Laboratory and field evaluation of entomopathogenic nematodes for control of Agriotes obscurus (L.) (Coleoptera: Elateridae)

The susceptibility of the dusky wireworm, Agriotes obscurus (L.) (Coleoptera: Elateridae), to different species and strains of entomopathogenic nematodes was tested in a virulence assay in the laboratory. Larvae were exposed to different nematode doses of 50 and 100 IJs/cm². At a dose of 50 IJs/cm², only a commercial strain Heterorhabditis bacteriophora Poinar and the native strain Steinernema carpocapsae (Weiser) B14 caused increased mortality compared with the control (11.1% and 13.3% mortality, respectively). At the higher dose tested, all strains (except Steinernema sp. D122) were virulent to A. obscurus larvae. Steinernema carpocapsae B14 caused higher mortality of wireworm (75.6%) and was used for the assay conducted in cages, with a dose of 100 IJs/cm², in field conditions. The results showed that S. carpocapsae B14 controlled 48.3% of A. obscurus larvae, demonstrating that some entomopathogenic nematodes have the potential to control larvae of A. obscurus. However, further work is needed to improve their efficacy.     

Infectivity of Steinernema carpocapsae and S. feltiae to Larvae and Adults of the Hazelnut Weevil,Curculio nucum: Differential Virulence and Entry Routes

We investigated the existing susceptibility differences of the hazelnut weevil, Curculio nucum L. (Coleoptera:, Curculionidae) to entomopathogenic nematodes by assessing the main route of entry of the nematodes, Steinernema carpocapsae strain B14 and S. feltiae strain D114, into larvae and adult insects, as well as host immune response. Our results suggested that S. carpocapsae B14 and S. feltiae D114 primarily entered adult insects and larvae through the anus. Larvae were more susceptible to S. feltiae D114 than S. carpocapsae B14 and adults were highly susceptible to S. carpocapsae B14 but displayed low susceptibility to S. feltiae D114. Penetration rate correlated with nematode virulence. We observed little evidence that hazelnut weevils mounted any cellular immune response toward S. carpocapsae B14 or S. feltiae D114. We conclude the differential susceptibility of hazelnut weevil larvae and adults to S. carpocapsae B14 and S. feltiae D114 primarily reflected differences in the ability of these two nematodes to penetrate the host.     

Stage-specific mortality of Rhagoletis indifferens (Diptera: Tephritidae) exposed to three species of Steinernema nematodes

Mortality of larval, pupal, and adult western cherry fruit fly, Rhagoletis indifferens (Tephritidae) exposed to the steinernematid nematodes Steinernema carpocapsae, Steinernema feltiae, and Steinernema intermedium, was determined in the laboratory and field. Larvae were the most susceptible stage, with mortality in the three nematode treatments ranging from 62 to 100%. S. carpocapsae and S. feltiae were equally effective against larvae at both 50 and 100 infective juveniles (IJs)/cm². S. intermedium was slightly less effective against larvae than the other two species. Mortalities of R. indifferens larvae at 0, 2, 4, and 6 days following their introduction into soil previously treated with S. carpocapsae and S. feltiae at 50 IJs/cm² were 78.6, 92.5, 95.0, and 77.5% and 87.5, 52.5, 92.5, and 70.0%, respectively, and at 100 IJs/cm² were 90.0, 92.0, 100.0, and 84.0% and 90.0, 50.0, 42.0, and 40.0%, respectively. There was no decline in mortality caused by S. carpocapsae as time progressed, whereas there was in one test with S. feltiae. Larval mortalities caused by the two species were the same in a 1:1:1 vermiculite:peat moss:sand soil mix and a more compact silt loam soil. In the field, S. carpocapsae and S. feltiae were equally effective against larvae. Pupae were not infected, but adult flies were infected by all three nematode species in the laboratory. S. carpocapsae was the most effective species at a concentration of 100 IJs/cm² and infected 11–53% of adults that emerged. The high pathogenicity of S. carpocapsae and S. feltiae against R. indifferens larvae and their persistence in soil as well as efficacy in different soil types indicate both nematodes hold promise as effective biological control agents of flies in isolated and abandoned lots or in yards of homeowners.     

