Dauer juvenile recovery transcriptome of two contrasting EMS mutants of the entomopathogenic nematode Heterorhabditis bacteriophora

The entomopathogenic nematode Heterorhabditis bacteriophora, symbiotically associated with enterobacteria of the genus Photorhabdus, is a biological control agent against many insect pests. Dauer Juveniles (DJ) of this nematode are produced in industrial-scale bioreactors up to 100 m3 in liquid culture processes lasting approximately 11 days. A high DJ yield (> 200,000 DJ·mL-1) determines the success of the process. To start the mass production, a DJ inoculum proceeding from a previous monoxenic culture is added to pre-cultured (24 h) Photorhabdus bacteria. Within minutes after contact with the bacteria, DJ are expected to perceive signals that trigger their further development (DJ recovery) to reproductive hermaphrodites. A rapid, synchronized, and high DJ recovery is a key factor for an efficient culture process. In case of low percentage of DJ recovery, the final DJ yield is drastically reduced, and the amount of non-desired stages (males and non-fertilized females) hinders the DJ harvest. In a preliminary work, a huge DJ recovery phenotypic variability in H. bacteriophora ethyl methanesulphonate (EMS) mutants was determined. In the present study, two EMS-mutant lines (M31 and M88) with high and low recovery phenotypes were analyzed concerning their differences in gene expression during the first hours of contact with Photorhabdus supernatant containing food signals triggering recovery. A snapshot (RNA-seq analysis) of their transcriptome was captured at 0.5, 1, 3 and 6 h after exposure. Transcripts (3060) with significant regulation changes were identified in the two lines. To analyze the RNA-seq data over time, we (1) divided the expression profiles into clusters of similar regulation, (2) identified over and under-represented gene ontology categories for each cluster, (3) identified Caenorhabditis elegans homologous genes with recovery-related function, and (4) combined the information with available single nucleotide polymorphism (SNP) data. We observed that the expression dynamics of the contrasting mutants (M31 and M88) differ the most within the first 3 h after Photorhabdus supernatant exposure, and during this time, genes related to changes in the DJ cuticle and molting are more active in the high-recovery line (M31). Comparing the gene expression of DJ exposed to the insect food signal in the haemolymph, genes related to host immunosuppressive factors were not found in DJ upon bacterial supernatant exposure. No link between the position of SNPs associated with high recovery and changes in gene expression was determined for genes with high differential expression. Concerning specific transcripts, nine H. bacteriophora gene models with differential expression are provided as candidate genes for further studies.

Monitoring the Photorhabdus spp. bacterial load in Heterorhabditis bacteriophora dauer juveniles over different storage times and temperatures: A molecular approach

Biological control products based on the entomopathogenic nematode Heterorhabditis bacteriophora can vary in virulence (quality). The influence of their symbiotic bacteria Photorhabdus spp. inside the infective dauer juvenile (DJ) on DJ quality has not received much attention in the past. The presence of the bacteria in the DJ is crucial for its biocontrol potential. This investigation provides a method to quantify the bacterial load inside the DJ based on a qPCR technique. Information from the genome of Photorhabdus laumondii strain DE2 was used to identify single copy genes with no homology to any other bacterial accessions. One gene (hereby named CG2) was selected for primers design and for further qPCR experiments. Cross-amplification tests with P. thracensis and P. kayaii, also symbionts of H. bacteriophora, were positive, whereas no amplicons were produced for P. temperata or Xenorhabdus nematophila. We tested our qPCR system in DJ populations carrying defined proportions of bacteria-free (axenic) vs bacteria-carrying nematodes. With an increasing proportion of axenic DJ in a population, virulence declined, and the virulence was proportional to the amount of bacterial DNA detected in the population by qPCR. Along liquid storage over long time, virulence also decreased, and this factor correlated with the reduction of bacterial DNA on the respective DJ population. We observed that stored DJ kept virulent up to 90 days and thereafter the virulence as well as the amount of bacterial DNA drastically decreased. Storage temperature also influenced the bacterial survival. Inside formulated DJ, the loss of bacterial DNA on the DJ population was accelerated under storage temperatures below 7.5 °C, suggesting that reproduction of the bacterial cells takes place when growth temperature is favorable. The role of bacterial survival inside stored DJ can now be adequately addressed using this molecular quality-control technique.

Pheno- and genotyping in vitro dauer juvenile recovery in the nematode Heterorhabditis bacteriophora

The entomopathogenic nematode (EPN) Heterorhabditis bacteriophora is an effective biological-control agent of insect pests. The dauer juveniles (DJs) seek for, infect insects, and release cells of the carried symbiotic bacterium of the genus Photorhabdus. Inside the host, the DJs perceive signals from the insect's haemolymph that trigger the exit from the arrested stage and the further development to mature adults. This developmental step is called DJ recovery. In commercial production, a high and synchronous DJ recovery determines the success of liquid-culture mass production. To enhance the understanding about genetic components regulating DJ recovery, more than 160 mutant- and 25 wild type inbred lines (WT ILs) were characterized for DJ recovery induced by cell-free bacterial supernatant. The mutant lines exhibited a broader DJ recovery range than WT ILs (4.6-67.2% vs 1.6-35.7%). A subset of mutant lines presented high variability of virulence against mealworm (Tenebrio molitor) (from 22 to 78% mortality) and mean time survival under oxidative stress (70 mM H2O2; from 10 to 151 h). Genotyping by sequencing of 96 mutant lines resulted in more than 150 single nucleotide polymorphisms (SNPs), of which four results are strongly associated with the DJ recovery trait. The present results are the basis for future approaches in improving DJ recovery by breeding under in vitro liquid-culture mass production in H. bacteriophora. This generated platform of EMS-mutants is as well a versatile tool for the investigation of many further traits of interest in EPNs. KEYPOINTS: • Exposure to bacterial supernatants of Photorhabdus laumondii induces the recovery of Heterorhabditis bacteriophora dauer juveniles (DJs). Both, the bacteria and the nematode partner, influence this response. However, the complete identity of its regulators is not known. • We dissected the genetic component of DJ recovery regulation in H. bacteriophora nematodes by generating a large array of EMS mutant lines and characterizing their recovery pheno- and genotypes. • We determined sets of mutants with contrasting DJ recovery and genotyped a subset of the EMS-mutant lines via genotyping by sequencing (GBS) and identified SNPs with significant correlation to the recovery trait.

