Entomopathogenic Nematode Populations Research Articles

Prospects and Present Status and of Entomopathogenic Nematodes (Steinernematidae and Heterorhabditidae) in Nepal

Naturally occurring entomopathogenic nematodes(EPN) and their symbiotic bacteria are important biotic factor in suppression of insect pest populations in soil and cryptic habitats. These nematodes can control pests due to their mutualistic association with bacteria that kill the hosts by septicemia and make the environment favorable for EPN development and reproduction.The virulent species of EPN can commercially be mass produced as biological control agents all over the world. Nepal has a great potential to exploit these beneficial nematodes for the control of insect pests. Exploration of indigenous EPN is receiving attention around th eworld. Numerous surveys conducted worldwide have detected many indigenous isolates. Altogether 115 species of EPN (97 of Steinernema and 18 of Heterorhabdits species) have been reported so far worldwide. However, very limited research on EPN in Nepal has been done with 29 species only with some new species. Hence,the isolation and description of the native efficacious species and populationsof EPN is the need of the hour,not only from a biodiversity view point but also from an environmental and biological control perspective.Thei dentification of EPNs, adapted to environmental and climatic conditions of cultivated areas, is important for sustainable pest suppression in integrated management programs in agricultural areas of Nepal. This article provides an overview of recent development on EPN research and evaluates their potential for use and exploitation in Nepal.

Long-term Persistence of Native New York Entomopathogenic Nematode Isolates Across Crop Rotation.

Entompathogenic nematodes are found worldwide in a wide array of soil habitats with a broad host range and significant variation in foraging strategies. The primary use of entomopathogenic nematodes (EPNs) in managed plant systems has been focused on inundative releases in a biopesticide strategy. Little effort has been placed in investigating the use of natural occurring or adapted EPN strains for long-term suppression of pest outbreaks in managed systems. This study examined the potential of EPN isolates from Northern New York (NNY), inoculated at a low level (250 million IJ/ha), which are climate adapted and their persistent characteristics preserved to maintain population levels in agricultural fields (N = 82) for multiple years and across crop rotation (alfalfa:corn:alfalfa). Persistence levels for Steinernema carpocapsae (Weiser) (Rhabditida:Steinernematidae) ranged between 8 and 12% of the soil cores assayed in continuous alfalfa and 1-14% of the soil cores assayed in continuous corn rotated from EPN treated alfalfa. Steinernema feltiae (Filipjev) (Rhabditida: Steinernematidae) residual persistence level ranged between 17 and 32% in continuous alfalfa and 22-41% in continuous corn rotated from EPN treated alfalfa. Combined EPN level ranged between 27 and 43% of the soil cores in continuous alfalfa and 28-55% in continuous corn rotated from EPN-treated alfalfa. Inspection of individual fields suggested EPN populations established in prior years at the residual soil core level of 18-35% can respond positively to an increase of susceptible hosts in both alfalfa and corn, often increasing their presence to 100%.

Mealworm Powder as Culture Media of Local Isolate Semarang Entomopathogenic Nematodes

Many researchers confirmed that entomopathogenic nematodes (EPNs) of the genera Heterorhabditis and Steinernema can be cultivated in vitro using artificial media that containing animal nutrition. However, artificial media with insect component hasn’t been widely studied. This research aims to analyze the population of EPNs isolate of Semarang cultivated in mealworm powder media. Five doses of mealworm powder (Tenebrio molitor) were tested in this research, i.e: 0.5; 1.0; 1.5; 2.0 g. Culture media using 1 g of mealworm larvae was used as control. The best treatment was further tested for its pathogenicity on Macrotermes sp. at seven levels of invective juveniles (IJs) : 0; 50; 100; 150; 200; 250; 300 IJs/mL. Each treatment was repeated five times. The EPNs population and termites mortality were analyzed using ANAVA, whereas pathogenic value was calculated using Probit analysis. The result showed EPNs population were significantly (LSD test; α> 0.05), likewise on termites mortality. The EPNs isolate of Semarang optimally at 0.5 g mealworm powder and pathogenicity against termites based on LD50 and LD90 values at 220 JI/mL and 410 JI/mL doses, respectively. In conclusion, this result can be an alternative to mass cultivation of EPNs, in effort of development of local bioinsecticides. The findings of this study also inform farmers that EPNs can be easily cultivated using the simple and available materials.