Efficacy of commercial formulations of entomopathogenic nematodes against tropical sod webworm,Herpetogramma phaeopteralis (Lepidoptera: Crambidae)

Large quantities of insecticides are used on warm season turfgrasses to combat pest infestations. To investigate the potential for microbial control, we screened commercially available entomopathogenic nematode products against Herpetogramma phaeopteralis Guenée, an economically injurious pest in the south‐eastern United States and Caribbean islands. All tested products, based on Steinernema carpocapsae, S. feltiae, Heterorhabditis bacteriophora, H. megidis and H. indica, were pathogenic to H. phaeopteralis larvae in the laboratory, but S. carpocapsae caused the highest mortality. Amongst nematode species, median lethal concentration (LC₅₀) was not different for three different larval sizes (based on 95% CL) with the exception of H. indica, which had higher LC₅₀ for small larvae. The number of infective juvenile stages (IJs) produced per White trap was significantly greater from larvae infected by H. bacteriophora and least for those infected by H. indica. A proprietary formulation of S. carpocapsae ‘Millenium^®’ was chosen for further greenhouse experiments. Overall, the neonicotinoid insecticide clothianidin provided the best control, but greenhouse experiments also revealed that the label rate of Millenium (10⁶ IJ/l at 2500 l/ha) reduced webworm populations by 83–93% and was as effective as clothianidin against larger‐size larvae. Our data suggest that commercial formulations of S. carpocapsae can be a good option for H. phaeopteralis biocontrol, but further field studies are warranted to confirm effectiveness under different environmental scenarios.     

Entomopathogenic nematodes fail to parasitize the woolly apple aphid Eriosoma lanigerum as their symbiotic bacteria are suppressed

The restriction of effective insecticides has facilitated the woolly apple aphid (WAA) Eriosoma lanigerum to become a major pest in apple orchards in Western Europe. It has also promoted alternative control strategies such as the use of entomopathogenic nematodes (EPN). We evaluated the control capacity of six commercially available EPN, viz. Heterorhabditis bacteriophora, Heterorhabditis megidis, Steinernema carpocapsae, Steinernema feltiae, Steinernema glaseri and Steinernema kraussei. We assessed the potential of these EPN to colonize and parasitize E. lanigerum in an in vitro multiwell test. Only S. carpocapsae caused higher mortality (20–40%) than the control treatment (water). However, the mortality observed with S. carpocapsae was found to be a test artefact and not induced by its specific entomopathogenic activity. A similar mortality range was recorded when applying the non‐entomopathogenic nematode Pratylenchus thornei in the same multiwell test set‐up. This result warrants careful interpretation of parasitism in these artificial test conditions. The failure of EPN activity was supported in further experiments by frequently finding S. carpocapsae inside living WAA. The presence of the EPN had no effect on aphid reproduction as numbers of ‘large’ embryos in EPN‐colonized and non‐colonized females were similar. In addition, the dauer juveniles did not recover in E. lanigerum reflecting that S. carpocapsae could not develop inside the WAA. We further demonstrated that growth of the EPN‐symbiotic bacteria Xenorhabdus nematophila and Photorhabdus luminescens is inhibited by the body fluid of the WAA, and we speculate that this antibacterial activity is the cause of the unsuccessful parasitization of the WAA by the EPN. This antibiosis inside the body of E. lanigerum would prevent production of the endotoxins by the bacterial symbionts that are essential for entomopathogenicity and insect control.     

