Genetic selection of the ambush foraging entomopathogenic nematode,Steinernema carpocapsae for enhanced dispersal and its associated trade-offs

Dispersal of organisms is influenced by environmental and innate population variability. It results in redistribution of populations with potential consequences for gene flow, population resilience and stability, and evolutionary diversification of traits in response to specific selection pressures. However, dispersal behavior in soil-dwelling organisms is understudied. Species of entomopathogenic nematodes, a group of soil-inhabiting lethal insect parasites used in biological pest control show a dichotomy in foraging behavior. Some species have been classified as ambushers while others as cruisers. We previously discovered that the ambush foraging Steinernema carpocapsae possesses a small group of sprinters that disperse faster than the fastest moving cruisers. In this study, we genetically selected S. carpocapsae for enhanced dispersal in the absence of hosts by capturing the fastest and farthest reaching infective juveniles (IJs) emanating from a nematode-infected Galleria mellonella cadaver, in soil. S. carpocapsae showed positive response to selection for dispersal with 13–23 and 21–37 fold increase in the percent IJs dispersing to the farthest distance from the source cadaver, after five and ten rounds of selection, respectively. There was also a significant increase in the average displacement of the selected lines (6.85–7.54 cm/day) than the foundation population (5.54 cm/day) maintained by passing through G. mellonella larvae in Petri dishes. The overall mean realized heritability for dispersal was 0.60. The farthest reaching IJs of the selected lines comprised more males (72 %) than the foundation population (44 %) at most time points. Trade-offs associated with enhanced dispersal included reduced reproduction capacity and nictation ability, a trait associated with ambush foraging. In conclusion, this study revealed the costs and benefits associated with selection for enhanced dispersal in a soil-dwelling insect parasite, enhancing our understanding of the evolution of new behavioral patterns, which could have important implications in biological control.     

Virulence and sex-dependent invasion efficiency of entomopathogenic nematodes on developmental stages of Phthorimaea operculella (Lep., Gelechidae)

Virulence and invasion efficiency of the three entomopathogenic nematodes, Heterorhabditis bacteriophora, Steinernema carpocapsae and S. feltiae against the potato tuber moth (PTM), Phthorimaea operculella was evaluated. Also evaluated were the sex ratio of Steinernema spp. and host stages to determine if 1) the developmental stage of the host affects sex ratio of nematodes; 2) infective juveniles (IJs) concentration affects sex ratio in host developmental stages and 3) the establishment of IJs is affected by developmental stages of host. The PTM pre-pupa and pupa were exposed to IJs in filter substrate petri dish bioassays. By increasing the IJs concentrations, the number of established Steinernema spp. in both PTM stages increased and only decreased at the highest concentration. No reduction in established nematode numbers at the highest concentration was observed for H. bacteriophora. Sex ratio of S. carpocapsae in pre-pupa was affected by IJ concentration. PTM was more susceptible to Steinernema spp. than H. bacteriophora. Pre-pupa were more susceptible to S. feltiae but S. carpocapsae recorded as the most virulent EPN on pupa. Invasion efficiencies were similar for Steinernema and considerably higher than for H. bacteriophora. Despite a higher invasion efficiency of Steinernema into pupae, mortality was lower compared to pre-pupa No correlation was recorded between the invasion efficiencies of the EPNs and mortalities of PTM. The results showed that the invasion efficiency is not appropriate criterion to reflect the virulence of studied EPNs. Compared to H. bacteriophora both tested Steinernema spp. were good candidates for further studies as biocontrol agents of PTM.     