Molecular Identification of Entomopathogenic Nematode Isolates from the Philippines and their Biological Control Potential Against Lepidopteran Pests of Corn.

In search for local entomopathogenic nematode (EPN) species as a biological control agent of lepidopterous insect pests of corn, a survey for EPN in the major islands in the Philippines was conducted. Seven EPN populations from 279 soil samples were isolated using Ostrinia furnacalis, the key target insect pest of corn in the country, as bait. Analysis of the ITS1-5.8S-ITS2 ribosomal DNA sequence revealed the presence of Steinernema abbasi, Steinernema minutum , Steinernema tami , and Heterorhabditis indica . The pathogenicity of these EPN was tested in Ostrinia furnacalis , Spodoptera litura , and Helicoverpa armigera larvae under laboratory conditions. All the EPN isolates were pathogenic to the lepidopteran species with, H. indica PBCB and S. abbasi MBLB exhibiting the highest virulence (88%-99.33% and 90%-100% mortality, respectively) at 48 hr post infection (HPI) and thus, further studies were done on these two EPN. The highest penetration rate at 48 HPI was observed in H. armigera infected with S. abbasi MBLB (28.15%), while the lowest was in O. furnacalis infected with H. indica PBCB (14.25%). Nonetheless, based on LC 50 at 48 HPI, H. indica PBCB was most virulent to S. litura (8.89 IJ per larva), but not significantly different from O. furnacalis (10.52 IJ per larva). Steinernema abbasi MBLB was most virulent to O. furnacalis (10.98 IJ per larva), but not significantly different to S. litura (17.08 IJ per larva). LT 50 estimates showed that O. furnacalis was significantly the most susceptible to H. indica PBCB (21.90 hr) and S. abbasi (21.18 hr). Our results suggest that H. indica PBCB and S. abbasi MBLB are good candidates as biological control agents against these insect pests of corn. Moreover, O. furnacalis as alternative bait for EPN was discussed. To date, this is the most extensive research on Philippine EPN, comprised of wide sampling coverage, molecular identification and bioefficacy assays.

Entomopathogenic nematode distribution and edaphoclimatic conditions in the Cerrado of Minas Gerais, Brazil

Based on the high diversity of Brazilian fauna and flora, edaphoclimatic conditions in the Cerrado of Minas Gerais, and the situation of utilization of EPNs in Brazil, a survey was conducted in order to relate the presence of these organisms with the physical and chemical attributes of the soil, combined with precipitation. To this end, soil samples were collected in areas with diversified vegetation types in Monte Carmelo, MG, at Juliana Farm. The samples were obtained every 15 days for 6 months. From each spot, soil samples (about 500 g) were collected for soil moisture characterization, nematode isolation and determination of pH, organic matter, potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), potential acidity (H + Al) and precipitation data (mm). Three populations of entomopathogenic nematodes of the Heterorhabditis amazonensis species were isolated in Cerrado stricto sensu and Gallery forest areas. The occurrence H. amazonensis could not be considered restricted to specific soil condition, as organic matter, humidity, pH, Ca, K, Mg and H + Al, especially considering the organic matter and K values, which had variable levels between the places of collection. The p values of the positive soil samples were at a lower level than the mean of the Gallery forest and Cerrado, and at the same level as maize and pasture area. The soil moisture in the Cerrado area increased with the higher values of precipitation; however, in the Gallery forest area this association was not observed. Also, the nematodes were isolated when the temperature began to decrease.

