Evaluation of the efficacy of the entomopathogenic nematodes Heterorhabditis sp. against sap beetles (Coleoptera: Nitidulidae)

Absract Nitidulid beetles (Coleoptera) are considered serious pests of date palms throughout the world. They attack the ripe fruit, causing it to rot, and damage is reflected in both reduced yield and lower fruit quality. Previous studies demonstrated the susceptibility of larvae of this pest to entomopathogenic nematodes from the genus Heterorhabditidis. In the present study nematode efficacy was evaluated in greenhouse and field. In containers filled with soil, moderate reduction in insect emergence was achieved when the nematodes were applied at concentrations of 25 and 50 IJs/cm². However, the highest concentration (100 IJs/cm²) treatment resulted in a drastic reduction (by 70–90%) in emergence of the beetles. The lowest emergence was achieved by the IS-19 and IS-21 strains (>10%). Efficacy of the IS-19 strain was retained up to 7 days after application at a rate of 100 IJs/cm². When the insect larvae were introduced to the soil 2 weeks after nematode application, the percentage emergence of insects increased by 2–2.5 fold as compared to previous introductions but was still lower than in the control. Insect density per container did not have an effect on efficacy of the nematodes when the strains IS-19 and IS-12 were used. Two field trials were conducted in different sites in Israel. In the first trail, conducted in date palm orchard, four strains of Heterorhabditis sp. were tested. No significant difference in insect emergence was recorded among the various treatments or the control. Whereas in the second trial conducted in a fig orchard, substantial reduction (by 50–70%) in insect emergence was recorded following nematode treatment. Further studies, under natural conditions, are needed to optimize application efficiency and evaluate the commercial utilization of these biological control agents.     

Comparative effects of Steinernema feltiae (Nematoda: Steinernematidae) and insecticides on yield and cropping of the mushroom Agaricus bisporus

In a mushroom crop (Agaricus bisporus) affected by a very low level of sciarid fly (Lycoriella auripila) infestation, the effects of an indigenous isolate of insect-parasitic nematode (Steinernema feltiae) and of two commonly used insecticides (diazinon and diflubenzuron) were studied. When compared with untreated plots, nematodes applied to the casing had no adverse effects on mushroom yields whereas insecticides decreased yields. At a rate of 3 × 10⁶ infective juveniles per tray (surface area = 0.56 m²), S. feltiae elicited increases of 28.5% and 19% in the mean total numbers and weights of mushrooms respectively. Treatment only with diflubenzuron resulted in 14.6% and 6% reductions in mean total numbers and weights of mushrooms, respectively; treatment with both diazinon and diflubenzuron caused 18.5% and 9.4% losses. Application of nematodes generally reduced the mean weight per mushroom whereas insecticides increased it; nematodes delayed the onset of mushroom production (first flush) whereas diflubenzuron delayed the third and fourth flushes. Nematode contamination of sporophores was minimal when S. feltiae was applied at casing. Although their numbers declined with time, the nematodes persisted, in the casing layer, throughout the cropping period of seven weeks. It is concluded that yield benefits associated with nematode application can result mainly from nematode effects on A. bisporus and not solely from suppression of a damaging pest population.     

Effects of Application Rates of Steinernema feltiae (Nematoda: Steinernematidae) on Biological Control of the Mushroom Fly Lycoriella auripila (Diptera: Sciaridae)

The potential of Steinernema feltiae for the biological control of Lycoriella auripila was tested in commercial mushroom‐growing conditions. The nematodes, applied at rates of 1.5, 3, 6 or 12 x 10 ⁶ infective juveniles per 34 kg tray of spawn‐run compost, were mixed into the casing material before it was spread over the compost surface. When compared with untreated control trays, any rate of nematode application significantly reduced fly emergence. Insecticides significantly reduced mushroom yields; nematodes significantly increased them. At a rate of 3 x 10 ⁶ infectives/tray S. feltiae elicited mean total increases in the weight and numbers of mushrooms produced of 8% and 11% respectively. The nematodes also reduced the incidence of mushrooms spoiled by tunnelling sciarid larvae. The early decline in the numbers of nematodes persisting in casing was a trend that was reversed later, when evidence was obtained that S. feltiae was recycling in insects that had been killed. When applied at a rate of 3 ‐10⁶infectives/tray of compost S. feltiae should provide reliable and cost‐effective biological control of L. auripila.     