Enhancing mass production of Heterorhabditis bacteriophora: influence of different bacterial symbionts (Photorhabdus spp.) and inoculum age on dauer juvenile recovery

The entomopathogenic nematode Heterorhabditis bacteriophora (Nematoda: Rhabditidae) is used in biological insect control. Their dauer juveniles (DJs) are free-living and developmentally arrested, invading host insects. They carry cells of their bacterial symbiont Photorhabdus spp. in the intestine. Once inside the insect´s hemolymph the DJs perceive a food signal, triggering them to exit the DJ stage and regurgitate the Photorhabdus cells into the insect's haemocoel, which kill the host and later provide essential nutrients for nematode reproduction. The exit from the DJ stage is called "recovery". For commercial pest control, nematodes are industrially produced in monoxenic liquid cultures. Artificial media are incubated with Photorhabdus before DJs are added. In absence of the insect's food signal, DJs depend on unknown bacterial food signals to trigger exit of the DJ stage. A synchronized and high DJ recovery determines the success of the industrial in vitro production and can significantly vary between nematode strains, inbred lines and mutants. In this study, fourteen bacterial strains from H. bacteriophora were isolated and identified as P. laumondii, P. kayaii and P. thracensis. Although the influence of bacterial supernatants on the DJ recovery of three inbred lines and two mutants differed significantly, the bacterial impact on recovery has a subordinate role whereas nematode factors have a superior influence. Recovery of inbred lines decreased with age of the DJs. One mutant (M31) had very high recovery in bacterial supernatant and spontaneous recovery in Ringer solution. Another mutant (M88) was recovery defective.

Stress tolerance in entomopathogenic nematodes: Engineering superior nematodes for precision agriculture

Entomopathogenic nematodes (EPNs) are soil-dwelling parasitic roundworms commonly used as biocontrol agents of insect pests in agriculture. EPN dauer juveniles locate and infect a host in which they will grow and multiply until resource depletion. During their free-living stage, EPNs face a series of internal and environmental stresses. Their ability to overcome these challenges is crucial to determine their infection success and survival. In this review, we provide a comprehensive overview of EPN response to stresses associated with starvation, low/elevated temperatures, desiccation, osmotic stress, hypoxia, and ultra-violet light. We further report EPN defense strategies to cope with biotic stressors such as viruses, bacteria, fungi, and predatory insects. By comparing the genetic and biochemical basis of these strategies to the nematode model Caenorhabditis elegans, we provide new avenues and targets to select and engineer precision nematodes adapted to specific field conditions.

Single nucleotide polymorphism markers in Heterorhabditis bacteriophora associated with virulence at low temperature

The entomopathogenic nematode (EPN), Heterorhabditis bacteriophora, is an important biological control agent worldwide. Industrially produced EPN need to meet the climatic requirements for the control of pests in field agriculture in autumn and spring when temperatures are low. For this trait (virulence at low temperature), previous EPN improvement attempts relied on phenotypic selection and the selected trait had low stability. The use of molecular markers can increase the efficacy of EPN breeding by tracking traits associated with specific genotypes. To date, fewer than 200 polymorphic and reproducible sequence-tagged molecular markers in H. bacteriophora have been reported. Here, we enhanced the palette of highly polymorphic genetic markers for this EPN by applying genotyping by sequencing (GBS). By analysing 48 H. bacteriophora homozygous wild-type inbred lines from different origins, we determined 4894 single nucleotide polymorphisms (SNPs) with at least one polymorphism along the tested set. For validation, we designed robust PCR assays for seven SNPs, finding 95% correspondence with the expected genotypes along 294 analysed alleles. We phenotyped all lines for their virulence at low temperature (15°C) against mealworm and observed infectivity ranging from 38 to 80%. Further, we carried out association analyses between genotypic and phenotypic data and determined two SNPs yielding potential association with H. bacteriophora virulence at low temperature. The use of these candidate SNPs as breeding markers will speed up the generation of strains better adapted to low temperature in this species. The generated set of lines and SNP data are a versatile tool applicable for further traits in this EPN.

Effects of soil texture and moisture on the host searching abilities of Steinernema siamkayai against Bactrocera latifrons

Soil texture and moisture have significant effects on the infectivity and persistence of entomopathogenic nematodes (EPN). Both EPN and chili fruit flies (Bactrocera latifrons) spend most of their lifetime in soil, therefore making it possible to consider the successful use of EPN against chili fruit flies. This study aimed to investigate the effects of soil texture and moisture contents on chili fruit fly pupation and adult emergence, and on the infectivity of the EPN, Steinernema siamkayai, against this pest. The results revealed that the highest percentage of pupation occurred at a depth of 2 cm in all soil types. At a depth of 5 cm in the sandy clay loam only, no pupation of B. latifrons was observed. The adult emergence ranged from 51.9 to 82.5%, with significant differences between soil types. The horizontal movement of S. siamkayai showed that the concentration of EPN at a rate of 20 000 dauer juveniles pupa−1 yielded maximum pupae mortality (77.5-91.3%) in all soil types containing 15% moisture. In the vertical displacement test, S. siamkayai was capable of killing pupae up to a distance of 15 cm in all soil types. These results confirmed the potential of S. siamkayai to be used within pest control programmes of various soil textures, as long as the soil moisture contents allow its dispersal and host infection.

Gluconacetobacter diazotrophicus Pal5 Enhances Plant Robustness Status under the Combination of Moderate Drought and Low Nitrogen Stress in Zea mays L

Plant growth promoting endophytic bacteria, which can fix nitrogen, plays a vital role in plant growth promotion. Previous authors have evaluated the effect of Gluconacetobacter diazotrophicus Pal5 inoculation on plants subjected to different sources of abiotic stress on an individual basis. The present study aimed to appraise the effect of G. diazotrophicus inoculation on the amelioration of the individual and combined effects of drought and nitrogen stress in maize plants (Zea mays L.). A pot experiment was conducted whereby treatments consisted of maize plants cultivated under drought stress, in soil with a low nitrogen concentration and these two stress sources combined, with and without G. diazotrophicus seed inoculation. The inoculated plants showed increased plant biomass, chlorophyll content, plant nitrogen uptake, and water use efficiency. A general increase in copy numbers of G. diazotrophicus, based on 16S rRNA gene quantification, was detected under combined moderate stress, in addition to an increase in the abundance of genes involved in N fixation (nifH). Endophytic colonization of bacteria was negatively affected by severe stress treatments. Overall, G. diazotrophicus Pal5 can be considered as an effective tool to increase maize crop production under drought conditions with low application of nitrogen fertilizer.