Effects of cover crops on the overwintering success of entomopathogenic nematodes and their antagonists

Conservation agriculture is arising as an alternative to conventional agriculture with the aim to have a reduced impact on the environment. This includes the use of cover crops to conserve soil quality by limiting soil erosion, adding organic matter, and enhancing soil nutrient content, as well as water availability, which are all factors that can greatly influence the presence of soil organisms. In the current study, we investigated whether winter cover crops can enhance the persistence of entomopathogenic nematodes (EPN) over the winter season. In a first trial we augmented EPN populations in plots without (bared) and with the cover crops, pea (Pisum sativum) or mustard (Brassica juncea). In late autumn, individual mini-plots in each of the three treatments were supplemented with infective juveniles (IJs) of either Heterorhabditis bacteriophora or Steinernema feltiae. In a second trial we studied naturally occurring EPN in plots without (bared) and with the cover crop pea (Pisum sativum) followed by planting of winter wheat. To identify and quantify EPN, we analyzed soil samples using quantitative real time PCR (qPCR) at three time points over the winter season. We also measured the activity of augmented EPN by baiting the soil with wax moth larvae, Galleria mellonella. In addition, we used the qPCR method to investigate the presence of nematophagous fungi (NF), free-living nematodes (FLN) and ectoparasitic bacteria, all of which can interfere with EPN performance. Numbers of naturally occurring EPN in the investigated fields were very low (<1 EPN per 100g of soil). The cover crops only had a significant positive effect on the numbers of augmented S. feltiae found in early winter. No striking effect was found for H. bacteriophora. Yet, augmentation was found to enhance the insect-suppressiveness of the soil, as the recorded EPN infectivity after the winter was slightly higher than what was observed in autumn, one month after application. The numbers of FLN, which compete for insect cadavers, was higher in spring than in early winter. These FLN and other antagonists may be important in reducing EPN numbers. In conclusion, the effect of cover crops on EPN persistence was only evident during early-winter and was only significant in the plots augmented with S. feltiae. Moreover, we found that higher numbers of EPN in agricultural soils do not necessarily translate into high infectivity, which is the key factor determining their effectiveness in controlling soil pests.

Isolation of entomopathogenic nematodes in an apple orchard in Southern Brazil and its virulence to Anastrepha fraterculus (Diptera: Tephritidae) larvae, under laboratory conditions.

Entomopathogenic nematodes (EPNs) are a promising alternative to integrated control in many fruit pests. Few studies were made on the relationship of Anastrepha fraterculus natural population with native EPNs population and other biotic and abiotic factors. The aim of this work was to verify the occurrence of endemic nematodes in an apple orchard, concerning environmental conditions and technical procedure, and access isolates virulence to A. fraterculus larvae. The experiment was conducted during a year taking monthly soil samples from an apple orchard, with and without fallen fruits just above the soil. Samples were baited with Tenebrium molitor and A. fraterculus larvae in laboratory. Canopy and fallen fruits were sampled to access the pest infestation. Seventy three EPN isolates were captured, in 23.2% soil samples, more with T. molitor than with A. fraterculus baits. From the 20 isolates tested against A. fraterculus, only five were pathogenic, and they were identified as Oscheius sp. The nematodes were captured during all seasons in a similar frequency. Soil and weather conditions, presence of fruit over the orchard soil, and A. fraterculus pupae in the fruits had no significant influence on the capture. As a conclusion, nematodes of the genera Oscheius are found in an apple orchard of Porto Amazonas constantly along the year, independently of fluctuations in A. fraterculus population, climate conditions and presence of fruit over the soil. Some of the isolates are pathogenic to A. fraterculus.