Effect of entomopathogenic nematodes on the fitness cost of resistance to Bt toxin crylac in pink bollworm (Lepidoptera: Gelechiidae)

The widespread use of crop plants genetically engineered to produce toxins from the bacterium Bacillus thuringiensis (Bt) imposes selection on insect populations to evolve resistance. The pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), is a major pest of cotton in the southwestern United States that is currently controlled with transgenic cotton that produces Bt toxin Cry1Ac. Previously reported theoretical work suggests that, in conjunction with a high dose/refuge strategy, fitness costs of Bt resistance can slow or prevent the evolution of resistance. We report here that the entomopathogenic nematode Steinernema riobrave (Rhabditida: Steinernematidae) increased the fitness cost of resistance to Cry1Ac in P. gossypiella. Mortality of P. gossypiella from fourth instar to adult eclosion was significantly higher for a Bt-resistant strain than a susceptible strain in tests with two to 14 infective juveniles of S. riobrave per larva, but it did not differ between strains when nematodes were absent. Nematodes established in P. gossypiella larvae at all concentrations tested, and nematode reproduction in infected P. gossypiella larvae occurred at nematode concentrations of four to 14 infective juveniles per larva. Our results suggest that incorporation of entomopathogenic nematodes into an integrated resistance management strategy could help to delay pest resistance to Bt toxins.     

Evaluation of a genetically selected strain of Steinernema feltiae against the mushroom sciarid Lycoriella mali

The control potential of a strain (ScP) of Steinernema feltiae genetically selected against the mushroom sciarid Lycoriella solani, was evaluated for the management of L. mali. Trials were conducted at two commercial mushroom farms with high and low levels of fly infestation. The efficacy of the ScP strain was compared with the SN strain of S. feltiae and the chitin synthesis inhibitor, diflubenzuron. At low densities of L. mali, the two strains did not differ in efficacy and caused 85–94% reduction in fly populations. At high fly densities with a mixed infestation of the phorid fly, Megaselia halterata, the ScP stain caused 56–83% reduction in L. mali populations whereas the SN strain caused 51–73% reduction. Two doses of the improved strain (0.5 times 10⁶ and 1.0 times 10⁶ infective juveniles per m² cropping area) did not differ in efficacy from diflubenzuron which caused 80% reduction in L. mali populations. The lower dose (0.5 times 10⁶) of the SN strain was less effective than diflubenzuron. The ScP strain had a major advantage over the SN strain in persistence in casing material. Nematodes had no significant effect on mushroom yields.     

Field efficacy of entomopathogenic nematodes against the beetle Maladera matrida (Coleoptera: Scarabaeidae)

Single, double and triple releases of the entomopathogenic nematode Heterorhabditis bacteriophora Poinar, reduced the population of the beetle Maladera matrida Argaman, infesting peanuts (’Shulamit’ cv.) by 70, 75 and 93% respectively in microplot tests. Simultaneous and late (2 weeks after infestation) applications reduced beetle numbers by 63 and 79% respectively, in the microplots, while early application (2 weeks prior to infestation) did not reduce the beetle population. In a field trial, reductions in insect population and damage to the crop were achieved by early treatment with the nematode as well as by Heptachlor, leading to reductions in the insect population of 60 and 90% respectively, when recorded 4 weeks after nematode application. However, the nematode treatment did not maintain its effectiveness for a longer period and pest damage increased to the same level as the untreated control after 7 weeks. When the nematodes were applied at different concentrations (0.25–1.0 x 10⁶ infective juveniles (IJs) m^‐2) their effectiveness was not related to the concentration level. The only significant (P < 0.05) reduction in insect levels was recorded in the treatment with 0.5 X 10⁶ IJs m^‐2. In a second field trial, both H. bacteriophora and Steinernema glaseri reduced insect populations significantly (P < 0.05) by approximately 50% in comparison to the control. In the third trial, treatment with H. bacteriophora resulted in a decrease in insect population of 90% while treatment with S. carpocapsae reduced the grub numbers by 40% in comparison to the control. A differential susceptibility of various grub developmental stages was recorded in the field. The small grubs (I‐4 mm long, lst‐2nd larval stage) were not affected by the nematode treatments while the numbers of medium and large size grubs were reduced by 2‐ and 3‐fold respectively in the various tests. Nematodes were recovered by ‘nematode traps’ containing Galleria mellonella larvae from treated field plots 78 days after application. The implications of the results from the present studies on the use of entomopathogenic nematodes are discussed in relation to the development of an integrated pest management programme.     