Virulence of entomopathogenic nematodes to larvae of the guava weevil, Conotrachelus psidii (Coleoptera: Curculionidae), in laboratory and greenhouse experiments

The guava weevil, Conotrachelus psidii, is a major pest of guava in Brazil and causes severe reduction in fruit quality. This weevil is difficult to control with insecticides because adults emerge over a long period, and larvae develop to the fourth-instar inside the fruit and move to the soil for pupation. We assessed the virulence of entomopathogenic nematodes to fourth-instar larvae in soil by comparing their susceptibility to nine species or strains: Heterorhabditis bacteriophora HP88, H. baujardi LPP7, and LPP1, H. indica Hom1, Steinernema carpocapsae All and Mexican, S. feltiae SN, S. glaseri NC, and S. riobrave 355. In petri dish assays with sterile sand at a concentration of 100 infective juveniles (IJs) of a given nematode species/strain, larval mortality ranged from 33.5 to 84.5%, with the heterorhabditids being the most virulent. In sand column assays with H. baujardi LPP7, H. indica Hom1, or S. riobrave 355 at concentrations of 100, 200, and 500 IJs, mortality was greater than the control only for H. baujardi (62.7%) and H. indica (68.3%) at the highest concentration. For H. baujardi LPP7 in a petri dish assay, the time required to kill 50 and 90% of the larvae (LT₅₀ and LT₉₀) for 100 IJs was 6.3 and 9.9 days, whereas the lethal concentration required to kill 50 and 90% of the larvae (LC₅₀ and LC₉₀) over 7 days was 52 and 122.2 IJs. In a greenhouse study with guava trees in 20-L pots, 10 weevil larvae per pot, and concentrations of 500, 1000 or 2000 IJs, H. baujardi LPP7 caused 30 and 58% mortality at the two highest concentrations. These results show that H. baujardi is virulent to fourth-instar larvae and has potential as a biological control agent in IPM programs.     

Comparative study of the entomopathogenic nematode, Steinernema carpocapsae, reared on mutant and wild-type Xenorhabdus nematophila

The symbiotic interaction between Steinernema carpocapsae and Xenorhabdus nematophila was investigated by comparing the reproduction, morphology, longevity, behavior, and efficacy of the infective juvenile (IJ) from nematodes reared on mutant or wild-type bacterium. Nematodes reared on the mutant X. nematophila HGB151, in which an insertion of the bacterial gene, rpoS, eliminates the retention of the bacterium in the intestinal vesicle of the nematode, produced IJs without their symbiotic bacterium. Nematodes reared on the wild-type bacterium (HGB007) produced IJs with their symbiotic bacterium. One or the other bacterial strain injected into Galleria mellonella larvae followed by exposing the larvae to IJs that were initially symbiotic bacterium free produced progeny IJs with or without their Xenorhabdus-symbiotic bacterium. The two bacterial strains were not significantly different in their effect on IJ production, sex ratio, or IJ morphology. IJ longevity in storage was not influenced by the presence or absence of the bacterial symbiont at 5 and 15 °C, but IJs without their bacterium had greater longevity than IJs with their bacterium at 25 and 30 °C, suggesting that there was a negative cost to the nematode for maintaining the bacterial symbiont at these temperatures. IJs with or without their symbiotic bacterium were equally infectious to Spodoptera exigua larvae in laboratory and greenhouse and across a range of soil moistures, but the absence of the bacterial symbiont inhibited nematodes from producing IJ progeny within the host cadavers. In some situations, such as where no establishment of an alien entomopathogenic nematode is desired in the environment, the use of S. carpocapsae IJs without their symbiotic bacterium may be used to control some soil insect pests.     

Efficacy of soil treatments of entomopathogenic nematodes against the larvae, pupae and adults of Tuta absoluta and their interaction with the insecticides used against this insect

