Desiccation tolerance among different isolates of the entomopathogenic nematode Steinernema feltiae (Fillipjev)

Poor storage capacity is a major constraint limiting further expansion of the use of entomopathogenic nematodes. In order to prolong shelf life, a quiescent state of the dauer juveniles (DJs) should be induced. This can be attained by means of desiccation of DJs. In this study, 24 natural isolations of Steinernema feltiae were exposed to desiccation stress in non-ionic polyethylene glycol 600. The dehydrating conditions were measured as water activity, a(w)-value. Non-adapted and adapted DJs were tested separately under a series of dehydrating conditions. The mean tolerated a(w)-value (MW50) ranged from 0.85 for the isolate NEP1 to 0.95 for FIN1, ISR5 and ITA2 when not adapted to desiccation stress and from MW50 of 0.822 for CR1 to 0.98 for ISR6 when adapted to the stress conditions. CR1 tolerated the lowest desiccation stress at an a(w)-value for the most tolerant 10% of the population (MW10) at 0.65 when DJs had been adapted to stress. No significant differences were recorded between all isolates in non-adapted DJs populations MW10 was compared. No correlation between tolerance under non-adapted and adapted conditions were found. Most tolerant isolates will now be used for cross-breeding and subsequent genetic selection to enhance desiccation tolerance.

Intraspecific variability of Steinernema feltiae strains from Cemoro Lawang, eastern Java, Indonesia

Four strains of Steinernema feltiae from Eastern Java, Indonesia were characterized based on morphometric, morphological and molecular data. In addition, their virulence against last instar Tenebrio molitor and heat tolerance was tested. Infective juvenile have a mean body length ranging from 749 to 792 microm. The maximum sequence difference among the four strains was 7 bp (8.8%) in the ITS and 2 bp (0.3%) in D2D3 regions of the rDNA. All the strains are not reproductively isolated and can reproduce with European strain S. feltiae Owiplant. The lowest LC50 was observed for strain SCM (373) and the highest for S. feltiae strain Owiplant (458) IJs/40 T. molitor. All four strains showed relatively better mean heat tolerance when compared with S. feltiae Owiplant, both in adapted and non-adapted heat tolerance experiments.

The influence of humidity on the effect of Steinernema feltiae against diapausing codling moth larvae (Cydia pomonella L.) (Lepidoptera: Tortricidae)

Codling moth (CM) is a serious and global pest of pome fruit. It overwinters in cryptic habitats as cocooned diapausing larvae. Field trials with the entomopathogenic nematode Steinernema feltiae (Rhabditida: Steinernematidae) report control of diapausing CM of up to 70%, but results are variable. The objective of this study was to define environmental conditions favouring the performance of the nematodes. Cocooned larvae were more susceptible than non-cocooned larvae. S. feltiae was unable to infect CM at a water activity (aw-values) < or = 0.9. Mortality of cocooned larvae was reported at lower aw-values than of non-cocooned larvae. Exposure time and impact of external relative humidity (RH) was studied. Mortality of cocooned larvae did not further increase after half an hour of exposure, whereas the mortality increased with increasing exposure time in non-cocooned larvae. LC50 and LC90 considerably decreased with increasing RH. The influence of the relative humidity was less pronounced when surpassing 80% than the effect of the volume of applied water. When S. feltiae was formulated in a surfactant-polymer-formulation (SPF), mortality significantly increased when compared to application in water.

Attraction of Heterorhabditis sp. toward synthetic (E)-beta-cariophyllene, a plant SOS signal emitted by maize on feeding by larvae of Diabrotica virgifera virgifera

Most plants, when damaged by herbivore insects, synthesize and release various chemicals as indirect defence mechanism that attract parasitic or predatory insects that are natural enemies of the herbivores. When attacked by Western Corn Rootworms, the roots of many maize plant varieties emit (E)-beta-caryophyllene that attracts the neighbouring entomopathogenic nematodes to kill the feeding pest. Through plant genetics and biotechnology it was possible to manipulate this volatile compound in order to increase the effectiveness of entomopathogenic nematodes in reducing the damage of the pest. In order to further use this strategy to improve the effectiveness of Heterorhabditis bacteriophora by selective breeding, we invesa tigated the applicability of the strategy in different standard laboratory bioassays using three different sand and agar plate assays. The synthetic form of (E)-beta-caryophyllene and H. megidis (the strain, which in previous investigation, showed significant attraction to caryophyllene) were used in the study. In all bioassays no significant difference was observed in attraction of nematodes between the caryophyllene treatments and the controls. The results contradict results of previous investigations done by other investigators (Rasmann et al., 2005). Future investigations for the genetic improvement of the host finding ability of entomopathogenic nematodes can therefore not target attraction to caryophyllene.