Attraction of entomopathogenic nematodes Steinernema carpocapsae and Heterorhabditis bacteriophora to the red palm weevil (Rhynchophorus ferrugineus)

The red palm weevil (RPW), Rhynchophorus ferrugineus, is a serious pest of date palms. Its larvae bore deep into the trunk disrupt the vascular tissues and kill the infested trees. Behavioral features of entomopathogenic nematodes (EPNs), reflected by attraction and distribution patterns, are fundamental aspect in determining their parasitic ability and potential management of RPW. We studied the attraction behavior of the EPNs Steinernema carpocapsae and Heterorhabditis bacteriophora to the RPW under simulated natural conditions in tubes to evaluate their infective potential. In all experiments, a certain proportion of infective juveniles (IJs) (16–20%) stayed near the inoculated site and a major proportion (38–48%) was attracted to the host end. Both H. bacteriophora and S. carpocapsae were efficient crawlers, climbing up and descending when locating their insect host. They were efficiently attracted to the various larval sizes and stages of the RPW life cycle. Host localization by ascending movement was more prominent in S. carpocapsae than in H. bacteriophora. In general, H. bacteriophora is classified as a cruiser forager and S. carpocapsae as an ambusher. However, in this study, we discovered a higher percentage of cruiser foragers among S. carpocapsae IJs. They dispersed much faster and their cruising behavior was prominent characteristic in controlling the cryptic RPW concealed in organic habitats.     

Efficacy of two entomopathogenic nematodes Heterorhabditis bacteriophora and Steinernema carpocapsae for control of the leopard moth borer Zeuzera pyrina (Lepidoptera: Cossidae) larvae under laboratory conditions

The biological traits of the entomopathogenic nematodes (EPNs), Steinernema carpocapsae and Heterorhabditis bacteriophora, against the larvae of the leopard moth, Zeuzera pyrina were evaluated in the laboratory. The traits included pathogenicity, penetration potential as well as foraging behaviour. Plate assays were performed using a range of EPN concentrations (5, 10, 20, 50 and 100 infective juveniles (IJs) per larva). The LC₅₀ values for S. carpocapsae and H. bacteriophora were 6.4 and 8.4 IJs larva^?1 after 72 h. Both EPN species caused high mortality in branch experiments. Significantly higher mortality rates occurred in the larger larvae after exposure to S. carpocapsae. Both EPN species successfully penetrated the Z. pyrina larvae as well as larvae of Galleria mellonella L. (Lepidoptera: Galleridae).The proportional response of H. bacteriophora to host-associated cues was strongly higher than S. carpocapsae in Petri dishes containing agar 1, 12 and 24 h after EPN application. These results highlight the efficiency of EPNs for the control of Z. pyrina larvae. However, due to the cryptic habitat of Z. pyrina larvae in their galleries in the trees, field trails need to be conducted to further evaluate this potential.     

Temperature and dose effects on the pathogenicity and reproduction of two Korean isolates of Heterorhabditis bacteriophora (Nematoda: Heterorhabditidae)

Out of some isolated Heterorhabditis bacteriophora from Korea, ecological study on two isolates which had different geographical features was investigated. That is, effects of temperature and dose on the pathogenicity and reproduction of two Korean isolates of H. bacteriophora were investigated using Galleria mellonella larvae in the laboratory. The median lethal dose (LD₅₀) decreased with increasing temperature, but increased at 35 °C. The optimal temperatures for infection were 30 °C for H. bacteriophora Jeju strain and 24 °C for H. bacteriophora Hamyang strain. The median lethal time, LT₅₀ of H. bacteriophora Hamyang strain was recorded at 13 °C to 35 °C and that of H. bacteriophora Jeju strain was recorded at 18 °C to 30 °C. The number of established nematodes in G. mellonella larvae was significantly different depending on temperature and dose. When G. mellonella larvae were exposed to 300 infective juveniles (IJs), mortality of G. mellonella gradually increased with exposure time with H. bacteriophora Jeju strain but not with H. bacteriophora Hamyang strain. 87.5% mortality of G. mellonella was recorded by H. bacteriophora Hamyang strain after 1440 min whereas 100% mortality was recorded by H. bacteriophora Jeju strain after 4320 min. The time from infection to the first emergence of nematodes decreased with increasing temperature. Duration of emergence of the two strains in the White traps also decreased with increasing temperature. The highest progeny numbers of H. bacteriophora Jeju strain were 264,602 while those of H. bacteriophora Hamyang strain were 275,744 at the rate of 160 IJs at 24 °C.     