Molecular identification, phylogeny and phylogeography of the entomopathogenic nematodes of the genus Heterorhabditis Poinar, 1976: a multigene approach

Presently, the genus Heterorhabditis contains 16 valid entomopathogenic nematode species. In this study we used samples from 11 species: H. amazonensis, H. bacteriophora, H. baujardi, H. beicherriana, H. downesi, H. floridensis, H. georgiana, H. indica, H. megidis, H. noenieputensis, and H. zealandica to amplify and sequence five gene fragments: the D2-D3 expansion segments of 28S rRNA, ITS rRNA, COI mtDNA genes and unc-87 and cmd-1 genes encoding thin filament (F-actin)-associated protein and calmodulin, respectively. Fifty new sequences for 11 species were generated. More than 980 sequences of five genes were analysed. Phylogenetic and sequence analysis of these genes using Bayesian inference, maximum likelihood and statistical parsimony confirmed a division of the genus into three clades (groups): ‘Indica’, ‘Bacteriophora’ and ‘Megidis’. The analysis of gene sequences downloaded from GenBank and identified as Heterorhabditis revealed many cases of species misidentifications and presence of reading mistakes in some sequences. Synonymisation of H. somsookae with H. baujardi, H. gerrardi, H. pakistanensis with H. indica, and H. sonorensis with H. taysearae, are confirmed by sequence and phylogenetic analysis. The ITS rRNA and COI genes could be considered as informative markers for species identification, barcoding and phylogeographical studies of Heterorhabditis.

Characterization of the phenotypic and genotypic tolerance to abiotic stresses of natural populations of Heterorhabditis bacteriophora

Entomopathogenic nematodes are effective biocontrol agents against arthropod pests. However, their efficacy is limited due to sensitivity to environmental extremes. The objective of the present study was to establish a foundation of genetic-based selection tools for beneficial traits of heat and desiccation tolerance in entomopathogenic nematodes. Screening of natural populations enabled us to create a diverse genetic and phenotypic pool. Gene expression patterns and genomic variation were studied in natural isolates. Heterorhabditis isolates were phenotyped by heat- and desiccation-stress bioassays to determine their survival rates compared to a commercial line. Transcriptomic study was carried out for the commercial line, a high heat-tolerant strain, and for the natural, low heat-tolerant isolate. The results revealed a higher number of upregulated vs. downregulated transcripts in both isolates vs. their respective controls. Functional annotation of the differentially expressed transcripts revealed several known stress-related genes and pathways uniquely expressed. Genome sequencing of isolates with varied degrees of stress tolerance indicated variation among the isolates regardless of their phenotypic characterization. The obtained data lays the groundwork for future studies aimed at identifying genes and molecular markers as genetic selection tools for enhancement of entomopathogenic nematodes ability to withstand environmental stress conditions.

Gene expression analysis of oxidative stress tolerance in the entomopathogenic nematode Heterorhabditis bacteriophora

The entomopathogenic nematode (EPN) Heterorhabditis bacteriophora is used as a biological control agent against diverse insect pests. Nematode-based products contain third-stage Dauer juveniles (DJ), which are adapted to long-term survival. Their longevity can be limited by stress conditions during industrial production up to field application. Oxidative stress has been recently reported to have a direct influence on the H. bacteriophora DJ longevity. Thus, understanding mechanisms by which DJ respond to oxidative stress can provide insights to improve DJ longevity. In this study, we carried out a comparative transcriptomic analysis on the early stage of oxidative stress induction (4 h) in two H. bacteriophora inbred lines with contrasting oxidative stress tolerance, HU2-IL1 (stress-tolerant) and PT1-IL1 (stress-sensitive). For assessing the transcriptome, MACE, a versatile RNA-seq variant was applied. Our de novo transcriptome assembly generated more than 20 000 transcripts, from which 10 290 were linked to 9776 different Uniprot accessions. The majority of the annotated transcripts presented high homology to parasitic nematodes within the genus Ancylostoma, whereas homology to the genus Caenorhabditis was negligible. A total of 630 and 461 transcripts were up-regulated (log2 fold-change (FC) ⩾ 2.0) in the stress-sensitive and the stress-tolerant line, respectively. The proportion of down-regulated transcripts was higher for both lines. However, down-regulation in the stress-sensitive line (5207 transcripts) exhibited a larger proportion than in the tolerant line (1844 transcripts), which indicates that targeted suppression of biological processes is also a crucial factor for the survival of H. bacteriophora under oxidative stress. Our global view of the transcriptome remodelling under oxidative stress suggests that the stress-sensitive line fails to maintain vital biological processes in contrast to the tolerant line. Interestingly, both lines activated similar biological processes directly involved in detoxification of reactive oxygen species, indicating that expression changes on transcript isoforms are of high relevance in this context. This study will open ways for the selection of DJ longevity predictor genes and allow the design of molecular markers for the breeding of improved lines.

Applying inbreeding, hybridization and mutagenesis to improve oxidative stress tolerance and longevity of the entomopathogenic nematode Heterorhabditis bacteriophora

Poor shelf-life and sensitivity to environmental stress of entomopathogenic nematodes (EPNs) are traits, which deserve attention for improvement. Recently, a strong positive correlation between oxidative stress tolerance and longevity of Heterorhabditis bacteriophora dauer juveniles (DJs) has been reported. In this study, the improvement of H. bacteriophora DJ longevity was achieved by hybridization and mutagenesis. A hybrid pool deriving from two oxidative stress tolerant and long-living parental strains was generated. This hybrid AU1 × HU2 survived 2.6 days and 18 days longer than its best parent under oxidative stress and control conditions, respectively. In addition to the natural genetic variability, an EMS-mutant pool (M-OXI) with high longevity was generated and one of the derived mutagenized inbred lines (MOX-IL6) survived 5.8 days and 28.4 days longer than its donor line (IL3) under oxidative stress and control conditions, respectively. A genetic cross between the mutagenized inbred line and its donor line (MOX-IL × IL3) still survived 2.5 days and 18.5 days longer than the donor line under oxidative stress and control conditions, respectively. Concerning virulence and reproductive potential, trade-off effects were not observed as a result of hybridization and mutagenesis. These results underline the potential of classical genetic approaches for trait improvement in the nematode H. bacteriophora.

