Infectivity Of Entomopathogenic Nematodes Research Articles

Evaluation of native entomopathogenic nematodes for the control of the European cherry fruit fly Rhagoletis cerasi L. (Diptera: Tephritidae) larvae in soil

Turkey is the world's largest producer of sweet cherries. The European cherry fruit fly, Rhagoletis cerasi L. (Diptera: Tephritidae), is a major pest of sweet cherry crops in Europe and Turkey. The detection of only one larva in the fruit by the processor can result in the rejection of the entire crop of that orchard and/or farm as \wormy and unmarketable. The main control tactic for R. cerasi is to prevent the females from laying eggs in the fruit. Currently, only a few insecticides are being used, and their application is debatable due to problems with residual ecotoxicity for humans and beneficial organisms. As an alternative to using insecticides for the reduction of adult populations, 4 indigenous entomopathogenic nematode species, Steinernema carpocapsae (Anamur isolate), S. feltiae (Rize isolate), Heterorhabditis bacteriophora (Ekecik isolate), and H. marelatus (Ankara isolate), were bioassayed against last-instar R. cerasi larvae at different temperatures (10, 15, and 25 °C) and nematode concentrations (0, 100, 500, and 1000 IJs/larva). Temperature and nematode concentration had a significant effect on the efficacy of nematode species. S. feltiae was the most virulent species at all temperatures and nematode concentrations. Only S. feltiae showed higher than 40% mortality at low temperatures (10 and 15 °C). At 25 °C, S. feltiae caused 95% mortality, followed by H. marelatus (82%) and H. bacteriophora (76%), at 1000 IJs/larva concentration. Our results indicate that R. cerasi larvae are highly susceptible to entomopathogenic nematode infection. In particular, S. feltiae has high potential for reducing last-instar larval populations, thus decreasing the adult population in the spring.

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.

Infectivity of entomopathogenic nematode Steinernema carpocapsae Pocheon strain (Nematoda: Steinernematidae) on the green peach aphid Myzus persicae (Hemiptera: Aphididae) and its parasitoids

Infectivity of entomopathogenic nematode (EPN) Steinernema carpocapsae Pocheon strain on the green peach aphid Myzus persicae and its parasitic wasps (e.g., Aphidius colemani, Aphidius gifuensis and Diaeretiella rapae) was evaluated under laboratory conditions. Infective juveniles (IJs) of S. carpocapsae Pocheon strain had low infectivity against nymph and adult stages of M. persicae, showing 2% and 6.7% of mortality, respectively. Application of the EPNs had little effect on mummies caused by the three parasitoid species, allowing them to remain intact. No IJ invaded the host, regardless of EPN application rate. The parasitoid emergence from mummies ranged from 80% to 85% in the presence of EPN while 79–86% was recorded in the absence of EPN. However, the presence of the IJs reduced oviposition by the three parasitoid species, decreasing the rate up to 59% when the nematodes were applied before parasitoid release, while little difference in oviposition was observed when nematodes were applied after parasitoid release.

Effect of spray volume on the deposition, viability and infectivity of entomopathogenic nematodes in a foliar spray on vegetables

Spray volume can influence the amount of free water on the leaf surface and subsequently the ability of entomopathogenic nematodes (EPNs) to move. In this study, an investigation was made of the effect of spray volume (548, 730 and 1095 L ha(-1) ) on the deposition, viability and infectivity of EPNs against Galleria mellonella on savoy cabbage, cauliflower and leek. Increasing spray volume decreased nematode deposition on 7.1 cm2 leek leaf discs at a 15° angle with the spray nozzle. Although the number of living nematodes observed on leek after 240 min of exposure was not significantly different between the low-volume application (548 L ha(-1) ) and the high-volume application (1095 L ha(-1) ), a greater infectivity was obtained in the latter application. The higher number of droplets deposited on the leek discs in the high-volume application may have stimulated nematode movement. No significant effect of spray volume was observed on the relative deposition of Steinernema carpocapsae on the bottom side of cauliflower and savoy cabbage leaf discs. In spite of the low S. carpocapsae deposition on the bottom side of the savoy cabbage discs, high infectivity was obtained against G. mellonella. Using the lowest spray volume on savoy cabbage, infectivity decreased with increasing exposure time, while infectivity was not affected by exposure time when a spray volume of 730 L ha(-1) or more was used. Spray volume is an important application parameter, as it affects nematode infectivity. Future research should investigate the effect of spray volume in the field and its influence on the effect of adjuvants.

Moisture desorption isotherms of Lavandula officinalis L. leaves at three temperatures

To investigate the mechanism of Galleria mellonella infected with entomopathogenic nematodes (EPNs), we studied the effects of a new species of EPNs, Heterorhabditis beicherriana n.sp., on the activities of Tyrosinase (TYR), acetylcholinesterase (AChE), carboxylesterase (CarE) and glutathione S-transferase (GST) in whole body of G. mellonella. The last instar larvae of G. mellonella (0.18 to 0.20 g/larva) were used as host insects and were randomized into four groups (n = 60 in each group): 0 IJ/larva control group, 20 IJs/larva group, 40 IJs/larva group and 80 IJs/larva group. Infective juveniles (IJs) were injected into each selected G. mellonella larva. At 0, 8, 16, 24, 32 and 40 h post-infection, ten injected larvae were randomly collected from each group, enzymes were extracted, and the enzyme activities of TYR, AChE, CarE and GSTs were assessed following standard methods. We found that the activities of TYR, AChE, CarE and GST were significantly enhanced in a dose-dependent manner in G. mellonella larvae infected with H. beicherriana, which may be the overeactive stress response to the EPNs infection and the overreaction could lead to the death of host insects. Key words: Entomopathogenic nematode, Galleria mellonella, tyrosinase, acetylcholinesterase, carboxylesterase, glutathione S-transferase.