Evaluation of Entomopathogenic Nematodes for the Control of Mediterranean Fruit Fly (Diptera: Tephritidae)

The virulence of various entomopathogenic nematode (EPN) strains was evaluated against the Mediterranean fruit fly, C. capitata . The selected nematodes were assessed for their infectivity for the final larval stage of the insect host and under varying environmental conditions. Among 12 EPN strains tested, Steinernema riobrave Texas ( Sr TX) and Heterorhabditis sp. IS-5 (H IS-5), showed high activity and induced >80% mortality. Six EPN strains showed limited activity (>30% mortality), and four strains had no effect (<20% mortality). Sr TX was more effective than H IS-5. Mature C. capitata larvae were most susceptible to nematode infection during the first 4h after they began to emerge from their diet to pupate. Activity of the two nematode strains at a constant inoculation rate was dependent on insect larval density. The highest activity was recorded at 1.88 larvae cm -2 and decreased at higher larval densities. EPN activity was also directly related to nematode density. Maximal activity was shown at a density of 150 infective juveniles cm -2 . A similar activity pattern was also recorded with Sr TX in four different soil types. The persistence of this EPN in the soil extended over 5 days but there was no activity after 14 days. Except for a lower activity under cool conditions (17°C), temperatures ranging between 22 and 41°C, or moisture levels in the treated soil ranging between 3 and 20%, had no significant effect on nematode activity. Our results suggest that application of Sr TX against C. capitata may have potential for controlling C. capitata .     

Susceptibility of Sap Beetles (Coleoptera: Nitidulidae) to Entomopathogenic Nematodes

Nitidulid beetles (Coleoptera) are considered to be serious pests of date palms throughout the world. They attack ripe fruit, causing it to rot, and damage is reflected in both reduced yield and lower fruit quality. The present study was aimed at an evaluation of the susceptibility to different sap beetles to entomopathogenic nematodes. We further tested nematode efficacy in pots filled with soil infested by third instar larvae of the two beetle species. In Petri dish assay, mortality levels of Carpophilus humeralis and C. hemipterus exposed to Heterorhabditis sp. IS-5 strain indicated that the latter is less susceptible to nematode infection. Exposure of both sap beetle species to different nematode strains gave moderate levels of mortality (35-65%) with the heterorhabditid strains HP88, IS-5 and IS-25. The IS-12 strain of Heterorhabditis sp. showed poor virulence (<35% mortality) against larvae of C. humeralis as well as larvae and pupae of C. humipterus. The nematode species S. riobrave showed moderate virulence (35-65%) mortality to larvae and pupae of S. humeralis as well as to larvae of C. hemipterus . Exposure of C. hemeralis to different concentrations of Heterorhabditis sp. IS-5 in pots containing soil resulted in high mortality (>65%). In contrast, the lower concentrations (500 and 1000 nematodes/pot) caused low mortality (35%) of C. hemipterus . Other heterorhabditid strains caused 95-100% mortality of C. humeralis in pot assay. The HP88 strain of H. bacteriophora and the Tx strain of Steinernema riobrave showed poor effectiveness. Incubation of different nematode strains with the C. humeralis larvae at high temperature (32 C) resulted in an increase in insect mortality with the IS-12 and IS-21 strains. Reduced mortality was recorded with the HP88 strain treatment at the higher temperature. The IS-5 and IS-12 strains were equally effective in all three soil types tested, whereas the IS-19 strain was more effective in the Almog type soil than in the others.     