Prevalence and activity of entomopathogenic nematodes and their antagonists in soils that are subject to different agricultural practices

Agricultural management practices can modify soil properties in ways that may disrupt the abundance and activity of beneficial organisms in the soil. We assessed the impact of different soil management practices on entomopathogenic nematodes (EPN), which have great potential as biological control agents against root-feeding insects. Soils were sampled during spring and autumn 2013 in all 96 plots of a long-term Swiss field trial (DOK experiment). By combining a traditional insect-baiting technique and real-time qPCR analyses, we identified and quantified over 20 soil-dwelling species (or genera). This allowed us to investigate how communities of natural EPN populations and their associated natural enemies and competitors are affected by (i) three crop types (wheat, maize and grass-clover ley) and (ii) farming systems, i.e. conventional, organic and biodynamic, which differed in fertilization, and pesticide use. We also determined the effects on soils’ microbial biomass in terms of carbon (Cmic) and nitrogen (Nmic) and applied spatial distribution analysis (SADIE) to uncover patterns of aggregations and associations of the study organisms. Although manure based farming systems increased microbial biomass, the systems did not influence the presence of EPN or their antagonists. EPN was more abundant in winter-wheat plots than in maize and grass-clover ley plots. Overall, very low numbers of EPN were recorded, implying that their natural presence would not be sufficient to have a satisfactory suppressive effect on root-feeding pests and the application of EPN would therefore be an appropriate measure to protect yields in case of root pest outbreaks.

Are Entomopathogenic Nematodes Effective Biological Control Agents Against the Carob Moth, Ectomyelois ceratoniae?

The carob moth (Ectomyelois ceratoniae) is the key pest of pomegranate, which causes a significant percentage of losses in pomegranate orchards and warehouses of Iran annually. The pest larvae are characterized by displaying a cryptic behavior within the fruit, which avoids most routine control techniques, especially chemical method. The low efficiency of traditional measurements and also the rich species diversity of natural enemies within the infested fruits highlight the necessity of exploring effective control methods, especially environmental friendly approaches. Entomopathogenic nematodes (EPNs) are a group of biological control agents that actively search for the host, including those in a cryptic habitat like the carob moth larvae within infested fruits. Here, we assumed that treatment of the infested and dropped fruits with EPNs may provide new insight into the management of the carob moth. Three species of EPNs, Steinernema feltiae, S. carpocapsae, and Heterorhabditis bacteriophora were selected and used in a series of in vitro and in vivo experiments. In preliminary assays, the EPNs species were used with different concentrations of infective juveniles (IJs) (0, 1, 5, 10, 25, and 50 IJ/larvae) in 2-cm diam. plates. The mortality rates of the laboratory tests were 79.75% and 76.5% for S. feltiae and S. carpocapsae, corresponded to LC50 value of 2.02 IJ/larva for S. feltiae and 2.05 IJ/larva for S. carpocapsae. On the contrary, H. bacteriophora demonstrated low virulence on the pest larvae in petri tests with a LC50 = 426.92 IJ/larva. Hence, both Steinernema species were selected for subsequent experiments. The penetration rate for S. feltiae and S. carpocapsae into the hemocoel of the pest was 43% and 31%, respectively, and the corresponding reproduction rate was 15,452 IJ/larva for S. feltiae and 18,456 IJ/larva for S. carpocapsae. The gathered data from those in vitro tests were used for a field assay. Different concentrations (5, 10, 50, 100, and 160 IJ/cm2 of the arena) of S. feltiae and S. carpocapsae were applied in the field test. The mean mortality results from the last test were 10.89% and 26.65% for S. feltiae and S. carpocapsae, respectively. Finally, we found that these low virulence rates of the nematodes were attributed to inhibitory/repellency effects of saprophytic fungi within the infested pomegranates, a usual status of the infested fruits in autumn or winter seasons. Future work on additional EPN populations more adapted to the extreme conditions of the pomegranate production area in Iran may provide sufficient evidence to continue the further investigation on the best EPN species populations and advanced formulations with high durability.