The efficacy of soil treatments of three native entomopathogenic nematodes (Steinernema carpocapsae, S. feltiae and Heterorhabditis bacteriophora) against Tuta absoluta larvae, pupae and adults was determined under laboratory conditions. The effect of three insecticides commonly used against T. absoluta, in the survival, infectivity and reproduction of these nematode strains was also evaluated. When dropped into soil to pupate, soil application of nematodes resulted in a high mortality of larvae: 100, 52.3 and 96.7 % efficacy for S. carpocapsae, S. feltiae and H. bacteriophora respectively. No mortality of pupae was observed and mortality of adults emerging from soil was 79.1 % for S. carpocapsae and 0.5 % for S. feltiae. The insecticides tested had a negligible effect on nematode survival, infectivity and reproduction. No sublethal effects were observed. Infective juveniles that survived to insecticide exposition were able to infect Galleria larvae with no significant differences from the control. The Galleria larvae affected by the three insecticides tested served as suitable hosts for the infection and reproduction of the nematodes. These results suggest that larvae of T. absoluta, falling from leaves following insecticide application, could be suitable hosts for nematodes, thereby increasing their concentration and persistence in the soil.     

Compatibility between Forficula auricularia and entomopathogenic nematodes to be used in pome fruit pest management

Use of predators, parasitoids and entomopathogens as biocontrol agents in pome fruit production can lead to more efficient and sustainable pest management programmes. The European earwig (Forficula auricularia Linnaeus [Dermaptera: Forficulidae]) is a major predator of key pests in pome fruit orchards, and entomopathogenic nematodes (EPNs) of the families Steinernematidae and Heterorhabditidae are obligate parasites of a large number of insect species. Therefore, the interaction between earwigs and EPNs can play an important role in pest management programmes. Susceptibility of the European earwig to Steinernema carpocapsae, Steinernema feltiae (Steinernematidae) and Heterorhabditis bacteriophora (Heterorhabditidae) was evaluated. S. carpocapsae was the only tested EPN capable of killing the European earwig. However, the European earwig can detect the presence of S. carpocapsae and therefore avoid nematode‐treated shelters. An earwig deterrent activity in EPN‐killed codling moth larvae that reduces the foraging of European earwig on insect cadavers containing nematodes and allows nematodes to complete their life cycle was also assessed with the three species of nematodes. These findings suggest a positive compatibility between the European earwig and EPNs.     

Efficacy of entomopathogenic nematodes against neonate larvae of <Emphasis Type="Italic">Capnodis tenebrionis</Emphasis> (L.) (Coleoptera: Buprestidae) in laboratory trials

The efficacy of five entomopathogenic nematode strains of the families Steinernematidae and Heterorhabditidae was tested against the neonate larvae of Capnodis tenebrionis. The nematode strains screened included two of Steinernema carpocapsae (Exhibit and M137), and one each of S. feltiae (S6), S. arenarium (S2), and Heterorhanditis bacteriophora (P4). Exposure of neonate larvae of Capnodis to 10 and 150 infective juveniles (IJs) per larva (equivalent to 3 and 48 IJs/cm² respectively) in test tubes with sterile sand, resulted in mortality between 60–91% and 96–100%, respectively. At a concentration of 150 IJs/larva, all of the nematode strains were highly virulent. Both S. carpocapsae strains (Exhibit and M137) caused infection and mortality to larvae more quickly than the other strains. However, at a lower concentration assay (10 IJs/larva), S. arenarium was the most virulent strain. The penetration rate as an indicator of entomopathogenic nematode infection was also evaluated. The highest value was recorded for S. arenarium (36%), followed by H. bacteriophora (30.6%), S. feltiae (23.1%), and S. carpocapsae (20.7%).     

Characterization of biocontrol traits in the entomopathogenic nematode Heterorhabditis georgiana (Kesha strain), and phylogenetic analysis of the nematode’s symbiotic bacteria