The structure of the Brassica napus seed microbiome is cultivar-dependent and affects the interactions of symbionts and pathogens

BACKGROUND

Although the plant microbiome is crucial for plant health, little is known about the significance of the seed microbiome. Here, we studied indigenous bacterial communities associated with the seeds in different cultivars of oilseed rape and their interactions with symbiotic and pathogenic microorganisms.

RESULTS

We found a high bacterial diversity expressed by tight bacterial co-occurrence networks within the rape seed microbiome, as identified by llumina MiSeq amplicon sequencing. In total, 8362 operational taxonomic units (OTUs) of 40 bacterial phyla with a predominance of Proteobacteria (56%) were found. The three cultivars that were analyzed shared only one third of the OTUs. The shared core of OTUs consisted mainly of Alphaproteobacteria (33%). Each cultivar was characterized by having its own unique bacterial structure, diversity, and proportion of unique microorganisms (25%). The cultivar with the lowest bacterial abundance, diversity, and the highest predicted bacterial metabolic activity rate contained the highest abundance of potential pathogens within the seed. This data corresponded with the observation that seedlings belonging to this cultivar responded more strongly to the seed treatments with bacterial inoculants than other cultivars. Cultivars containing higher indigenous diversity were characterized as having a higher colonization resistance against beneficial and pathogenic microorganisms. Our results were confirmed by microscopic images of the seed microbiota.

CONCLUSIONS

The structure of the seed microbiome is an important factor in the development of colonization resistance against pathogens. It also has a strong influence on the response of seedlings to biological seed treatments. These novel insights into seed microbiome structure will enable the development of next generation strategies combining both biocontrol and breeding approaches to address world agricultural challenges.

Phenotyping dauer juvenile oxidative stress tolerance, longevity and persistence within wild type and inbred lines of the entomopathogenic nematode Heterorhabditis bacteriophora

The commercial use of the entomopathogenic nematodeHeterorhabditis bacteriophoraas a biocontrol agent against noxious insects is limited due to its relatively short shelf-life. Longevity of dauer juveniles (DJ) during storage and in transit to end users is considerably restricted by environmental stresses. As a derivative stress triggered by environmental factors, oxidative stress causes a strong internal metabolic imbalance leading to lifespan reduction. In this study, the relation between DJ oxidative stress tolerance and longevity inH. bacteriophorawas investigated at 25 and 7°C. A strong and significant correlation between DJ oxidative stress tolerance and longevity during storage in Ringer’s solution ( at 7°C; at 25°C) was recorded. Phenotyping of these traits was performed for 40H. bacteriophorawild type strain and inbred line collections. At 25°C, the mean time survived in Ringer’s by 50% of the DJ (MTS50) ranged from 21 to 57 days, whereas under oxidative stress, survival was from 3 to 22 days. At 7°C, a maximum MTS50of 94 days was assessed when DJ were stored in Ringer’s, while the maximum MTS50was only 25 days with oxidative stress induction. The heritability of DJ tolerance to oxidative stress, determined by using homozygous inbred lines, is high (), an indication of a high probability for successful selective breeding. In a subset of preselectedH. bacteriophorainbred lines, DJ oxidative stress tolerance correlated with the DJ survival (persistence) after application to sand (). The study provides fundamental data required for a genetic breeding programme to produce hybrids with improved stress tolerance and prolonged shelf-life and soil persistence.

Photorhabdus luminescens LN2 requires rpoS for nematicidal activity and nematode development

Photorhabdus (Enterobacteriaceae) bacteria are pathogenic to insects and mutualistic with entomopathogenic Heterorhabditis nematodes. Photorhabdus luminescens subsp. akhurstii LN2, associated with Heterorhabditis indica LN2, shows nematicidal activity against H. bacteriophora H06 infective juveniles (IJs). In the present study, an rpoS mutant of P. luminescens LN2 was generated through allelic exchange to examine the effects of rpoS deletion on the nematicidal activity and nematode development. The results showed that P. luminescens LN2 required rpoS for nematicidal activity against H06 nematodes, normal IJ recovery and development of H. indica LN2, however, not for the bacterial colonization in LN2 and H06 IJs. This provides cues for further understanding the role of rpoS in the mutualistic association between entomopathogenic nematodes and their symbionts.

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.

Influence of bacterial density and mating on life history traits of Heterorhabditis bacteriophora

Heterorhabditis bacteriophora, associated with Photorhabdus luminescens, is commonly used against insect pests. Dauer juveniles (DJ) develop into self-fertilising hermaphrodites that lay eggs until juveniles hatch inside the uterus and feed on the body content of the mother (endotokia matricida). The life history traits of H. bacteriophora were studied at 2.5 × 109, 5 × 109, 10 × 109 and 20 × 109 cells ml−1 of P. luminescens at 25°C using a hanging drop technique. The number of offspring produced per hermaphrodite increased from 50 at 2.5 × 109 cells ml−1 to 269 at 20 × 109 cells ml−1 of P. luminescens. The bacterial density did not influence the beginning of endotokia matricida, hermaphrodite death, DJ release from the maternal carcass and the percentage of juveniles obtained through endotokia matricida. Mating of automictic females could not increase offspring production and survival. Endotokia matricida is an obligatory developmental step in H. bacteriophora.

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.