Unraveling the intraguild competition between Oscheius spp. nematodes and entomopathogenic nematodes: Implications for their natural distribution in Swiss agricultural soils

Entomopathogenic nematodes (EPN) are excellent biological control agents to fight soil-dwelling insect pests. In a previous survey of agricultural soils of Switzerland, we found mixtures of free-living nematodes (FLN) in the genus Oscheius, which appeared to be in intense competition with EPN. As this may have important implications for the long-term persistence of EPN, we studied this intraguild competition in detail. We hypothesized that (i) Oscheius spp. isolates act as scavengers rather than entomopathogens, and (ii) cadavers with relatively small numbers of EPN are highly suitable resources for Oscheius spp. reproduction. To study this, we identified Oscheius spp. isolated from Swiss soils, quantified the outcome of EPN/Oscheius competition in laboratory experiments, developed species-specific primers and probe for quantitative real-time PCR, and evaluated their relative occurrence in the field in the context of the soil food web. Molecular analysis (ITS/D2D3) identified MG-67/MG-69 as Oscheius onirici and MG-68 as O. tipulae (Dolichura-group). Oscheius spp. indeed behaved as scavengers, reproducing in ∼64% of frozen-killed cadavers from controlled experiments. Mixed infection in the laboratory by Oscheius spp. with low (3 IJs) or high (20 IJs) initial EPN numbers revealed simultaneous reproduction in double-exposed cadavers which resulted in a substantial reduction in the number of EPN progeny from the cadaver. This effect depended on the number of EPN in the initial inoculum and differed by EPN species; Heterorhabditis megidis was better at overcoming competition. This study reveals Oscheius spp. as facultative kleptoparasites that compete with EPN for insect cadavers. Using real-time qPCR, we were able to accurately quantify this strong competition between FLN and EPN in cadavers that were recovered after soil baiting (∼86% cadavers with >50% FLN production). The severe competition within the host cadavers and the intense management of the soils in annual crops readily explain the low EPN numbers in Swiss field samples. The developed molecular tools can be used to elucidate the extent to which the competitive interactions affect EPN populations. This can help to develop strategies to achieve good persistence and natural EPN recycling, in particular in systems where native EPN levels are low, such as annual crops.

Effect of soil management on Heterorhabditis bacteriophora GPS11 persistence and biological control in a vegetable production system

Soil habitat conditions that promote abundance and persistence of entomopathogenic nematodes (EPNs) might be encouraged by reduced tillage and compost amendments. We hypothesized that altered soil management with reduced tillage, cover crops (clover and barley), and compost (100kg of N/ha), would increase survival and biocontrol services of EPNs, compared with conventional management. Field trials were conducted at the Ohio Agricultural Research and Development Center Muck Crops Research Station, Huron County, OH in 2010 and 2011. Plots were planted with carrots. EPNs, Heterorhabditis bacteriophora GPS11, were released and their survival was compared between the two soil management regimes by sampling over a period of 8weeks using in situ bait traps containing Galleria mellonella. Repeated measures analysis of variance did not show significant differences between the two soil management regimes in the pattern of H. bacteriophora survival over time or during any evaluation in either year, except 2weeks after cadaver application in 2010 when the EPN population was significantly greater in the conventional than in the alternative soil management regime. Although treatment effects were generally not significant, statistically significant increase in nematode population densities between the 2years of the study, despite generally poor weather conditions following EPN release in the second year, provide encouraging evidence that populations of these biological control agents could increase in vegetable production fields. However, longer periods may be required for clearly distinguishable increase in EPN population density, persistence, and biological control services in the alternative soil management treatments.

Entomopathogenic Nematodes Isolated from Gansu Province and Four New Species in China

With the morphological characters, anatomy and molecular biology research, 29 entomopathogenic nematodes populations isolated from Gansu province were identified and the resoult showed that they were 8 species of EPN,Steinernema feltiae,Steinernemabicornutum,Steinernemaaffin,SteinernemakrusseandSteinernemakarilwere belong toSteinernemawhileHeterorhabditis brevicaudis,HeterorhabditismegidiandHeterorhabditismarelatuwere belong toHeterorhabditisrespectively. And what more research found thatS. affin,S. krusse,S. karilandH. marelatuwere new species in China.