Evaluation of the virulence of entomopathogenic nematodes as a biological control agents against Gryllotalpa gryllotalpa (Gryllotalpidae)

The European mole cricket Gryllotalpa gryllotalpa Linnaeus (Orthoptera: Gryllotalpidae) is a key pest of several crops in different regions of the world. The mole crickets usually damage seedlings, feeding aboveground on foliage or stem tissue and belowground on roots and tubers. In biological control, entomopathogenic nematodes (EPNs) are used against many insect pests inhabiting the soil as safe alternatives to chemical pesticides. The present study aimed to evaluate the virulence of the EPNs Heterorhabditis bacteriophora strain (HP88) and Steinernema carpocapsae strain (All) against the adults of European mole cricket G. gryllotalpa under laboratory and field conditions. Three concentrations of each of the EPNs H. bacteriophora and S. carpocapsae were tested against G. gryllotalpa. Infected adults of the pest with nematodes, at the concentration of 2000/cup, attained a mortality rate of (96%), when treated with S. carpocapsae and (72%) when treated with H. bacteriophora 8 days post-treatments. Mortality percentage of G. gryllotalpa adults increased as the concentration of the nematodes increased. Adults' mortality of G. gryllotalpa reached 100% when treated with the combination of the 2 nematode's strains at all concentrations 4 days post-treatments. At the 3 tested concentrations, 500, 1,000 and 2,000 IJs/adults, H. bacteriophora showed a higher reproductive rate in vivo than S. carpocapsae. A single adult of G. gryllotalpa treated with H. bacteriophora produced the average of 29,972, 74,768 and 140,474 IJs/adult, at the 3 concentrations, respectively. Under field application, a combination of EPNs successfully reduced the loss of the cabbage seedlings and the population's number of the mole crickets after 3 applications in the 2 seasons of (2019 and 2020). Therefore, EPNs as biological control agents can be recommended against the mole cricket G. gryllotalpa in vegetable crops as s pest management practice in an integrated pest management programme.     

Development Rates in Entomopathogenic Nematodes: Infected Hosts vs. Aqueous Suspension

Rearing conditions have been shown to affect several aspects of entomopathogenic nematode biology, including dispersal behavior and infectivity. The present study explores the differences in development rate of Heterorhabditis bacteriophora and Steinernema carpocapsae when infective juveniles (IJ) were collected in water using the standard White trap method vs. natural emergence from cadavers into sand. We exposed Galleria mellonella to IJ entompopathogenic nematodes treated in one of three ways: collected in a White trap, allowed to emerge directly into sand, or collected in a White trap and treated with a cadaver homogenate. When S. carpocapsae IJ were allowed to emerge from cadavers directly into sand and then allowed to infect new hosts, they developed into adults at a faster rate than IJ that were collected with White traps. The difference in development was not due to differential infection rates. No difference in development stages was detected amount the same H. bacteriophora treatments.     

Effect of insect cadaver desiccation and soil water potential during rehydration on entomopathogenic nematode (Rhabditida: Steinernematidae and Heterorhabditidae) production and virulence

We examined the influence of insect cadaver desiccation on the virulence and production of entomopathogenic nematodes (EPNs), common natural enemies of many soil-dwelling insects. EPNs are often used in biological control, and we investigated the feasibility of applying EPNs within desiccated insect cadavers. Desiccation studies were conducted using the factitious host, Galleria mellonella (Lepidoptera: Pyralidae, wax moth larvae) and three EPN species (Heterorhabditis bacteriophora ‘HB1’, Steinernema carpocapsae ‘All’, and Steinernema riobrave). Weights of individual insect cadavers were tracked daily during the desiccation process, and cohorts were placed into emergence traps when average mass losses reached 50%, 60%, and 70% levels. We tracked the proportion of insect cadavers producing infective juveniles (IJs), the number and virulence of IJs produced from desiccated insect cadavers, and the influence of soil water potentials on IJ production of desiccated insect cadavers. We observed apparent differences in the desiccation rate of the insect cadavers among the three species, as well as apparent differences among the three species in both the proportion of insect cadavers producing IJs and IJ production per insect cadaver. Exposure of desiccated insect cadavers to water potentials greater than −2.75 kPa stimulated IJ emergence. Among the nematode species examined, H. bacteriophora exhibited lower proportions of desiccated insect cadavers producing IJs than the other two species. Desiccation significantly reduced the number of IJs produced from insect cadavers. At the 60% mass loss level, however, desiccated insect cadavers from each of the three species successfully produced IJs when exposed to moist sand, suggesting that insect cadaver desiccation may be a useful approach for biological control of soil insect pests.     