Life history trait analysis of the entomopathogenic nematode Steinernema feltiae provides the basis for prediction of dauer juvenile yields in monoxenic liquid culture

Entomopathogenic nematodes (Steinernema spp.) are used in integrated pest management to control insect pests in cryptic environments. The nematodes are mass produced in monoxenic liquid culture with their symbiotic bacteria Xenorhabdus spp. For a better understanding of nematode population dynamics, the life history traits (LHTs) of the entomopathogenic nematode Steinernema feltiae were assessed at 25 °C by observing single pairs of male and female nematodes using a hanging drop technique. To investigate the influence of different food supplies on nematode reproduction, the LHTs were assessed with a daily supply of 5 ×, 10 × and 20 × 10(9) cells ml(-1) of the nematode's bacterial symbiont Xenorhabdus bovienii in semi-solid nematode growth gelrite (NGG) medium. Increasing bacterial density had a significant positive influence on the average number of offspring produced, which ranged from 359 to 813 per female. The intrinsic rate of natural increase r m, which ranges from 1.10 to 1.19 day(-1), was neither influenced by the bacterial density, nor was the mean generation time T (5.12-5.25 days) and population doubling time (PDT) (0.64-0.59 days). The average lifespan of reproductive females, which ranged from 6.7 to 7.3 days, was positively correlated with bacterial density. A positive correlation between female body volume and bacterial density was recorded (R = 0.67) as well as a significant positive correlation between female body size and offspring production (R = 0.89) in hanging drops. Whether these data can be used to predict nematode yields in liquid culture was tested. The total female body volume calculated as the average female body volume × total number of parental females per millilitre 3 days after nematode inoculation was positively correlated (R = 0.72) with nematode yields. The total female body volume on process day 3 is thus a good indicator for the estimation of nematode yield at the end of the process (12-15 days post dauer juvenile (DJ) inoculation) in both Erlenmeyer flasks and bioreactors. With a mean deviation of 9467 DJs ml(-1), the error resembles approximately 5 % of the final DJ yields.

Life history trait analysis of the entomopathogenic nematode Steinernema riobrave

Life history traits (LHT) of Steinernema riobrave strain Sr 7-12 and Sr HYB19 were assessed at 25°C in monoxenic culture using a hanging drop technique. The LHT were studied with 5 × 109, 10 × 109 and 20 × 109 cells ml−1 of Xenorhabdus cabanillasii in semi-solid Nematode Growth Gelrite. Increased X. cabanillasii densities had a significant positive influence on offspring production on both Sr 7-12 and the hybrid Sr HYB19. At the higher bacterial food density, the total fertility rate (TFR) per female of Sr 7-12 was 2022 offspring and the net reproductive rate () reached 1904 offspring. Similarly, for Sr HYB19, the TFR per female was 2434 and was 1903. The percentage offspring produced via intra-uterine development (endotokia matricida) was relatively higher at 5 × 109 bacterial cells ml−1 than at higher bacterial density, with 64% and 66% of the total offspring produced by Sr 7-12 and Sr HYB19, respectively. A positive correlation () was recorded for offspring production and bacterial food density and for female body volume with bacterial density (). Female nematodes survived longer at higher bacterial food density; however, females of both strains could not survive longer than 7.8 days, beginning from hatching and always ending 1 day after initiation of endotokia matricida. Based on LHT results, there is a potential to maximise yield of dauer juveniles in liquid culture through increasing bacterial food density.

Influence of cell density of Escherichia coli and the dinoflagellate Crypthecodinium cohnii on life history traits of the nematode Panagrolaimus sp. strain NFS 24-5, a potential larval food for marine aquaculture

The nematode Panagrolaimus sp. NFS 24-5 has potential for use as living food for larval shrimps and fish in marine aquaculture. The nematodes are usually produced on bacterial or yeast cells. Nematodes cannot synthesise the long chain fatty acid docosahexaenoic acid (DHA) which is essential for feeding marine aquaculture organisms. The eukaryotic, heterotrophic dinoflagellate Crypthecodinium cohnii consists of approximately 20% DHA. To culture the nematodes and simultaneously enrich them with DHA, single adult male and female individuals were cultured in hanging drops with variable cell density of C. cohnii. Life history traits, such as net reproductive rate (), population doubling time (PDT) and intrinsic rate of natural increase (), were assessed and compared with data obtained from cultures on Escherichia coli. A maximum was recorded at a cell density of 4 × 106 C. cohnii cells ml−1, corresponding to 2478.82 μg dry mass ml−1. The same was achieved with 7× lower biomass of E. coli at a cell density of 3 × 109 cells ml−1, corresponding to 335.63 μg dry mass ml−1. The results exclude the use of the dinoflagellate culture from application in mass production of the nematode for aquaculture food and limit the use to post-harvest enrichment of the nematodes with essential fatty acids. At a density of 3 × 109 E. coli cells ml−1 the PDT was lowest and the was highest, indicating that this cell density might be closest to optimum conditions for nematode reproduction. Exceeding this cell density yielded fewer offspring within a longer time period. Implications for mass production in monoxenic liquid cultures are discussed.

Monoxenic liquid culture with Escherichia coli of the free-living nematode Panagrolaimus sp. (strain NFS 24-5), a potential live food candidate for marine fish and shrimp larvae

The free-living, bacterial-feeding nematode Panagrolaimus sp. (strain NFS 24-5) has potential for use as live food for marine shrimp and fish larvae. Mass production in liquid culture is a prerequisite for its commercial exploitation. Panagrolaimus sp. was propagated in monoxenic liquid culture on Escherichia coli and parameters, like nematode density, population dynamics and biomass were recorded and compared with life history table data. A mean maximum nematode density of 174,278 mL(-1) and a maximum of 251,000 mL(-1) were recorded on day 17 after inoculation. Highest average biomass was 40 g L(-1) at day 13. The comparison with life history table data indicated that the hypothetical potential of liquid culture is much higher than documented during this investigation. Nematode development is delayed in liquid culture and egg production per female is more than five times lower than reported from life history trait analysis. The latter assessed a nematode generation time of 7.1 days, whereas the process time at maximum nematode density in liquid culture was 16 days indicating that a reduction of the process time can be achieved by further investigating the influence of nematode inoculum density on population development. The results challenge future research to reduce process time and variability and improve population dynamics also during scale-up of the liquid culture process.

Influence of temperature on life history traits of the free-living, bacterial-feeding nematode Panagrolaimus sp. strain NFS-24

Life cycle analysis data of the free-living, bacterial-feeding Panagrolaimus sp. strain NFS 24-5 were assessed at different temperatures using a hanging drop method with single male and female individuals and a food density of 3 × 109 Escherichia coli cells ml−1. Lifespan at the moment when the first egg was laid was 5.7 days at 21°C and 4 days at 25, 27 and 29°C. The intrinsic rate of natural increase () was 0.53 at 21°C, 0.81 at 25°C, 0.93 at 27°C and 0.81 at 29°C, corresponding to population doubling times () of 1.3, 0.9, 0.7 and 0.9 days, respectively. Over 200 offspring per female were produced at 27°C. All other temperatures yielded fewer offspring. When females were kept without males, the life span was 49 days, whereas the last reproductive female (hanging drop with male individual) died after 16.5 days. These data will contribute to the interpretation of nematode population dynamics recorded in liquid culture.