Natural enemies of natural enemies: the potential top‐down impact of predators on entomopathogenic nematode populations

Top‐down population regulation can influence the success of biological control agents when they are released into the field. Entomopathogenic nematodes (EPNs) are used commonly in biological control programmes, but their efficacy suffers from poor persistence. Although abiotic soil conditions have been shown to reduceEPNpersistence, consumption of infected insects by scavengers and of infective juvenile (IJ) nematodes by predators may also regulate these populations. In the present study, the effects of different soil arthropods onEPNs in laboratory conditions were measured. It was hypothesised that arthropods commonly found in soil communities whereEPNs are applied would consume cadavers of insects parasitised by the nematodes and theIJs themselves.Some species of scavengers consumeEPN‐infected insects. Crickets (Gryllus bimaculatusDe Geer),American cockroaches [Periplaneta Americana(Linnaeus)], ants [Tetramorium chefketiForel andPheidole pallidula(Nylander)], earwigs (Labidura ripariaPallas), mites (Sancassania polyphyllaeZachvatkin), and springtails (Sinella curvisetaBrook andFolsomia candidaWillem) have different responses to nematode‐killed insects. Results suggested that ants (T. chefketi), cockroaches, mites, and earwigs fed onSteinernema‐killed insects whereas neither crickets nor springtails consumed them.In the second part of the study, experiments were conducted to determine whether mites and springtails consumedIJ EPNs. Results showed thatS. polyphyllaemites do not consume infective juveniles in soil, whereas both springtail species consumed significant numbers of theIJs.Top‐down regulatory processes can be a limiting factor forEPNpopulations under laboratory conditions. Both host cadavers andIJs are consumed (albeit by different arthropods), so these results may help explain the difficulties associated with the persistence ofEPNapplication to soil.

New citriculture system suppresses native and augmented entomopathogenic nematodes

Since its first detection in 2005, the bacterial disease huanglongbing (HLB or citrus greening) has emerged as a critical threat to the citrus industry in Florida. An “Advanced Production System” (APS) could mitigate the impact of HLB by bringing citrus trees into production more quickly and economically than conventional citriculture methods. However, unlike conventional practices, APS fertigates plants daily, thereby changing the soil properties in ways that might impact soil biota. We tested the hypothesis that changes to soil properties caused by APS would affect the abundance of native entomopathogenic nematodes (EPNs) and/or the survival of augmented EPNs. The densities of organisms at different trophic levels were measured by real-time qPCR in three experiments conducted in an ongoing field experiment. Target organisms included 6 entompathogenic nematodes, 5 nematophagous fungi (NF) and a phoretic bacterium, Paenibacillus sp. Soil properties, free-living nematodes and citrus fibrous roots were also measured. Compared to soil under conventional citriculture (CC), APS increased soil pH and Mg content, while reducing the electrical conductivity, and content of K, Mn and Fe. The naturally occurring EPN Steinernema diaprepesi was 5times less abundant in APS plots where these nematodes were more heavily encumbered by the phoretic bacterium Paenibacillus sp., which limits the foraging success of EPNs. In general, when EPNs were augmented in either treatment, fewer Steinernema riobrave than Heterorhabditis indica were recovered and recovery of both species declined rapidly over time. As seen with native S. diaprepesi, fewer augmented S. riobrave were recovered from APS plots in two of the three experiments, whereas the management system did not affect the recovery of H. indica. More of some endoparasitic and trapping NF were recovered from soil augmented with S. riobrave than with H. indica. However, variation in the responses of NF to the management systems suggests that these NF species were not primarily responsible for the steinernematid responses to APS. Although APS has the potential to reduce EPN populations and exacerbate herbivory by subterranean pests such as the root weevil Diaprepes abbreviatus, additional study of the physical causes of this effect may reveal ways to avoid the problem.