Combining Steinernema carpocapsae and Bacillus thuringienis strains for control of diamondback moth (Plutella xylostella)

The diamondback moth (DBM), Plutella xylostella, is a major pest of crucifers and has developed resistance against all chemical insecticides, even against the biological insecticide Bacillus thuringiensis (Bt). Entomopathogenic nematodes (EPN) as novel biological insecticide were successfully used against DBM. We investigated the potential of Bt to enhance the virulence of EPN and the possibility to reduce the application density by exploiting synergistic effects. The interaction between different combinations of Bt and EPN were tested against early 3rd instar P. xylostella on 2 cm2 cabbage leaf discs. All of the combinations had additive effects. Synergistic results were exceptional. The result indicate that a joint application of Bt and EPN is not recommended. But alternating applications are a measure to manage resistance development against Bt.

Entomopathogenic nematodes in the European biocontrol market

In Europe total revenues in the biocontrol market have reached approximately 200 million Euros. The sector with the highest turn-over is the market for beneficial invertebrates with a 55% share, followed by microbial agents with approximately 25%. Annual growth rates of up to 20% have been estimated. Besides microbial plant protection products that are currently in the process of re-registration, several microbial products have been registered or are in the process of registration, following the EU directive 91/414. Entomopathogenic nematodes (EPN) are exceptionally safe biocontrol agents. Until today, they are exempted from registration in most European countries, the reason why SMEs were able to offer economically reasonable nematode-based products. The development of technology for mass production in liquid media significantly reduced the product costs and accelerated the introduction of nematode products in tree nurseries, ornamentals, strawberries, mushrooms, citrus and turf. Progress in storage and formulation technology has resulted in high quality products which are more resistant to environmental extremes occurring during transportation to the user. The cooperation between science, industry and extension within the EU COST Action 819 has supported the development of quality control methods. Today four companies produce EPN in liquid culture, offering 8 different nematode species. Problems with soil insects are increasing. Grubs, like Melolontha melolontha and other scarabaeidae cause damage in orchards and turf. Since the introduction of the Western Corn Rootworm Diabrotica virgifera into Serbia in 1992, this pests as spread all over the Balkan Region and has reached Italy, France and Austria. These soil insect pests are potential targets for EPN. The development of insecticide resistance has opened another sector for EPN. Novel adjuvants used to improve formulation of EPN have enabled the foliar application against Western Flower Thrips and Plutella xylostella. To reach these markets, the product costs for EPN will have to further decrease in the future. One possibility to reduce application costs related with the use of EPN is the inoculative application to cause long term effects on pest populations.

Mass production of entomopathogenic nematodes for plant protection

Entomopathogenic nematodes of the genera Heterorhabditis and Steinernema are commercially used to control pest insects. They are symbiotically associated with bacteria of the genera Photorhabdus and Xenorhabdus, respectively, which are the major food source for the nematodes. The biology of the nematode-bacterium complex is described, a historical review of the development of in vitro cultivation techniques is given and the current use in agriculture is summarised. Cultures of the complex are pre-incubated with the symbiotic bacteria before the nematodes are inoculated. Whereas the inoculum preparation and preservation of bacterial stocks follow standard rules, nematodes need special treatment. Media development is mainly directed towards cost reduction, as the bacteria are able to metabolise a variety of protein sources to provide optimal conditions for nematode reproduction. The process technology is described, discussing the influence of bioreactor design and process parameters required to obtain high nematode yields. As two organisms are grown in one vessel and one of them is a multicellular organism, the population dynamics and symbiotic interactions need to be understood in order to improve process management. Major problems can originate from the delayed or slow development of the nematode inoculum and from phase variants of the symbiotic bacteria that have negative effects on nematode development and reproduction. Recent scientific progress has helped to understand the biological and technical parameters that influence the process, thus enabling transfer to an industrial scale. As a consequence, costs for nematode-based products could be significantly reduced.

Growth of Photorhabdus luminescens in batch and glucose fed-batch culture

Photorhabdus luminescens, a bacterial symbiont of entomopathogenic biocontrol nematodes, was grown in batch and glucose fed-batch culture. The cell density, bioluminescence, production of antibiotic substances, number of cells with inclusion bodies, glucose concentration and oxygen uptake rate were recorded. The addition of 12.4 g 1(-1) glucose prolonged the growth, and the yield almost doubled, from 6.85 g 1(-1) to 12.45 g 1(-1) dry mass. The production of antibiotic substances increased by 140%. Bioluminescence was higher in the batch culture. A shift of P. luminescens to phase II variants was not detected.