Our objective was to estimate the biocontrol potential of the recently discovered entomopathogenic nematode species Heterorhabditis georgiana (Kesha strain). Additionally, we conducted a phylogenetic characterization of the nematode’s symbiotic bacterium. In laboratory experiments, we compared H. georgiana to other entomopathogenic nematodes for virulence, environmental tolerance (to heat, desiccation, and cold), and host seeking ability. Virulence assays targeted Acheta domesticus, Agrotis ipsilon, Diaprepes abbreviatus, Musca domestica, Plodia interpunctella, Solenopsis invicta, and Tenebrio molitor. Each assay included H. georgiana and five or six of the following species: Heterorhabditis floridensis, Heterorhabditis indica, Heterorhabditis mexicana, Steinernema carpocapsae, Steinernema feltiae, Steinernema rarum, and Steinernema riobrave. Environmental tolerance assays included Heterorhabditis bacteriophora, H. georgiana, H. indica, S. carpocapsae, S. feltiae, and S. riobrave (except cold tolerance did not include S. carpocapsae or S. riobrave). Host seeking ability was assessed in H. bacteriophora, H. georgiana, S. carpocapsae, and Steinernema glaseri, all of which showed positive orientation to the host with S. glaseri having greater movement toward the host than S. carpocapsae (and the heterorhabditids being intermediate). Temperature range data (tested at 10, 13, 17, 25, 30 and 35 °C) indicated that H. georgiana can infect Galleria mellonella between 13 and 35 °C (with higher infection at 17–30 °C), and could reproduce between 17 and 30 °C (with higher nematode yields at 25 °C). Compared with other nematode species, H. georgiana expressed low or intermediate capabilities in all virulence and environmental tolerance assays indicating a relatively low biocontrol potential. Some novel observations resulted from comparisons among other species tested. In virulence assays, H. indica caused the highest mortality in P. interpunctella followed by S. riobrave; S. carpocapsae caused the highest mortality in A. domesticus followed by H. indica; and S. riobrave was the most virulent nematode to S. invicta. In cold tolerance, S. feltiae exhibited superior ability to cause mortality in G. mellonella (100%) at 10 °C, yet H. bacteriophora and H. georgiana exhibited the ability to produce attenuated infections at 10 °C, i.e., the infections resumed and produced mortality at 25 °C. In contrast, H. indica did not show an ability to cause attenuated infections. Based on the phylogenetic analysis, the bacterium associated with H. georgiana was identified as Photorhabdus luminescens akhurstii.     

Effect of potting media on the efficacy and dispersal of entomopathogenic nematodes for the control of black vine weevil, Otiorhynchus sulcatus (Coleoptera: Curculionidae)

The effect of five commercial potting media, peat, bark, coir, and peat blended with 10% and 20% compost green waste (CGW) on the virulence of six commercially available entomopathogenic nematodes (EPN), Heterorhabditis bacteriophora UWS1, Heterorhabditis megidis, Heterorhabditis downesi, Steinernema feltiae, Steinernema carpocapsae, and Steinernema kraussei was tested against third-instar black vine weevil (BVW), Otiorhynchus sulcatus. Media type was shown to significantly affect EPN virulence. Heterorhabditis species caused 100% larval mortality in all media whereas Steinernema species caused 100% larval mortality only in the peat blended with 20% CGW. A later experiment investigated the effect of potting media on the virulence of EPN species against BVW by comparing the vertical dispersal of EPN in the presence and absence of BVW larva. Media type significantly influenced EPN dispersal. Dispersal of H. bacteriophora was higher than H. megidis, H. downesi, or S. kraussei in all media, whereas, S. feltiae and S. carpocapsae dispersal was much reduced and restricted to peat blended with 20% CGW and coir, respectively. In the absence of larvae, most of the EPN species remained in the same segment they were applied in, suggesting that the larvae responded to host volatile cues. Greenhouse trials were conducted to evaluate the efficacy of most virulent strain, H. bacteriophora in conditions more representative of those in the field, using 2.5 × 10⁹ infective juveniles/ha. The efficacy of H. bacteriophora UWS1 against third-instar BVW was 100% in peat, and peat blended with 10% and 20% CGW but only 70% in bark and coir, 2 weeks after application. These studies suggest that potting media significantly affects the efficacy and dispersal of EPN for BVW control.     