Infection of Anastrepha ludens following soil applications of Heterorhabditis bacteriophora in a mango orchard

To determine the efficacy of Heterorhabditis bacteriophora Poinar (Nematoda: Heterorhabditidae) for control of Anastrepha ludens (Loew) (Diptera: Tephritidae), field experiments were performed in a mango orchard with soil temperatures of 24–29 °C. The density of third‐instar A. ludens (50–500 larvae per plot) released into 0.25 m² wood‐framed experimental plots containing soil (16% wt/wt moisture) previously treated with 125 infective juveniles per square centimetre soil surface did not significantly influence the prevalence of infection by H. bacteriophora. In subsequent experiments, the percentages of infection of fly pupae were positively correlated with the concentration of infective stages applied to soil plots. The highest average percentage of infection (74% at 250 infective juveniles per square centimetre) was observed when fly larvae were released simultaneously onto soil, compared to larvae that emerged from laboratory‐infested mangoes over a period of 8 days (52% infection at 500 infective juveniles per square centimetre). Double applications of infective juveniles at an interval of 4 days did not greatly improve the prevalence of infection (～10% higher) compared to single applications. Between 9 and 15% of larvae that remained within infested mangoes became infected by nematodes, irrespective of the concentration of nematodes applied to each experimental plot. We conclude that effective control of A. ludens requires very high densities of H. bacteriophora. The successful use of this nematode for biocontrol of A. ludens will depend on identifying ways of overcoming the fly's ability to avoid infection.     

Prey preference and life tables of the predatory mite Parasitus bituberosus (Acari: Parasitidae) when offered various prey combinations

Parasitus bituberosus Karg (Acari: Parasitidae) is one of the predatory mite species inhabiting mushroom houses. It is known to accept a wide range of prey, suggesting that it may be a promising candidate for the biological control of key pests of mushroom culture. In our study it did not show any prey preference among four groups of small organisms often occurring in mushroom growth medium, namely rhabditid nematodes, pygmephorid mites, and sciarid and phorid fly larvae. Nevertheless, the type of food these predators fed on affects their development. The shortest egg-to-adult development time was obtained on a nematode diet. On a diet of phorid larvae, mite development stopped at the deutonymph stage; none reached adulthood. All other diets sufficed to reach the adult phase. Female fecundity when fed nematodes and sciarid larvae did not differ, but it was much lower when fed pygmephorid mites. Other life table parameters confirmed that pygmephorid mites constituted the worst diet for P. bituberosus. The highest intrinsic rate of population increase (r _m  = 0.34) was obtained on the nematode diet; when fed sciarid larvae and pygmephorid mites it was 0.25 and 0.14, respectively. Our study provides good reasons to further test P. bituberosus as biocontrol agent of especially sciarid flies and nematodes, especially when the compost is well colonized by mushroom mycelium (which retards nematode growth).     

Factors affecting entomophilic activity of Neoaplectana feltiae in mushroom compost

The host-searching ability of Neoaplectana feltiae Filipjev (= S. bibionis Bovien) (Nematoda: Steinernematidae) in response to larvae of a mushroom fly, Lycoriella solani Winn. was examined in a mushroom substrate. Individuals of L. solani were less attractive for the parasite than larvae of Galleria mellonella L. The nematode juveniles penetrated a 22 cm layer of casing mixture within 2–4 days. In the casing alone nematode effectiveness was better than in mushroom compost or in compost and casing together. In the casing mixture parasite dosages of 20 and 100 juveniles per cm² led to 22% and 45% parasitization of L. solani respectively, while all G. mellonella larvae were parasitized at both dosages. The prevalence of nematode infection depended on the content of water in the mushroom substrate. The highest N. feltiae infectivity was observed, when the ratio of the dry casing weight to the weight of water content was 1: 2.5. The practical aspects of the observed phenomena, essential for the use of N. feltiae in the protection of commercial mushroom cultivation are discussed.     