Liquid culture of Panagrolaimus sp. for use as food for marine aquaculture shrimp and fish species

Free-living nematodes have potential to be used as live food for early life stages of several species in marine aquaculture. Panagrolaimus sp. displays several favourable characteristics for this application. The present study proved the feasibility of propagation in monoxenic liquid culture on Saccharomyces cerevisiae. The development of yeast cell density, nematode numbers and size distribution was assessed daily for 15 days. After a lag phase of 4 days the inoculated first-stage juveniles started development to adults. Yields in terms of nematode number as well as biomass were highly variable. The maximum number of nematodes varied from 45 000 to 238 000 ml−1 and maximum biomass from 49 to 143 g l−1. Information on size, dry and wet weight of the nematodes is provided. The size spectrum of Panagrolaimus sp. individuals ranged from 176 × 8 μm to 1377 × 61 μm and 8.15 to 3202.39 ng wet weight. Water content of the nematodes was 71.7 ± 2.5%, so dry weight per individual was 2.31-905.95 ng. Differentiation of juvenile stages by body length was not possible. Based on comparison of dry weight per individual the Panagrolaimus sp. might be used as a substitute for rotifers, a commonly used live food organism.

Desiccation tolerance of dauers of entomopathogenic nematodes of the genus Steinernema

For transport of entomopathogenic nematodes to the user, developmentally arrested dauer juveniles (DJ) are mixed with inert carriers at high density. If quiescence is not induced, DJ will quickly lose energy reserves and die. To induce quiescence DJ can be moderately desiccated. This study investigated the desiccation tolerance by measurement of water activity (-value) tolerated by 50% of populations (WA50) of different Steinernema species and strains. DJ were tested with or without prior adaptation to desiccation stress. Stress conditions were produced by exposure to various concentrations of poly(ethylene glycol) 600. Significant differences in desiccation tolerance were recorded between strains and species, but, except for Steinernema abbasi, not within strains of one species. Without adaptation to stress conditions, the most tolerant species was S. carpocapsae (WA50 = 0.836) followed by S. abbasi (0.86). Adaption to stress by exposure to an -value of 0.95 for 48 h increased tolerance to 0.68 and 0.66, respectively. The least tolerant species were S. kraussei, S. glaseri and S. ethiopiense. Tolerance recorded would allow storage at a water activity that would inhibit growth of bacteria but not of fungi. Analysis of water activity tolerated by only 10% of the population indicated potential for genetic improvement by selective breeding for S. carpocapsae, S. abbasi and S. arenarium to reach water activity levels that would also inhibit fungal growth.

Desiccation and storage of Panagrolaimus sp. (strain NFS-24-5)

Panagrolaimus sp. strain NFS-24-5 has potential to be used as live food for early stages of fish and crustacean species in marine aquaculture. One constraint to its commercialisation is the lack of a method that enables storage of nematodes over a longer time span. The objective of this study was to develop a procedure to transfer nematodes into a dormant state by desiccation. The nematodes were concentrated at densities of 25, 50, 100 and 200 × 103 individials cm−2 on nylon net or cellulose paper, preconditioned for 72 h at 97.3% relative humidity (RH) and then stored at 52.9 or 32.8% RH for 1 week. Cellulose was a better carrier for the nematodes. Survival of the nematodes was reduced only at the highest nematode density on both materials. The water activity of desiccated nematodes was 0.44 and 0.33 at 52.9% and 32.8% RH, respectively, well beyond a point to prevent microbial growth. After storage over a period of 10 weeks at 25 × 103 nematodes cm−2 at 52.9 and 32.8% RH, 92% of the nematodes were still alive. Monitoring the size distribution revealed no changes at 52.9% RH, but there were more of the larger nematodes dying at 32.8% RH in two out of three experiments. The method can be used to store quiescent Panagrolaimus sp. (strain NFS-24-5) for transportation and use in small scale feeding experiments for marine fish and crustacean larvae.

Steinernema ethiopiense sp. n. (Rhabditida: Steinernematidae), a new entomopathogenic nematode from Ethiopia

Three isolates (Dero-1, Dero-8 and Mosisa-1) of a new entomopathogenic nematode, S. ethiopiense sp. n., were isolated by baiting soil samples from the Mendi area, Western Wollega, Ethiopia, with last instar wax moth larvae Galleria mellonella. Infective juveniles of S. ethiopiense sp. n. have a body length of 898 (768-1010) μm, a maximum of eight identical ridges (i.e., nine lines) in the lateral field, excretory pore located at mid-pharynx, hyaline layer occupying approximately half of the tail and c′ = 3.2. First generation males lack a caudal mucron, whereas second generation males possess a short spine-like mucron. The spicules are slightly arcuate, golden-brown in colour and have an ellipsoid or oblong manubrium. First generation females lack a postanal swelling and have a minute protuberance on the tail tip whereas second generation females have a postanal swelling and protruding vulva. Based on the morphology, morphometrics and DNA analysis, the new species belongs to the glaseri group. The closest relative species is the afro-tropical S. karii recorded from Kenya. The BLAST analysis of the ITS region of the rDNA revealed a similarity of 93% with S. karii, supporting the validity of S. ethiopiense sp. n. as a new species. In the phylogenetic trees the new species groups together only with S. karii (bootstrap value of 100%), but is also separated from S. karii by a bootstrap value of 100% or 70%.

A realistic appraisal of methods to enhance desiccation tolerance of entomopathogenic nematodes

Understanding the desiccation survival attributes of infective juveniles of entomopathogenic nematodes (EPN) of the genera Steinernema and Heterorhabditis, is central to evaluating the reality of enhancing the shelf-life and field persistence of commercial formulations. Early work on the structural and physiological aspects of desiccation survival focused on the role of the molted cuticle in controlling the rate of water loss and the importance of energy reserves, particularly neutral lipids. The accumulation of trehalose was also found to enhance desiccation survival. Isolation of natural populations that can survive harsh environments, such as deserts, indicated that some populations have enhanced abilities to survive desiccation. However, survival abilities of EPN are limited compared with those of some species of plant-parasitic nematodes inhabiting aerial parts of plants. Research on EPN stress tolerance has expanded on two main lines: i) to select strains of species, currently in use commercially, which have increased tolerance to environmental extremes; and ii) to utilize molecular information, including expressed sequence tags and genome sequence data, to determine the underlying genetic factors that control longevity and stress tolerance of EPN. However, given the inherent limitations of EPN survival ability, it is likely that improved formulation will be the major factor to enhance EPN longevity and, perhaps, increase the range of applications.