Use of real-time PCR to discriminate parasitic and saprophagous behaviour by nematophagous fungi

Entomopathogenic nematodes (EPNs) are important pathogens of soilborne insects and are sometimes developed commercially to manage insect pests. Numerous nematophagous fungal species (NF) prey on nematodes and are thought to be important in regulating natural or introduced EPN populations. However, nematophagy by these fungi in nature cannot be inferred using existing methods to estimate their abundance in soil because many of these fungi are saprophytes, resorting to parasitism primarily when certain nutrients are limiting. Therefore, we developed an assay to quantify NF DNA in samples of nematodes. Species-specific primers and TaqMan probes were designed from the ITS rDNA regions of Arthrobotrys dactyloides, Arthrobotrys oligospora, Arthrobotrys musiformis, Gamsylella gephyropagum and Catenaria sp. When tested against 23 non-target fungi, the TaqMan real-time PCR assay provided sensitive and target-specific quantification over a linear range. The amount of A. dactyloides or Catenaria sp. DNA in 20 infected nematodes, measured by real-time PCR, differed between fungal species (P=0.001), but not between experiments (P>0.05). However, estimates of relative NF parasitism using a bioassay with 20 nematodes infected by either species, differed greatly (P<0.001) depending on whether the fungi were alone or combined in the samples used in the assay. Tests done to simulate detection of NF DNA in environmental samples showed that, for all species, background genomic DNA and/or soil contaminants reduced the quantity of DNA detected. Nested PCR was ineffective for increasing the detection of NF in environmental samples. Indeed, real-time PCR detected higher amounts of NF DNA than did nested PCR. The spatial patterns of NF parasitism in a citrus orchard were derived using real-time PCR and samples of nematodes extracted from soil. The parasitism by Catenaria sp. was positively related to the abundance of both heterorhabditid and steinernematid EPNs. The possible significance of the associations is ambiguous because NF attack a broad range of nematode taxa whereas EPNs are a small minority of the total nematode population in a soil sample. These studies demonstrate the potential of real-time PCR to study the role of NF parasitism in soil food webs.

Spatial pattern analysis of entomopathogenic and other free‐living nematodes at landscape scales

AbstractSpatial patterns of entomopathogenic and other free‐living nematode populations were analyzed at the landscape scale. Free‐living bacterial feeder populations can be classified into trophic groups based on their functional and life history characteristics. Differences in life history traits were hypothesized to result in different spatial structures of populations. Spatial autocorrelations (Moran's I index) and exponential variogram models were calculated and estimated for each trophic group, including entomopathogenic nematodes, and comparisons were made among them. Spatial autocorrelation results showed that the spatial structure of entomopathogenic nematode populations is similar to that of relatively r‐selected free‐living bacterial feeders, which share several life history traits with entomopathogenic nematodes.

Characterization of populations of entomopathogenic nematodes isolated at diverse sites across Israel

Entomopathogenic nematodes (EPNs) are known as effective biological agents against soil-dwelling stages of insect pests. In the present study new EPN populations from the Steinernematidae and Heterorhabditidae families were isolated at different locations in Israel. The newly isolated populations were subjected to a series of bioassays for beneficial traits, for selection of a superior isolate for biological control of insect pests. Nematode motility was measured in sand columns, with and without the susceptible insect Galleria mellonella. Infectivity was evaluated using an invasion rate assay, as well as a dose response assay in which LD50 values were calculated. Nematode tolerance to environmental stresses was examined in heat and desiccation tolerance assays. The reproductive potentials of the new isolates were assessed in cadavers of G. mellonella. All Steinernematid populations were identified as Steinernema feltiae and the Heterorhabditids as Heterorhabditis sp. Large variation between the various populations was recorded in all assays. In order to identify a superior isolate, each strain was scored as 1 if its performance did not differ significantly from the highest level recorded in the particular assay; −1 if its performance did not differ significantly from the lowest level; and 0 if its performance was between the highest and lowest levels or did not differ from either. The scores for the different traits were then summed for each population and the totals were compared. The best score (+4) was obtained for a S. feltiae population isolated in northern Israel. These results will be used for the utilization of the new population as a biological control agent.