Pathogenicity of Heterorhabditis nematodes isolated from north-western Himalaya against the larvae of Plutella xylostella (L.) (Lepidoptera: Plutellidae)

The efficacy of three entomopathogenic nematodes (Heterorhabditis spp.), from north western Himalaya, India was studied against the diamondback moth, Plutella xylostella (Linnaeus, 1758) (Lepidoptera: Plutellidae), under laboratory conditions. The larvae were exposed to 10, 20, 30 and 40 infective juveniles (IJs) of each nematode species for different time periods and they were found to be susceptible to all the EPNs tested. However, the susceptibility of larvae to nematode infection varied according to the dosages of IJs and their exposure periods. The efficacy of these indigenous entomopathogenic nematodes was also evaluated against the commercially available entomopathogenic nematode H. indica. An indigenous isolate, H. bacteriophora (HRJ), along with the commercial isolate H. indica recorded 100.0% mortality of insect larvae in 96 h exposure time against third instar larvae of P. xylostella. However, it was noticed that with the advancement of larval stage its mortality rate reduces and vice versa with the exposure period. All the tested nematode species were also found to reproduce within the host and produced infective juveniles. In conclusion, the evidence obtained in this study suggests that all the three indigenous EPN species are virulent enough to produce 100% mortality of larvae of P. xylostella. These EPN species thus have potential for the management of P. xylostella under integrated management practices.     

Biological control potentials of insect-parasitic nematode Rhabditis blumi (Nematoda: Rhabditida) for major cruciferous vegetable insect pests

Pathogenicity of Rhabditis blumi Sudhaus against major cruciferous insect pests was evaluated in the lab and greenhouse. In Petri-dish tests against the insects, including Artogeia rapae L., Mamestra brassicae L., and Plutella xylostella L., insect mortality by R. blumi and its associated bacteria was dose and time dependent, which increased with dose (0?C80 dauer juveniles/larva) and time increments. Pathogenicity against fourth-instar larvae was higher than the rate of corresponding third-instar larvae. The highest insect mortality rate was observed in fourth-instar larvae of P. xylostella, followed by A. rapae, and M. brassicae, with mortality rates of 93.5, 88.2, and 77.8?%, respectively. Lethal dose values at 50?% (LD₅₀) of R. blumi were 25.7 dauer juveniles/larva on P. xylostella; 28.0 dauer juveniles/larva on A. rapae; and 40.6 dauer juveniles/larva on M. brassicae, respectively. In greenhouse tests, P. xylostella larvae were most susceptible to nematodes, with insect reduction rate of 88.0?%. The rate varied with vegetable species and persistence time of live nematodes on vegetable leaves after spraying. Nematodes established in cadavers showed positive correlation with nematode dose, whereas nematode persistence on the leaf was inversely related to hours after treatment.     

Biocontrol potential of the entomopathogenic nematodes Heterorhabditis bacteriophora and Steinernema carpocapsae on cucurbit fly,Dacus ciliatus (Diptera: Tephritidae)

Entomopathogenic nematodes (EPNs) from the families Steinernematidae and Hererorhabditidae are considered excellent biological control agents against many insects that damage the roots of crops. In a regional survey, native EPNs were isolated, and laboratory and greenhouse experiments were conducted to determine the infectivity of EPNs against the cucurbit fly, Dacus ciliatus Loew (Diptera: Tephritidae). Preliminary experiments showed high virulence by a native strain of Heterorhabditis bacteriophora Poinar (Rhabditida: Heterorhabditidae) and a commercial strain of Steinernema carpocapsae Weiser (Rhabditida: Steinernematidae). These two strains were employed for further analysis while another native species, Steinernema feltiae, was excluded due to low virulence. In laboratory experiments, larvae and adult flies were susceptible to nematode infection, but both nematode species induced low mortality on pupae. S. carpocapsae had a significantly lower LC₅₀ value against larvae than H. bacteriophora in filter paper assays. Both species of EPNs were effective against adult flies but S. carpocapsae caused higher adult mortality. When EPN species were applied to naturally infested fruit (150 and 300 IJs/cm²), the mortality rates of D. ciliatus larvae were 28% for S. carpocapsae and 12% for H. bacteriophora. Both EPN strains successfully reproduced and emerged from larvae of D. ciliates. In a greenhouse experiment, H. bacteriophora and S. carpocapsae had similar effects on fly larvae. Higher rates of larval mortality were observed in sandy loam and sand soils than in clay loam. The efficacy of S. carpocapsae and H. bacteriophora was higher at 25 and 30°C than at 19°C. The results indicated that S. carpocapsae had the best potential as a biocontrol agent of D. ciliatus, based on its higher virulence and better ability to locate the fly larvae within infected fruits.     