Entomopathogenic nematode foraging strategies – is Steinernema carpocapsae really an ambush forager?

For many years, entomopathogenic nematodes have been classified as either ‘ambush’, ‘cruise’ or ‘intermediate’ foragers. Here, we critically examine the evidence that Steinernema carpocapsae, the most studied ‘ambush’ forager, does actually use an ambush foraging strategy in nature. We propose an alternative hypothesis for the characteristic behaviours of S. carpocapsae (and other supposed ‘ambush’ foragers) based on adaptation to habitats other than mineral soils in which S. carpocapsae can ‘cruise’. Several papers are reviewed in which S. carpocapsae was used successfully to control sedentary or cryptic pests in organic habitats, thus supporting our hypothesis. If this hypothesis is correct, it does not preclude S. carpocapsae (or any entomopathogenic nematode species) using an ambushing strategy under certain circumstances, but we believe on current evidence that the classification of S. carpocapsae as an ambush forager cannot be sustained.

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.

Variability in desiccation tolerance among different strains of the entomopathogenic nematode Heterorhabditis bacteriophora

Abstract

The shelf life of biological control products based on the entomopathogenic nematode Heterorhabditis bacteriophora is rather short. In order to prolong shelf life, the metabolism of nematodes during storage must be reduced. This can be achieved by means of desiccation of the infective third-stage dauer juveniles (DJ). The tolerance can be increased by an adaptation to moderate desiccation conditions. Previous investigations indicate that the heritability of the desiccation tolerance is high, justifying a genetic selection for enhanced tolerance. This investigation screened the desiccation tolerance of 43 strains of Heterorhabditis spp. and 18 hybrid/inbred strains of H. bacteriophora. Dehydrating conditions measured as water activity (aw values) were produced by treating DJ with different concentrations of the non-ionic polymer poly(ethylene glycol) 600. Significant inter-specific variation was recorded between nematode strains and species. The mean tolerated aw value (MW50) ranged from 0.90 to 0.95 for non-adapted and 0.67 to 0.99 for adapted nematode populations. For selective breeding, only the 10% most tolerant individuals would be used. The lowest aw value tolerated by 10% of a population (MW10) ranged from of 0.845 to 0.932 for non-adapted nematode populations and 0.603 to 0.950 for adapted nematode populations. Adaptation significantly increased the desiccation tolerance and a weak correlation was recorded for tolerance with and without adaptations. The most tolerant nematode strains will form the basis for the foundation of a parental stock produced by cross-breeding and following genetic selection for enhanced tolerance. Thus, this investigation is another milestone on the road to domestication of H. bacteriophora for commercial use in sustainable pest management.

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.

Life cycle and population development of the entomopathogenic nematodes Steinernema carpocapsae and S. feltiae (Nematoda, Rhabditida) in monoxenic liquid culture

Abstract

The life cycle and population dynamics of the entomopathogenic nematodes Steinernema carpocapsae and S. feltiae were studied in monoxenic liquid culture with their symbiotic bacteria Xenorhabdus nematophila and X. bovienii. To distinguish between the different juvenile and adult stages, their size was recorded. No differences were observed between the species in the size of the juvenile stages but significant differences were recorded in the length of the F1 adults, pre-dauer (J2d) and dauer juvenile stages (DJ). On average, 90% of inoculated DJ of S. feltiae recovered and 77% of S. carpocapsae. In general, S. feltiae developed from the inoculum DJ to the adult approximately 1 day faster than S. carpocapsae. The sex ratio was female-biased (59.2 ± 2.2% in S. carpocapsae, 66.7 ± 2.6% in S. feltiae) in the parental population but not in the F1 generations. Steinernematid adults, like heterorhaditids, respond to depleting food resources with the cessation of egg laying. Juveniles hatch inside the uterus and develop at cost of the maternal body content causing the death of the adult (endotokia matricida). In contrast to Heterorhabditis spp. and in vivo observations of steinernematids by other authors, who reported that readily developed DJ leave the carcass of the dead adult, in this study J2d emerged 12 h after cessation of egg laying. The density of both bacterial cultures decreased due to the feeding of the parental juveniles. However, X. nematophila continued at very low density, whereas the density of X. bovienii increased again until 15 days post-inoculation. The vast majority of F1 S. carpocapsae offspring developed to DJ, whereas in S. feltiae a significant second and third generation of adults was observed, probably due to the increasing bacterial population. However, second and third generation adults in S. feltiae cultures did not contribute significantly to the DJ yield. Mean yields of 158 × 103 DJ ml–1 were recorded for S. carpocapsae and 106 × 103 DJ ml–1 for S. feltiae. The results provide valuable information for future process improvement.

Binding of Cyt1Aa and Cry11Aa toxins of Bacillus thuringiensis serovar israelensis to brush border membrane vesicles of Tipula paludosa (Diptera: Nematocera) and subsequent pore formation

Bacillus thuringiensis serovar israelensis (B. thuringiensis subsp. israelensis) produces four insecticidal crystal proteins (ICPs) (Cry4A, Cry4B, Cry11A, and Cyt1A). Toxicity of recombinant B. thuringiensis subsp. israelensis strains expressing only one of the toxins was determined with first instars of Tipula paludosa (Diptera: Nematocera). Cyt1A was the most toxic protein, whereas Cry4A, Cry4B, and Cry11A were virtually nontoxic. Synergistic effects were recorded when Cry4A and/or Cry4B was combined with Cyt1A but not with Cry11A. The binding and pore formation are key steps in the mode of action of B. thuringiensis subsp. israelensis ICPs. Binding and pore-forming activity of Cry11Aa, which is the most toxic protein against mosquitoes, and Cyt1Aa to brush border membrane vesicles (BBMVs) of T. paludosa were analyzed. Solubilization of Cry11Aa resulted in two fragments, with apparent molecular masses of 32 and 36 kDa. No binding of the 36-kDa fragment to T. paludosa BBMVs was detected, whereas the 32-kDa fragment bound to T. paludosa BBMVs. Only a partial reduction of binding of this fragment was observed in competition experiments, indicating a low specificity of the binding. In contrast to results for mosquitoes, the Cyt1Aa protein bound specifically to the BBMVs of T. paludosa, suggesting an insecticidal mechanism based on a receptor-mediated action, as described for Cry proteins. Cry11Aa and Cyt1Aa toxins were both able to produce pores in T. paludosa BBMVs. Protease treatment with trypsin and proteinase K, previously reported to activate Cry11Aa and Cyt1Aa toxins, respectively, had the opposite effect. A higher efficiency in pore formation was observed when Cyt1A was proteinase K treated, while the activity of trypsin-treated Cry11Aa was reduced. Results on binding and pore formation are consistent with results on ICP toxicity and synergistic effect with Cyt1Aa in T. paludosa.