Optimization of Inoculation for In Vivo Production of Entomopathogenic Nematodes

Entomopathogenic nematodes are potent biopesticides that can be mass-produced by in vitro or in vivo methods. For in vivo production, consistently high infection rates are critical to efficiency of the process. Our objective was to optimize in vivo inoculation of Steinernema carpocapsae and Heterorhabditis bacteriophora in Galleria mellonella and Tenebrio molitor by determining effects of inoculation method, nematode concentration, and host density. We found immersing hosts in a nematode suspension to be approximately four times more efficient in time than pipeting inoculum onto the hosts. The number of hosts exhibiting signs of nematode infection increased with nematode concentration and decreased with host density per unit area. This is the first report indicating an effect of host density on inoculation efficiency. We did not detect an effect of nematode inoculum concentration on nematode yield per host or per gram of host. Yield was affected by host density in one of the four nematode-host combinations (S. carpocapsae and T. molitor). We conclude that optimization of inoculation parameters is a necessary component of developing an in vivo production system for entomopathogenic nematodes.     

Food preference of Sancassania polyphyllae (Acari: Acaridae): living entomopathogenic nematodes or insect tissues?

Sancassania polyphyllae (Acari: Acaridae) adult female mites will feed on insect cadavers and infective juveniles (IJs) of entomopathogenic nematodes. Our objective was to determine whether S. polyphyllae has a food preference when offered a choice between tissues of Polyphylla fullo (Coleoptera: Scarabaeidae) or Galleria mellonella (Lepidoptera: Pyralidae), or IJs of Steinernema feltiae (Rhabditida: Steinernematidae) or IJs of Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae). When offered a choice between no food and one of the different food sources, P. fullo, G. mellonella or S. feltiae IJs, S. polyphyllae had a significant preference for food sources compared to no food. When it was offered either no food or H. bacteriophora, there was no significant difference in the mite distribution. When offered two different food choices, P. fullo or G. mellonella, P. fullo or S. feltiae, and P. fullo or H. bacteriophora, the mite showed significant preferences for P. fullo larvae. In S. feltiae vs. G. mellonella and S. feltiae vs. H. bacteriophora experiments, S. polyphyllae showed significant a preference for S. feltiae. In three-choice experiments, S. polyphyllae had a preference for P. fullo, followed by S. feltiae, G. mellonella and H. bacteriophora, respectively. Our data confirm, in part, our hypothesis that when offered different food choices, this mite species prefers tissues of its phoretic host, P. fullo over lepidopteran host tissues or living IJs. Based on these laboratory data, H. bacteriophora should be used as a biological control agent against P. fullo over a Steinernema species.     

Susceptibility to entomopathogens and modulation of basal immunity in two insect models at different temperatures

In this work, we analysed the efficacy of different commercial bio-insecticides (Steinernema feltiae, Steinernema carpocapsae, Heterorhabditis bacteriophora and Bacillus thuringiensis) by valuating the mortality induced on two insect models, Galleria mellonella (Lepidoptera) and Sarcophaga africa (Diptera) after exposure to different temperatures (10, 20 and 30 °C). Moreover, we investigated the effects of temperature on the basal humoral immunity of the two target insects; particularly, phenoloxidase (PO) and lysozyme activity. Our results show that G. mellonella is susceptible to all bio-insecticides at all the examined temperatures, except when infected at 10 °C with S. carpocapsae and at 30 °C with S. feltiae and B. thuringiensis. S. africa is more susceptible at 30 °C to all bioinsecticides; whereas, when infected at 10 and 20 °C, H. bacteriophora is the most efficient. Temperature modulates PO activity of both G. mellonella and S. africa, otherwise variations in lysozyme activity is observed only in G. mellonella. Except for a possible correlation between the increased lysozyme activity and the delayed Bt efficacy recorded on G. mellonella at 30 °C, a different resistance to bio-insecticides at different temperatures does not seem to be associated to variations of the host basal immunity, probably due to immunoevasive and immunodepressive strategies of these entomopathogens.     