Influence of the aeration rate on the yields of the biocontrol nematode Heterorhabditis megidis in monoxenic liquid cultures

The entomopathogenic nematode-bacterium complex Heterorhabditis megidis Photorhabdus luminescens was cultured in 10-1 internal loop bioreactors with marine impellers at aeration rates of 0.3 vvm and 0.7 vvm. Process parameters like impeller velocity and oxygen saturation were controlled at equal set points. The bacterial density was assessed at 24 h. Nematode dauer juveniles (DJ) were then inoculated and the development to adults after 8 days and final DJ yields after 16 days were recorded. The bacterial population density and the nematode inoculum development was variable and was not influenced by the aeration rate. A significant effect on the yield was recorded at the highest aeration rate. This result was confirmed by a direct comparison in two 5-1 internal loop glass bioreactors at 0.3 vvm and 1.0 vvm, which were inoculated with nematode and bacterium pre-cultures from the same flask culture. Possible reasons for the positive correlation between aeration rate and DJ yield are discussed.

Pathogenicity, development, and reproduction of Heterorhabditis bacteriophora and Steinernema carpocapsae under axenic in vivo conditions

Galleria mellonella larvae cultured axenically were treated with axenic dauer juveniles of Heterorhabditis bacteriophora and Steinernema carpocapsae. After 3 days S. carpocapsae had killed all insects, with 9.4 +/- 4.3 nematodes per larva. H. bacteriophora were unable to kill G. mellonella, although 13.3 +/- 6.4 nematodes per Galleria were found in the hemocoel. Invading nematodes of both strains recovered from the dauer stage. H. bacteriophora developed into hermaphrodites with eggs and J1 in the uterus and in the hemolymph of the living insects. Development beyond the J1 stage was not recorded. An injection of supernatants from different Photorhabdus luminescens cultures killed the insects but could not provide nutrients to support a further development. Only the injection of bacterial cells supported production of dauers in the axenic insects. Axenic S. carpocapsae developed to adults and produced offspring. After 3 weeks an average of 5275 nematodes per larva were counted, of which 6.7% were dauer juveniles, 39.2% other juvenile stages, 11.9% males, and 42.2% females. Compared to in vivo reproduction in the presence of the symbiotic bacterium Xenorhabdus nematophilus the dauer juvenile yields were low. Even after 5 weeks the percentage of dauer juveniles did not surpass 10%.

Molecular identification of Photorhabdus luminescens strains by amplification of specific fragments of the 16S ribosomal DNA

Sequence variation within the variable region of the 16S rRNA at position 440 to 480 allowed the synthesis of specific PCR primers for the identification of groups within the species Photorhabdus luminescens, symbionts of entomopathogenic nematodes of the genus Heterorhabditis. For the second PCR primer the highly conserved region at 755 to 795 was used. The P. luminescens type strain specific primer could not recognize any other P. luminescens strain. The primer TEMPERATUS based on the sequence of strain DSM12190 (isolated from North West European H. megidis strain HSH2) identified all P. luminescens associated with H. megidis from North West Europe and two isolates from closely the related nematode strains from Ireland. The primer TROPICUS based on strain DSM12191 (isolated from the nematode type strain H. indica strain LN2) identified P. luminescens of tropical origin isolated from H. indica. Symbionts of H. bacteriophora could not yet be separated into well described groups with the primers used. A comparison of sequence data resulted in the identification of additional groups. The non-symbiotic P. luminescens isolates are distinct in the variable region. The group HELIOTHIDIS contains 15 P. luminescens associated with H. bacteriophora from North East America. The MARELATUS group contains symbionts of the nematode H. marelatus from the West Coast of the US. The data together with the specific symbiotic association of P. luminescens strains with different nematode species support the division of the taxon P. luminescens into different species.

Influence of culture method on Steinernema glaseri lipids

Entomopathogenic nematodes can be mass produced in artificial media for use as biological insecticides. Nematode in vitro media have been primarily developed on the basis of yield without fully considering nematode nutritional requirements. We investigated the quality and quantity of lipids in the entomopathogenic nematode Steinernema glaseri when grown in vivo in Popillia japonica (a natural host), Galleria mellonella (a factitious host), and in solid and liquid media. Nematode yield (infective juveniles per mg dry organic material) was 4 times higher in the in vivo compared with the in vitro cultures. Nematodes produced in vivo using P. japonica accumulated a significantly higher amount of lipids compared with nematodes grown using G. mellonella or in vitro solid and liquid methods, respectively. Fractionation of S. glaseri total lipids revealed that nematodes produced using P. japonica accumulated significantly higher phospholipids and sterols compared with other methods. C:18 fatty acids were the predominant class of lipids in S. glaseri irrespective of production method. In vivo-produced nematodes had oleic 18:1 acid as the major fatty acid, whereas in vitro-produced S. glaseri had a mixture of oleic 18:1 and linoleic 18:2 acids as the predominant fatty acids. We conclude that the lipid composition of entomopathogenic nematode is host or medium dependent. We suggest that adjusting the in vivo medium by addition of components similar to a natural host nutritional composition should improve nematode production.

