective of this study was to investigate pH tolerance of 11 steinernematids and six heterorhabditids post exposure to different pH levels. Entomopathogenic nematode populations were exposed to varying pH levels (pH2 to pH11) made up from two different chemical solutions (ammonium-acetate and citrate-phosphate). Entomopathogenic nematode populations are expected to have varying tolerance to different pH levels. The highest infective juvenile survival was obtained from pH3 to pH10 in citrate-phosphate, where all populations displayed >50% survival. Steinernema carpocapsae populations had >90% survival at pH3 to pH11 in citrate-phosphate solutions. Overall, the steinernematids had a higher survival range in ammonium-acetate pH solutions compared with the heterorhabditids. Moreover, Steinernema spp., S. carpocapsae (ScCxrd, ScAll, and ScItalian) and S. riobrave showed consistently higher survival in both acidic and alkaline solutions, when compared to the other steinernematids, suggesting that they may be applied in both acidic and alkaline soils. These findings can be of use when selecting EPNs for biological control purposes in the two countries, respectively.

Characterization of a new isolate of entomopathogenic nematode, Steinernema sangi (Rhabditida, Steinernematidae), and its symbiotic bacteria Xenorhabdus vietnamensis (γ-Proteobacteria) from Mizoram, northeastern India

A survey was conducted to isolate indigenous EPN, specifically from the northeastern part of India, a biodiversity hotspot region, to record the occurrence and their further use as biological control agent. The morphological and molecular analysis (ITS rDNA for Steinernema and 16S rRNA for Xenorhabdus) revealed that the entomopathogenic nematodes isolated from four different habitats and its symbiotic bacteria are conspecific with Steinernema sangi and Xenorhabdus vietnamensis respectively. The phylogenetic analysis based on maximum parsimony (MP) revealed that Steinernema sangi belongs to feltiae-kraussei-oregonense group. The study constitutes the first report of Steinernema sangi and its symbiotic bacteria Xenorhabdus vietnamensis outside the type locality, Vietnam, and in particular from India.

Assessing the Role of Environmental Conditions on Efficacy Rates of Heterorhabditis indica (Nematoda: Heterorhabditidae) for Controlling Aethina tumida (Coleoptera: Nitidulidae) in Honey Bee (Hymenoptera: Apidae) Colonies: a Citizen Science Approach

Certain species of entomopathogenic nematodes, such as Heterorhabditis indica Poinar, Karunakar & David, have the potential to be effective controls for Aethina tumida (Murray), or small hive beetles, when applied to the soil surrounding honey bee (Apis mellifera L.) hives. Despite the efficacy of H. indica, beekeepers have struggled to use them successfully as a biocontrol. It is believed that the sensitivity of H. indica to certain environmental conditions is the primary reason for this lack of success. Although research has been conducted to explore the impact of specific environmental conditions--such as soil moisture or soil temperature-on entomopathogenic nematode infectivity, no study to date has taken a comprehensive approach that considers the impact of multiple environmental conditions simultaneously. In exploring this, a multivariate logistic regression model was used to determine what environmental conditions resulted in reductions of A. tumida populations in honey bee colonies. To obtain the sample sizes necessary to run a multivariate logistic regression, this study utilized citizen scientist beekeepers and their hives from across the mid-Atlantic region of the United States. Results suggest that soil moisture, soil temperatures, sunlight exposure, and groundcover contribute to the efficacy of H. indica in reducing A. tumida populations in A. mellifera colonies. The results of this study offer direction for future research on the environmental preferences of H. indica and can be used to educate beekeepers about methods for better utilizing H. indica as a biological control.

Effects of storage temperature on survival and infectivity of three indigenous entomopathogenic nematodes strains (Steinernematidae and Heterorhabditidae) from Meghalaya, India

Three locally isolated strains of entomopathogenic nematodes (EPNs), viz. Heterorhabditis indica, Steinernema thermophilum and Steinernema glaseri, from Meghalaya, India were characterized in terms of storage temperature and survival and infectivity of their infective juveniles (IJs). The survival and infectivity of nematode IJs was studied at, 5 ± 2 and 25 ± 2 °C, for a period of 120 days, using deionized water as storage medium. The viability of nematode IJs was checked by mobility criterion at different storage periods, while the infectivity of nematode IJs was ascertained on the basis of establishment of IJs, using Galleria mellonella larva mortality tests in petridishes. The results of this study revealed that storage temperature markedly affects the survival as well as the establishment of nematode IJs of the three EPN species. At 5 °C, comparatively higher rate of IJ's survival (i.e. 74-86 %) was observed for 15 days of storage, but the same reduced drastically to 28-32 % after 30 days of storage for H. indica and S. thermophilum. On the other hand, at 25 °C, the survival of nematode IJs was observed till 120 days for all the three studied EPNs. In case of S. thermophilum and S. glaseri, higher rate of IJs survival (>75 %) was observed respectively at 15 and 30 days of observation. The study also showed that the establishment of IJs of the three EPN species declines with increase in storage periods, at both the test temperatures. In general, the nematodes stored at 25 °C showed comparatively better establishment than those stored at 5 °C. Among the three EPN studied, the establishment of S. glaseri was comparatively better than the rest of the species at both the temperatures and for different storage durations. In conclusion, our study adds further valuable information about the effect of storage temperature on survival and infectivity of three indigenous EPN species of Meghalaya, India which appears to be promising biocontrol agents of local insect pests.