Comparison of Two Steinernematid Species for Control of the Root Weevil Diaprepes abbreviatus

Steinernema carpocapsae Weiser All strain was compared to Steinernema riobravis Cabanillas, Poinar, and Raulston for control of the root weevil, Diaprepes abbreviatus (L.), in the laboratory and in potted citrus. In the laboratory bioassay, D. abbreviatus larvae were exposed to 30, 60, and 120 nematodes/cm³ in sand. Insect mortality 1 week after application was greater (P ≤ 0.05) for S. riobravis than for S. carpocapsae in the laboratory bioassay. In the greenhouse bioassay, D. abbreviatus larvae were exposed to 3 and 9 nematodes per cm³ of soil in potted citrus. Again, at each rate, mortality was greater (P ≤ 0.05) in pots treated with S. riobravis than in pots treated with S. carpocapsae. The results of this study suggest that S. riobravis is a better biological control agent against D. abbreviatus larvae in potted plants than S. carpocapsae.     

New entomopathogenic nematodes from semi-natural and small-holder farming habitats of Rwanda

Five field surveys for indigenous entomopathogenic nematodes (EPNs) were conducted in 22 semi-natural and 17 small-holder farming habitats across 16 districts of different altitudes in the northern, eastern, southern and Kigali city provinces of Rwanda. In 2014, 216 mixed soil samples were collected and subsamples thereof baited with Galleria mellonella or Tenebrio molitor larvae. Five samples from five locations and habitats were positive for nematodes (2.8%). Nine nematode species/strains were isolated and five successfully maintained. DNA sequence comparisons and morphological examinations revealed Steinernema carpocapsae, Heterorhabditis bacteriophora, as well as two steinernematids and one heterorhabditid with no species designation. The isolates (strains) were named Steinernema sp. RW14-M-C2a-3, Steinernema sp. RW14-M-C2b-1, Steinernema carpocapsae RW14-G-R3a-2, H. bacteriophora RW14-N-C4a and Heterorhabditis sp. RW14-K-Ca. These are the first records of naturally occurring EPNs in Rwanda. It is also the first record of S. carpocapsae from Africa. Finding H. bacteriophora from tropical rather than temperate Africa was surprising. The found nematodes will serve as the basis for efficacy screening, and for mass production in a biocontrol agent factory at Rubona Research Centre of the Rwanda Agriculture Board with the ultimate aim of delivering effective, safe and environmentally benign pest control for soil-inhabiting pests.     

Evaluation of entomopathogenic nematodes for suppression of carrot weevil

The potential of entomopathogenic nematodes as biologicalcontrol agents for carrot weevil (Listronotus oregonensis) was evaluated throughboth laboratory and field experiments. In thelaboratory, Steinernema carpocapsae, S. riobrave, S. feltiae, Heterorhabditis megidis, H. bacteriophora, and a control (water only) werecompared in sand and muck soil against adults,and in sand against larvae. All nematodespecies produced high levels of larvalmortality. S. carpocapsae producedsignificantly greater adult mortality in sandthan other species or the untreated control. H. bacteriophora caused low adultmortality in sand, but the greatest adultmortality among treatments in a similar testthat used muck soil; S. carpocapsae wasranked second on muck soil. Other speciesconsistently produced intermediate (H.megidis and S. riobrave) or low (S.feltiae) levels of mortality on bothsubstrates. In the field, we compared theeffect of early season vs. late seasonapplications of H. bacteriophora or S. carpocapsae on carrot weevil mortality andparsley survival and yield. Significantdifferences among treatments in plant survivaland yield were not found; however treatmentsinvolving H. bacteriophora had higherplant survival than other treatments. Earlierapplication of this species was associated withhigher plant survival. S. carpocapsaetreatments had similar plant survival to thecontrol. Mortality of larvae and combinedstages of carrot weevil was significantlygreater at 1 week following H.bacteriophora application than for othertreatments. H. bacteriophora also showedgreater persistence than S. carpocapsaein treated plots. We conclude that H.bacteriophora is a good candidate for furtherevaluation as a biological control agentagainst carrot weevil on muck soils in theGreat Lakes region.     