Comparative Assessment of biological (Nematoda: Steinernema feltiae) and chemical methods of control for the mushroom fly Lycoriella auripila (Diptera: Sciaridae)

The sciarid fly Lycoriella auripila is the major pest of mushrooms cultivated in the UK. Its larvae, which are capable of damaging the crop at all stages of production, may cause severe yield losses and can only be controlled with chemical pesticides. An indigenous isolate of the insect‐parasitic nematode Steinemema feltiae was tested as a biological control agent and its effects compared with two commonly used insecticides, diazinon and diflubenzuron. The timing of application of nematodes was found to affect their efficacy. When applied to compost during spawning, nematodes did not significantly reduce fly emergence, but they did if applied at casing when they were almost as effective as diflubenzuron. Diazinon incorporated into compost did not reduce fly emergence and was also the only treatment that did not lower the incidence of mushrooms spoiled by tunnelling of the larvae of L. auripila. When compared with untreated control plots those treated both with diazinon and diflubenzuron showed significant mean losses in yield of 10% in total weight and 17% in total numbers of mushrooms picked. In contrast, when S. feltiae was applied at casing significant mean increases in yield of 7% and 19%, respectively, were attained. Infective nematodes persisted well in casing, very few were found on sporophores.     

Biological control potential of native entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) against Spodoptera cilium (Lepidoptera: Noctuidae) in turfgrass

In laboratory studies, we demonstrated that five native entomopathogenic nematode species/isolates caused 100% mortality of Spodoptera cilium larvae, a soil surface-feeding pest of turfgrass. At 25 infective juveniles/cm² applied to sod, two selected Turkish species, Steinernema carpocapsae and Heterorhabditis bacteriophora (Sarigerme isolate), averaged 77% and 29% larval mortality, respectively.     

Screening Spanish isolates of steinernematid nematodes for use as biological control agents through laboratory and greenhouse microcosm studies

Entomopathogenic nematodes (EPNs) are one of the best non-chemical alternatives for insect pest control, with native EPN strains that are adapted to local conditions considered to be ideal candidates for regional biological control programs. Virulence screening of 17 native Mediterranean EPN strains was performed to select the most promising strain for regional insect pest control. Steinernema feltiae (Filipjev) (Rhabditida: Steinernematidae) Rioja strain produced 7%, 91% and 33% larval mortality for the insects Agriotes sordidus (Illiger) (Coleoptera: Elateridae), Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) and Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), respectively, and was selected as the most promising strain. The S. feltiae Rioja strain-S. littoralis combination was considered the most suitable to develop the Rioja strain as a biocontrol agent for soil applications. The effect of soil texture on the virulence of the Rioja strain against S. littoralis was determined through dose-response experiments. The estimated LC₉₀ to kill larvae in two days was 220, 753 and 4178 IJs/cm² for soils with a clay content of 5%, 14% and 24%, respectively, which indicates that heavy soils produced negative effects on the virulence of the Rioja strain. The nematode dose corresponding to the LC₉₀ for soils with a 5% and 14% clay content reduced insect damage to Capsicum annuum Linnaeus (Solanales: Solanaceae) plants under greenhouse microcosm conditions. The results of this research suggest that an accurate characterization of new EPN strains to select the most suitable combination of insect, nematode and soil texture might provide valuable data to obtain successful biological control under different ecological scenarios in future field applications.     