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.

Evaluation of adjuvants for foliar application of Steinernema carpocapsae against larvae of the diamondback moth (Plutella xylostella)

Abstract

Increasing resistance to chemical insecticides in field populations of the diamondback moth (Plutella xylostella) has stimulated research on alternative control measures. The entomopathogenic nematode Steinernema carpocapsae may be one such alternative, particularly against the third larval instar of P. xylostella. The LC50 for the second instar is 38, the third 13 and the fourth 22 nematodes/larva. Plutella xylostella pupae were not affected by the nematodes, although mortality in leaf disk bioassays after application of nematodes in water seldom surpassed 50%. Therefore, additives were tested to improve nematode performance. Only Triton X-100 (0.3%) caused phytotoxic effects. The addition of xanthan gum or potassium alginate resulted in a two-fold increase of insect mortality at 80% relative humidity (RH) and a five-fold increase at 60% RH. Mixtures of 0.3% xanthan or alginate with 0.3% surfactants further improved efficacy. In water the LT50 for S. carpocapsae against P. xylostella larvae was > 40 h. Using a mixture of 0.3% xanthan or 0.3% alginate with 0.3% surfactant, the LT50 was reduced to < 25 h.

Interactions between host plants, the subterranean burrower bug, Cyrtomenus bergi, and the entomopathogenic nematode Heterorhabditis megidis

Abstract

Interactions between seedlings of Zea mays L. as a host plant, the subterranean burrower bug, Cyrtomenus bergi Froeschner, as a pest, and Heterorhabditis megidis Poinar, an entomopathogenic nematode, as a biological control agent, were studied, with a particular emphasis on the host finding behaviour of the nematode. For the trials a sand-filled Y-tube olfactometer was used. In the Y-arms, a bug, a maize seedling, a combination of both, or sand only was offered. Six experiments were carried out with differently treated Y-arms. Nematodes were significantly attracted to maize as well as to the combination of maize and bug, but not to C. bergi alone or to sand only. The influence of the feeding strategy of C. bergi on the host finding behaviour of H. megidis is discussed.

Two new Subspecies of Photorhabdus luminescens, Isolated from Heterorhabditis bacteriophora (Nematoda: Heterorhabditidae): Photorhabdus luminescens subsp. kayaii subsp. nov. and Photorhabdus luminescens subsp. thracensis subsp. nov

Bacterial isolates from nematodes from Turkish soil samples were initially characterized by molecular methods and seven members of the genus Photorhabdus identified to the species level, using riboprint analyses and metabolic properties. Strain 07-5 (DSM 15195) was highly related to the type strain of Photorhabdus luminescens subsp. laumondii DSM 15139T, and was regarded a strain of this subspecies. Strains 1121T (DSM 15194T), 68-3 (DSM 15198) and 47-10 (DSM 15197) formed one, strain 39-8T (DSM 15199T), 39-7 (DSM 15196) and 01-12 (DSM 15193) formed a second cluster that branched intermediate the three subspecies of Photorhabdus luminescens. Based upon moderate 16S rRNA gene sequence similarities and differences in metabolic properties among themselves and with type strains of the three subspecies we consider the two clusters to represent two new subspecies of Photorhabdus luminescens for which the names Photorhabdus luminescens subsp. kayaii, type strain 1121T (DSM 15194T, NCIMB 13951T), and Photorhabdus luminescens subsp. thracensis subsp. nov., type strain 39-8T (DSM 15199T, NCIMB 13952T) are proposed.

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.

Laboratory bioassays of virulence of entomopathogenic nematodes against soil-inhabiting stages of Frankliniella occidentalis Pergande (Thysanoptera: Thripidae)

Abstract

The efficacy of entomopathogenic nematodes (EPN) was evaluated in a laboratory trial against soil-dwelling stages, late second instar larvae and pupal stages of western flower thrips (WFT), Frankliniella occidentalis Pergande. Among the six EPN strains assessed for the first time, Steinernema feltiae (Nemaplus®) and Heterorhabditis bacteriophora (HD01) caused 65 and 59% mortality, respectively. Steinernema carpocapsae (Agriotos) and S. arenarium (Anomali) caused moderate mortality (40-45%) while Steinernema spp. (Morocco) and H. bacteriophora (Nematop®) had little effect. In a dose response study with concentrations of 100, 400 and 800 infective juveniles (IJ) per cm2 soil of H. bacteriophora (HK3), S. feltiae (Nemaplus®) and H. bacteriophora (HD01), mortality increased only up to 400 IJ cm-2. The rate of infectivity of H. bacteriophora (HK3) and S. feltiae (Nemaplus®) indicated that both strains could survive at least 6 days in the soil and infect WFT immature stages.

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.

Physico-chemical properties and mode of action of a signal from the symbiotic bacterium Photorhabdus luminescens inducing dauer juvenile recovery in the entomopathogenic nematode Heterorhabditis bacteriophora

Abstract

Recovery in entomopathogenic nematodes is the exit from the dauer juvenile stage. It is a response to environmental queues signalling the presence of food sources (e.g., insect haemolymph). The bacterium Photorhabdus luminescens excretes a signal which also induces recovery of its symbiotic Heterorhabditis bacteriophora dauer juveniles. This bacterial signal is composed of at least two compounds with different polarity. The symbiotic bacteria also secrete an antagonistic signal which inhibits nematode recovery. The recovery-inducing signal compounds have a molecular mass of less than 20 kDa and are negatively charged. The data indicate that at least one compound is smaller than 5 kDa. The bacterial signal triggers by receptor binding, the first step in a recovery-inducing muscarinic signalling pathway.