Encapsulation of the entomopathogenic nematode Steinernema feltiae in Tipula oleracea

The encapsulation response of Tipula oleracea to the entomopathogenic nematode Steinernema feltiae was investigated by exposing the insects to nematode dauer juveniles (DJs) and by injecting DJs with and without the symbiotic bacteria Xenorhabdus bovienii. The encapsulation response varied considerably between individual insect larvae. The variation could not be attributed to a more or less scattered nematode invasion over time since it was also recorded after simultaneous injection of a fixed DJ dose. The proportion of encapsulated nematodes declined with increasing dose (injected DJs/larva) from approx 80% for 1 DJ/larva to 33-34% for 20 DJ/larva. Tipula oleracea larvae were capable of encapsulating nematodes with and without symbionts inside the hemocoel; however, at doses of 10 and 20 DJ/larva, axenic nematodes were encapsulated less frequently than monoxenic nematodes. Injected axenic nematodes that were not encapsulated did not develop in T. oleracea larvae but disappeared from the insect's hemocoel. Coinjection of symbiotic bacteria increased encapsulation of axenic nematodes, showing that X. bovienii is triggering the encapsulation response of T. oleracea against S. feltiae.

Pathogenicity of axenic Steinernema feltiae, Xenorhabdus bovienii, and the bacto-helminthic complex to larvae of Tipula oleracea (Diptera) and Galleria mellonella (Lepidoptera)

The pathogenicity of the nematode-bacterium complex Steinernema feltiae-Xenorhabdus bovienii to larvae of Tipula oleracea and Galleria mellonella was investigated by injection of dauer juvenile nematodes carrying their bacterial symbiont cells (monoxenic nematodes). Axenic nematodes (free of bacteria) and the symbiotic bacteria themselves were tested. The LC50 of X. bovienii in T. oleracea was 15,700 colony forming units (CFU)/larva compared to < or = 8 CFU in G. mellonella. Xenorhabdus bovienii is apparently removed from the tipulids hemolymph, possibly by cellular defense mechanisms. Axenic nematodes were less pathogenic than monoxenic nematodes for both insects. The difference was less pronounced in G. mellonella larvae: one axenic nematode was sufficient to kill 80% in 1 day. The remaining insects found dead after 50 days were developmentally arrested. In T. oleracea 20 axenic nematodes caused 39% whereas 20 monoxenic dauer juveniles caused 90% mortality within 8 days. The data indicate that the virulence of the S. feltiae/X. bovienii complex is greater than the additive effect of the nematodes and their bacteria, further evidence for the synergistic activity of the symbiotic bacto-helminthic complex during pathogenesis.

Inability of the polyphasic approach to systematics to determine the relatedness of the genera Xenorhabdus and Photorhabdus

Comparative analysis of the genes coding for 16S rRNA of the type strains of Xenorhabdus and Photorhabdus species indicates the close phylogenetic relationship of these two genera. However, distance matrix analyses do not unambiguously separate the symbionts of entomopathogenic nematodes according to their assignment into different genera. When various 16S rRNA gene sequences from a selection of members of the gamma subclass of Proteobacteria and outgroup taxa were used, the intrageneric relationships of Xenorhabdus species and the positions of Photorhabdus luminescens and related species changed significantly.

Development and application of oligonucleotide probes for molecular identification of Xenorhabdus species

Synthetic deoxyoligonucleotide probes that hybridized against the region at positions 455 through 480 of 16S rRNA were developed for the identification of all five Xenorhabdus species. Sequence variation in the respective rRNA region between two strains of Xenorhabdus luminescens in addition allowed the construction of two strain-specific probes. Of 27 isolates determined to be Xenorhabdus strains by phenotypic characterization, 24 could be assigned to four of the five species. Two strains (HL-1 and HL-2) isolated from a Heterorhabditis sp. and a single strain (D-1.1) isolated from Steinernema affinis showed no hybridization signal with any of the five species-specific probes. With regard to the available species descriptions of nematodes, the results presented here confirm that, except for Steinernema affinis, the individual nematode hosts harbor only a single Xenorhabdus species.