Attraction of four entomopathogenic nematodes to four white grub species

To better understand the differences in the efficacy of entomopathogenic nematode species against white grub species, we are studying the various steps of the infection process of entomopathogenic nematodes into different white grub species using nematode species/strains with particular promise as white grub control agents. In this study we compared the attraction of the entomopathogenic nematodes Steinernema scarabaei (AMK001 strain), Steinernema glaseri (NC1 strain), Heterorhabditis zealandica (X1 strain), and Heterorhabditis bacteriophora (GPS11 strain) to third-instars of the scarabs Popillia japonica, Anomala orientalis, Cyclocephala borealis, and Rhizotrogus majalis, and late-instar greater wax moth, Galleria mellonella, larvae. Individual larvae were confined at the bottom of 5.5 cm vertical sand columns, nematodes added to the sand surface after 24 h, and nematodes extracted after another 24 h. Nematode attraction to hosts was strongly affected by nematode species but the effect of insect species varied with nematode species. S. glaseri had a high innate dispersal rate (i.e., in absence of insects) and was strongly attracted to insects without significant differences among insect species. S. scarabaei had a very low innate dispersal rate so that even a strong relative response to insects resulted in low absolute dispersal rates toward insects. S. scarabaei tended to be most attracted to G. mellonella and least attracted to C. borealis. H. zealandica had a high innate dispersal rate but only responded weakly to insects without significant differences among species. H. bacteriophora had limited innate dispersal and only weakly responded to insects with G. mellonella tending to be the most attractive and C. borealis the least attractive insect. It has to be noted that we cannot exclude that the use of different rearing hosts (A. orientalis and P. japonica larvae for S. scarabaei, G. mellonella larvae for the other nematodes) might have had an impact on the nematodes dispersal and relative attraction behavior. This study indicates that host attractiveness and nematode dispersal rates may contribute but do not play a major role in the variability in white grub susceptibility and/or nematode virulence.     

Entomopathogenic nematodes induce components of systemic resistance in plants: Biochemical and molecular evidence

Antagonism between entomopathogenic nematodes (EPNs) and plant-parasitic nematodes (PPNs) has been documented over the past two decades but its mechanism and ecological significance remain elusive. We investigated the effects of Steinernema carpocapsae and its symbiotic bacterium, Xenorhabdus nematophila applied to the potting medium on pyrogallol peroxidase (P-peroxidase), guaiacol peroxidase (G-peroxidase) and catalase activities in Hosta sp. and Arabidopsis thaliana leaves as components of induced systemic resistance. We found that P-peroxidase activity was significantly higher in the leaves from hosta plants treated with S. carpocapsae infective juveniles (IJs) and S. carpocapsae infected insect cadavers than in the leaves from the control plants 2 weeks after treatment. The G-peroxidase activity was significantly higher in S. carpocapsae infected cadaver and X. nematophila treatments 10 and 15 days after treatment (DAT) and in S. carpocapsae IJs treatment 5 and 15 DAT. The catalase activity in hosta leaves was significantly higher in S. carpocapsae infected cadaver and X. nematophilus treatments compared with the control 5 and 15 DAT and in S. carpocapsae IJs treatment 5 and 10 DAT. Further, the catalase activity in A. thaliana leaves was significantly higher in S. carpocapsae IJs treatment than in the control 7 DAT. We also determined the effects of S. carpocapsae infected cadavers and S. carpocapsae IJs on PR1-gene expression in transgenic A. thaliana leaves through GUS (β-glucuronidase) activity assay and found that the PR1-gene was expressed in leaves from all treatments except the control. Thus, we conclude that the EPNs and their symbiotic bacteria can induce systemic resistance in plants which may explain the elusive antagonistic effect of EPNs on PPNs.     