Investigations on the parasitism ofOtiorrhynchus salicicola andO. Sulcatus [Col.: Curculionidae] byHeterorhabditis sp. [Nematoda]

Experiments were conducted to study the efficacy of the insect parasitic nematodeHeterorhabditis sp. (HW79) as a biological control agent ofOtiorrhynchus salicicola. This weevil species is reported as a pest of ornamental plants in Switzerland and Italy. Dipping plastic boxes containing heavily infested cuttings of laurel (Prunus laurocerasus) in a nematode suspension resulted in approximately 100% parasitisation of full-grown larvae, pupae and non-emerged young adults. The average dose resulting from dipping varied between 56,000 and 62,000 nematodes per liter soil. This experiment was run under natural outdoor conditions. In a further outdoor experiment, pottedLigustrum plants were inoculated with eggs ofO. salicicola and later 20,000 infective juvenile nematodes per liter soil were added to the soil surface. The resulting weevil mortality in the treated pots was 78%. In seven greenhouse tests using the same nematode dose in pots with horticultural soil to which weevil larvae had been added, weevil mortality varied between 76% and 100%, the arithmetic average being 90%. These results indicate that Heterorhabditid nematodes may provide an effective means of controllingO. salicicola. In an other experiment usingO. sulcatus larvae, the influence of application time on nematode efficacy was investigated. When nematodes were added a few days before weevil larvae had hatched from the eggs, no parasitic effect was obtained. Nematode applications done shortly after larval hatching however, resulted in complete weevil control. These results are of significance in timing nematode applications in practice.      

Susceptibility of the Plum Curculio,Conotrachelus nenuphar,to Entomopathogenic Nematodes

The plum curculio, Conotrachelus nenuphar, is a major pest of pome and stone fruit. Our objective was to determine virulence and reproductive potential of six commercially available nematode species in C. nenuphar larvae and adults. Nematodes tested were Heterorhabditis bacteriophora (Hb strain), H. marelatus (Point Reyes strains), H. megidis (UK211 strain), Steinernema riobrave (355 strain), S. carpocapsae (All strain), and S. feltiae (SN strain). Survival of C. nenuphar larvae treated with S. feltiae and S. riobrave, and survival of adults treated with S. carpocapsae and S. riobrave, was reduced relative to non-treated insects. Other nematode treatments were not different from the control. Conotrachelus nenuphar larvae were more susceptible to S. feltiae infection than were adults, but for other nematode species there was no significant insect-stage effect. Reproduction in C. nenuphar was greatest for H. marelatus, which produced approximately 10,000 nematodes in larvae and 5,500 in adults. Other nematodes produced approximately 1,000 to 3,700 infective juveniles per C. nenuphar with no significant differences among nematode species or insect stages. We conclude that S. carpocapsae or S. riobrave appears to have the most potential for controlling adults, whereas S. feltiae or S. riobrave appears to have the most potential for larval control.     

Influences of host size and host species on theinfectivity and development of Heterorhabditismegidis (strain NLH-E87.3)

The infectivity, time to first emergence of infective juveniles (IJs), total number of IJs per insect and IJs body length of the entomopathogenic nematode Heterorhabditis megidis (strain NLH-E87.3) after development in larvae of two insect hosts, Galleria mellonella (greater wax moth) and Otiorhynchus sulcatus (vine weevil) was studied. At a dose of 30 IJs, larvae of G. mellonella show to be significantly more susceptible than O. sulcatus larvae. At a dose of one IJ, vine weevil larvae were more susceptible. The number of invading infective juveniles (IJs) increased with host size while the host mortality at a dose of one IJ decreased with the increase of host size. Time to first emergence was longer at a dose of one IJ per larva and increased with the increase of host size in both insect species. Reproduction of IJs differed between host species, host sizes and doses of nematodes. Generally, the IJs body size increased with an increasing host size. The longest infective juveniles were produced at the lowest IJ doses. Results are discussed in relation to the influence of different host species and their different sizes on the performance of H. megidis (strain NLH-E87.3) as a biological control agent.