Influence of cell density and phase variants of bacterial symbionts (Xenorhabdus spp.) on dauer juvenile recovery and development of biocontrol nematodes Steinernema carpocapsae and S. feltiae (Nematoda: Rhabditida)

The rhabditid nematodes Steinernema carpocapsae and Steinernema feltiae are used in biological control of insect pests. Mass production is done in liquid culture media pre-incubated with their bacterial symbionts Xenorhabdus nematophila and Xenorhabdus bovienii, respectively, before nematode dauer juveniles (DJs) are inoculated. As a response to food signals produced by the bacterial symbionts, the DJs exit from the developmentally arrested dauer stage (they recover development) and grow to adults, which produce DJ offspring. Variable DJ recovery after inoculation often causes process failure due to non-synchronous population development and low numbers of adult nematodes. This contribution investigated the influence of the bacterial cell density on DJ recovery and development to adults. At higher density of 10¹⁰ bacterial cells ml⁻¹, a higher percentage of DJ recovery was induced, and adults occurred earlier in both Steinernema spp. than at lower density of 10⁹ and 10⁸ cells ml⁻¹. Xenorhabdus symbionts produce phase variants. Recovery in bacteria-free supernatants was lower than in supernatants containing bacterial cells for both primary and secondary phase Xenorhabdus spp. and lower in secondary than in primary phase supernatants or cell suspensions. In general, recovery was lower for Steinernema feltiae and the time at which 50% of the population had recovered after exposure to the food signal was longer (RT₅₀ = 17.1 h) than for Steinernema carpocapsae (RT₅₀ = 6.6 h). Whereas >90% S. carpocapsae DJs recovered in hemolymph serum of the lepidopteran insect Galleria mellonella, recovery of S. feltiae only reached 31%. Penetration into a host insect prior to exposure to the insect’s food signal did not enhance DJ recovery. Consequences for liquid culture mass production of the nematodes and differences between species of the genera Steinernema and Heterorhabditis are discussed.     

A mechanism of acquired resistance against an entomopathogenic nematode by Agrotis ipsilon feeding on perennial ryegrass harboring a fungal endophyte

Perennial ryegrass forms a symbiotic relationship with the fungus Neotyphodium lolii, which provides many benefits including resistance to herbivory through the production of alkaloids. The impact of endophytic grass on the third trophic level has received little attention. The black cutworm, Agrotis ipsilon, is less susceptible to the entomopathogenic nematode, Steinernema carpocapsae, when it consumes the endophytic grass. We examined the potential mechanisms of the resistance exhibited by A. ipsilon against S. carpocapsae. Although A. ipsilon larvae fed on endophytic grass had similar numbers of nematodes attached and that successfully developed into adults, they had significantly lower mortality than larvae fed on endophyte-free grass when exposed to nematodes for 1.5 h. We examined the effects of N. lolii produced ergot alkaloids, ergotamine tartrate, ergonovine maleate, ergocryptine, and erogcristine on nematode viability and infectivity. Ergonovine malate increased and ergocristine decreased the rates of nematode infectivity, whereas other treatments had no significant effect. We also investigated the effects of ergocristine on Xenorhabdus nematophila, the symbiotic bacterium of S. carpocapsae. Bacterial growth and pathogenicity were significantly reduced when the bacterium was grown in nutrient broth containing 200 μg/ml concentration of ergocristine. We conclude that herbivores capable of developing on endophytic grasses may acquire some level of resistance against S. carpocapsae due to the toxic effects of ergocristine on the bacterium, X. nematophila. Our results underscore the ability of N. lolii to affect trophic interactions through the production of alkaloids.     