Susceptibility of three lepidopteran pests to five entomopathogenic nematodes and in vivo mass production of these nematodes

Abstract

An investigation was conducted in pots to access the susceptibility of three lepidopteran pests, namely, gram pod borer, Helicoverpa armigera, greater wax moth, Galleria mellonella, and rice moth, Corcyra cephalonica, to two recently described species, Steinernema masoodi, S. seemae, and three indigenous S. carpocapsae, S. glaseri and S. thermophilum entomopathogenic nematodes (EPN). The suitability of these lepidopterans for the in vivo mass production of the nematodes was also estimated. Among the five species of EPN, S. masoodi, S. seemae and S. carpocapsae were found most pathogenic to C. cephalonica, bringing about mortality within 24 h, followed by H. armigera (36, 38 and 48 h, respectively) and G. mellonella (30, 36 and 48 h, respectively). The other species of EPN, viz., S. glaseri and S. thermophilum was the least pathogenic, which killed the larvae of C. cephalonica in 29 and 36 h, respectively, G. mellonella in 48 h, and H. armigera in 38 and 56 h, respectively. Galleria mellonella was found the most suitable host for the mass production of infective juveniles (IJs) of S. seemae, which yielded higher IJs than S. carpocapsae. Helicoverpa armigera was the next best suitable alternate host, which produced maximum IJs in case of S. seemae followed by S. masoodi, S. carpocapsae, S. glaseri and S. thermophilum. Rice moth, Corcyra cephalonica was the least suitable host. The susceptibility of H. armigera to five tested EPN species and susceptibility of G. mellonella and C. cephalonica to S. masoodi and S. seemae are new records.     

Effect of Entomopathogenic Nematode Concentration on Survival during Cryopreservation in Liquid Nitrogen

Entomopathogenic nematodes are used for biological control of insect pests. A method for improved cryopreservation of infective juvenile stage nematodes has been developed using Steinernema carpocapsae and Heterorhabditis bacteriophora. Optimum survival for both species was achieved with 12,000 infective juveniles/ml in glycerol and 7,500/ml in Ringer''s solution. For S. carpocapsae, maximum survival also was observed with 60,000 infective juveniles/ml in glycerol and 25,000/ml in Ringer''s solution. These concentrations resulted in 100% post-cryopreservation survival of S. carpocapsae and 100% retention of original virulence to Galleria mellonella larvae. This is the first report of achieving 100% survival of an entomopathogenic nematode after preservation in liquid nitrogen. Maximum survival of H. bacteriophora following cryopreservation was 87%.     

Immune defence components of Spodoptera exigua larvae against entomopathogenic nematodes and symbiotic bacteria

The current work investigated the immune response of Spodoptera exigua Hübner (Lepidoptera: Noctuidae) when challenged with two entomopathogenic nematodes (EPNs), Steinernema carpocapsae (Weiser) and Heterorhabditis bacteriophora (Poinar). The cellular and humoral defences were considered in this study. The haemocytes were observed around H. bacteriophora, but no haemocyte was found around S. carpocapsae. In larvae treated with H. bacteriophora and S. carpocapsae, total haemocyte counts (THCs) reached maximum levels at 4 and 12 hours post-injection (hpi), respectively, but decreased with the proliferation of symbiotic bacteria. In the humoral defence, there was no significant difference between EPNs on phenoloxidase (PO) activity. Phospholipase A₂ (PLA₂) and protease activity levels in the initial time post-injection were higher in the larvae treated with S. carpocapsae than in H. bacteriophora. In the following, the roles of symbiotic bacteria and axenic infective juveniles (IJs) in suppressing the immune system were studied separately. Maximum THC levels were observed in larvae treated with axenic nematodes and minimum THC levels were recorded in the live Xenorhabdus nematophila treatment. In the humoral defence, PLA₂ activity with axenic S. carpocapsae was suppressed at 4 hpi, while in monoxenic S. carpocapsae the PLA₂ level was increased to the maximum amount at 8 hpi. PO activity with monoxenic S. carpocapsae decreased gradually by 4 hpi; in live X. nematophila, it decreased from 0.5 to 16 hpi, while in axenic S. carpocapsae, it increased slowly from 0.5 to 16 hpi. The current work showed the synergistic effect of nematode and its bacterium in the suppression of the immune system and highlighted the role of the symbiont in inhibition of immune responses.     