Intraspecific variation among isolates of the entomopathogenic nematode Steinernema feltiae from Bull Island, Ireland

AbstractThe application of large numbers of entomopathogenic nematodes (EPN) to control insect pests of agriculture is likely to have an impact on the local EPN fauna, yet little is known about the intraspecific relationships between EPN populations, particularly with regard to phylogeny and outbreeding. Here we assess the fitness, with regards to fecundity, host insect mortality and time taken to produce progeny, of isolates of Steinernema feltiae from Bull Island, Ireland. Exon-primed, intron-crossing (EPIC) PCR was used to examine intraspecific phylogenies between S. feltiae isolates, and identified up to three possible colonisation events of Bull Island. EPIC-PCR grouped two isolates, 33.D.(2) and 59.F.(2), separately from the remaining ten S. feltiae isolates These same two isolates consistently performed poorly in all fitness assessments. Following the crossbreeding of all isolates in Galleria mellonella, the number of host cadavers exhibiting emerging infective juveniles was significantly fewer than expected and there were significant differences between isolates in the number of days until progeny were observed. Host insect mortality varied between 40 and 87%. Such intraspecific variation may be a result of adaptation to different microhabitats of Bull Island, which in turn may be accentuated by laboratory culture practices.

METAPOPULATION DYNAMICS OVERRIDE LOCAL LIMITS ON LONG‐TERM PARASITE PERSISTENCE

A simple null model, particularly germane to small and vulnerable organisms such as parasites, is that local conditions set a stage upon which larger-scale dynamics play out. Soil moisture strongly influences survival of entomopathogenic nematodes (EPN), which in turn drive trophic cascades by protecting vegetation from root-feeding herbivores. In this study, we examine the mechanisms responsible for patchy occurrence of an entomopathogenic nematode, Heterorhabditis marelatus, in a California coastal prairie. One hypothesis proposes that biotic factors such as competition and natural enemies could regulate occurrence of EPN populations. We found that fungi and other enemies of EPN, although locally potent, did not explain the patterns of incidence across sites. Abiotic factors also have strong effects on EPN persistence, especially for vulnerable free-living stages. Thus, we tested the hypothesis that patchy occurrence of EPN on a large landscape was driven by differences in soil moisture. Our research uses long-term data on nematode incidence in combination with a landscape-level experiment to demonstrate the lack of a correlation between soil moisture and long-term persistence. A year-long experiment showed EPN mortality was weakly correlated with soil moisture among our study sites. Thirteen years of data, however, showed that colonization rates were highly correlated with long-term persistence. Sites with highest long-term persistence experienced the highest rates of rhizosphere colonization, extinction, and turnover. As a result, we concluded that metapopulation dynamics override limitations set by local and short-term abiotic conditions to determine long-term persistence in this parasite-driven trophic cascade.

Canonical correspondence analysis demonstrates unique soil conditions for entomopathogenic nematode species compared with other free-living nematode species

Entomopathogenic nematodes are ubiquitous yet relatively little is known of their naturally occurring spatial distributions and ecological functions compared with more easily observable taxa. This study provides the most detailed accounting to date of the soil habitat conditions associated with presence and abundance of naturally occurring populations of entomopathogenic nematodes compared with other free-living nematode species. We sampled at least 100 sites representing six habitats in an agroecosystem. Nematodes were extracted and soil physical and chemical conditions were measured for each site and the species–environment relationship was analyzed by canonical correspondence analysis. Both biotic and abiotic components of the soils were important in the relationship between free-living nematodes and soil conditions. Entomopathogenic nematodes were associated with a different set of soil conditions than other free-living species. The conditions favoring entomopathogenic nematodes were most similar to the most r-selected bacterial feeders and may result from an adaptive syndrome associated with their unique life history. Soil conditions associated with entomopathogenic nematode abundance included increased enrichment and food web structure, lower P, higher K, and lower C:N ratio. Soils in agroecosystems might be manipulated through soil fertility management practices to increase the ecosystem services provided by entomopathogenic nematodes.