Entomopathogenic nematodes control Plum Sawflies (Hoplocampa minuta and H. flava)

Plum sawflies are among the most damaging pests of European plum. Current control strategy implies insecticide application. Three species of entomopathogenic nematodes (EPN), Steinernema feltiae Filipjev, S. carpocapsae Weiser and Heterorhabditis bacteriophora Poinar were tested under laboratory and field conditions to assess effectiveness against larval and adult stages. Laboratory tests resulted in up to 100% mortality of last instar larvae before construction of a cocoon. However, the nematodes were not able to penetrate the cocoon. Foliar application did not result in plum sawflies larvae infestation by EPNs. Under field conditions, the nematodes reduced the number of emerging adults by application against sawfly larvae in the previous year before migration into the soil for overwintering by 62%–92%. Application of the nematodes against adults just before their anticipated emergence resulted in reduction of fruit infestation up to 100%. Mean results of 5 trials using caged trees were 47.8% with S. feltiae, 56.3% with S. carpocapsae and 62.9% with H. bacteriophora. In open field trails, control of adults obtained with S. feltiae at 0.5 million nematodes/m² was 98.2 and 67.8% and at 0.25 million m⁻² 41.7 and 41.2%. Forecasting adult emergence and optimal soil moisture conditions are essential for success of the nematode application.     

Effect of temperature on the development of <Emphasis Type="Italic">Steinernema carpocapsae</Emphasis> and <Emphasis Type="Italic">Steinernema feltiae</Emphasis> (Nematoda: Rhabditida) in liquid culture

For commercial use of the entomopathogenic nematodes Steinernema carpocapsae and Steinernema feltiae in biological control of insect pests, they are produced in liquid culture on artificial media pre-incubated with their symbiotic bacteria Xenorhabdus nematophila and Xenorhabdus bovienii, respectively. After 1 day of the bacterial culture, nematode dauer juveniles (DJs) are inoculated, which recover development. The adult nematodes produce DJ offspring, which are harvested and can be sprayed. This study determined optimal temperatures to obtain high DJ progeny within a short process time. Temperatures assessed were 23°C, 25°C, 27°C, and 29°C for S. carpocapsae and 20°C, 23°C, 25°C, and 27°C for S. feltiae. The recovery of inoculated DJs was hardly affected and was reduced only in S. carpocapsae at 29°C. The fecundity (eggs in uterus) in S. carpocapsae reached a maximum at 27°C; whereas, maximum yields were recorded at 25°C. For both Steinernema spp., highest DJ densities were obtained after 15 days incubation at 25°C. Optimal culture temperature for both nematode species is 25°C. S. carpocapsae was more sensible to suboptimal temperature than S. feltiae. Results on total DJ density and DJ proportion of the total nematode population were more variable at non-optimal temperature condition for S. carpocapsae than for S. feltiae. Suboptimal culture temperature also reduced DJ infectivity.     

The potential use of entomopathogenic nematodesagainst Typhaea stercorea

Four entomopathogenic nematode species, Steinernema carpocapsae, S. feltiae, Heterorhabditis bacteriophoraand H. megidis, were tested in a petri dish assay against larvae and adults of the hairy fungus beetle Typhaea stercorea. In general, adults were less susceptible than larvae and the LC₅₀ decreased with the duration of the exposure to nematodes. S. carpocapsae was the most effective species against adult beetles (LC₅₀ after 96 hours exposure =67 nematodes/adult). Against larvae S.carpocapsae and H. megidis were comparablyeffective with an LC₅₀ of 30 and 55nematodes/larvae, respectively. S. carpocapsaewas tested at 70 and 100% RH against adults in baits of either chicken feed or crushed wheat, both supplemented with horticultural capillary matting pieces in order to obtain a wet weight of 50–60%. At70% RH no significant effect of the nematodes was obtained due to desiccation of the bait. In chickenfeed at 100% RH the mortality reached 80% with 500nematodes/adult. In wheat significant mortality was obtained only at 5000 nematodes/adult. Heavy growth of mould probably limited the nematode infection. When the bait was used in tube traps, desiccation and growth of mould was prevented, but nematode efficacy dropped to 4.4% in the traps and 12% in the surrounding litter. This revised version was published online in July 2006 with corrections to the Cover Date.