Biological Control of the Pecan Weevil,Curculio caryae (Coleoptera: Curculionidae), with Entomopathogenic Nematodes

Steinernema carpocapsae (Weiser) strain A11, S. feltiae (Filipjev) strain SN, and Heterorhabditis bacteriophora Poinar strains HP88 and Georgia were tested for their efficacy as biological control agents of the pecan weevil, Curculio caryae (Horn), in pecan orchard soil-profile containers under greenhouse conditions. Percentage C. caryae parasitism by S. carpocapsae and H. bacteriophora strain HP88 and Georgia was consistently poor when applied either prior to or following C. caryae entry into the soil, suggesting that these nematode species and (or) their enterobacteria are poor biological control agents of weevil larvae. Soil taken 21 days following application of S. carpocapsae or H. bacteriophora strain HP88 induced a low rate of infection of Galleria mellonella larvae, whereas soil that had been similarily treated with H. bacteriophora strain Georgia induced a moderate rate of infection. Percentage C. caryae parasitism by S. feltiae was consistently low when applied following C. caryae entry into the soil and was inconsistent when applied as a barrier prior to entry of weevil larvae into the soil. Soil taken 21 days following application of S. feltiae induced a high rate of infection of G. mellonella larvae.     

Variations in Immune Response of Popillia japonica and Acheta domesticus to Heterorhabditis bacteriophora and Steinernema Species

The infectivities of Steinernema carpocapsae, S. glaseri, S. scapterisci, and Heterorhabditis bacteriophora to Japanese beetle larvae, Popillia japonica, and house cricket adults, Acheta domesticus, were compared using external exposure and hemocoelic injection. Only H. bacteriophora and S. glaseri caused high P. japonica mortality after external exposure. When nematodes were injected, P. japonica had a strong encapsulation and melanization response to all species except S. glaseri. Heterorhabditis bacteriophora and S. carpocapsae were able to overcome the immune response, but S. scapterisci was not. All species except S. scapterisci were able to kill and reproduce within the host. Only S. scapterisci and S. carpocapsae caused A. domesticus mortality after external exposure. When nematodes were injected, A. domesticus had a strong immune response to all species except S. scapterisci. Steinernema carpocapsae effectively overcame the strong immune response and caused high host mortality, but S. glaseri and H. bacteriophora did not. Steinernema scapterisci caused high host mortality and reproduced, S. glaseri and H. bacteriophora caused low host mortality but only S. glaseri reproduced, and S. carpocapsae was able to kill the host but reproduced poorly. Most (ca. 90%) of the S. carpocapsae in the hemocoel of P. japonica became encapsulated and melanized within 8 hours postinjection. The symbiotic bacterium, Xenorhabduf nematophilus, was often released before this encapsulation and melanization.     

Infection of the Entomopathogenic Nematode,Steinernema carpocapsae,as Affected by the Presence of Steinernema glaseri

The infection behavior of Steinernema carpocapsae infective juveniles (IJ) was investigated in the presence and absence of S. glaseri. Mixed inoculation of S. carpocapsae with S. glaseri IJ significantly raised the nictation rates of S. carpocapsae IJ. Significantly more S. carpocapsae IJ migrated to the host insect in the mixed inoculation with S. glaseri IJ on agar plates. More S. carpocapsae IJ penetrated into the host insect placed 2 cm below the surface in the mixed inoculation with S. glaseri IJ. More S. glaseri than S. carpocapsae IJ penetrated into hosts placed 7 cm deep. Irrespective of host location, the male ratio of S. carpocapsae IJ established in the host body was always higher in the mixed inoculation with S. glaseri IJ.