Infectivity of Steinernema mushtaqi (Rhabditida: Steinernematidae) against insect pests and their mass production

The infectivity of entomopathogenic nematode (EPN), Steinernema mushtaqi was tested against legume pod borer, Maruca vitrata, tobacco caterpillar, Spodoptera litura, blue butterfly, Lampides boeticus, red hairy caterpillar, Amsacta moorei, brown bug, Clavigralla gibbosa, mealy bug, Centrococcus somatics, fruit borer, Earias vittella and green bug, Nezara viridula larvae and in vivo mass production of the above-tested species of EPN have been carried out during 2008. S. mushtaqi was found to be more pathogenic to A. moorei, as it brought about 100% mortality within 48 h, than to S. litura, L. boeticus, N. viriduala and E. vittella, as mortality occurred within 72 h; whereas this level of mortality was recorded in C. somatis, C. gibbosa and M. vitrata within 144 h. No mortality was observed in the control treatment. Multiplication of S. mushtaqi and the yield of infective juveniles (IJs) on these insects was the highest (0.94 × 105 IJs/cadaver) from N. viriduala, followed by S. litura (0.76 × 105 IJs/cadaver), L. boeticus as also C. gibbosa (0.31 × 105 IJs/cadaver) and M. vitrata (0.20 × 105 IJs/cadaver). Very poor populations of IJs were found from A. moorei (0.15 × 105 IJs/cadaver) and C. somatics (0.01 × 105 IJs/cadaver). No multiplication of IJs was found from the cadaver of E. vittella. This opens a new hope of utilising S. mushtaqi in the insect pests management programme.

Laboratory efficacy against neonate larvae of the banana weevil Cosmopolites sordidus of two indigenous entomopathogenic nematode species from the Canary Islands (Spain)

We evaluated the efficacy, against neonate larvae of the banana weevil, Cosmopolites sordidus (Germar) (Coleoptera, Curculionidae), of two indigenous entomopathogenic nematode species of the families Steinernematidae and Heterorhabditidae. The nematodes, isolated in the Canary Islands (Spain), were Heterorhabditis bacteriophora (TF19) and an undescribed species (TF135) of the Steinernema feltiae- group. Exposure of C. sordidus neonate larvae to 5 concentrations (2.5, 5, 25, 50 and 100 infective juveniles/cm2) in Petri dishes with sterile sand resulted in 100% mortality, with each of the two nematode species at all concentrations tested. The lowest median lethal time (LT50) for Steinernema sp. was 2.6 d at a dose of 25 infective juveniles/cm2 and 3.6 d for H. bacteriophora at a dose of 2.5 and 5 infective juveniles/cm2. The penetration rate was also evaluated as an indicator of entomopathogenic nematode infectivity. The highest values for penetration rate were recorded at lowest concentrations of infection in Steinernema sp. and in H. bacteriophora, but rates differed regardless of the LT50. Our results suggest that these two nematode species may improve the biological control of neonate larvae of C. sordidus in banana crops.

The lethal and sub-lethal consequences of entomopathogenic nematode infestation and exposure for adult pine weevils, Hylobius abietis (Coleoptera: Curculionidae)

Entomopathogenic nematodes (EPN) frequently kill their host within 1–2 days, and interest in EPN focuses mainly on their lethality. However, insects may take longer to die, or may fail to die despite being infected, but little is known about the effects of EPN infection on insects, other than death. Here we investigate both lethal and sub-lethal effects of infection by two EPN species , Steinernema carpocapsae and Heterorhabditis downesi, on adults of the large pine weevil, Hylobius abietis. Following 12 h nematode–weevil contact in peat, S. carpocapsae killed a significantly higher proportion of weevils (87–93%) than H. downesi (43–57%) at all concentrations tested. Less than 10% of weevils were dead within 2 days, and weevils continued to die for up to 10 days after exposure (LT 50 of 3 days or more). In a separate experiment, live weevils dissected 6 days after a 24 h exposure to nematodes on filter paper harbored encapsulated and dead nematodes, showing that weevils could defend themselves against infection. Some live weevils also harbored live nematodes 6 days after they had been removed from the nematode infested medium. Feeding by weevils was not affected by infection with, or exposure to, either species of EPN. We discuss these results in relation to the use of EPN in biological control against H. abietis.

Evaluation of entomopathogenic nematodes against the olive fruit fly, Bactrocera oleae (Diptera: Tephritidae)

Infectivity of six entomopathogenic nematode (EPNs) species against Bactrocera oleae was compared. Similar infection levels were observed when third-instar larvae were exposed to infective juveniles (IJs) on a sand-potting soil substrate. When IJs were sprayed over naturally infested fallen olives, many larvae died within treated olives as well as in the soil; Steinernema feltiae caused the highest overall mortality of 67.9%. In addition, three laboratory experiments were conducted to optimize a time period for S. feltiae field application. (1) Abundance of fly larvae inside fallen olives was estimated over the 2006–2007 season with the highest number of susceptible larvae (3 mm and larger) per 100 olives being observed during December, 2006. (2) S. feltiae efficacy against fly larvae dropped to the soil post-IJ-application was determined. B. oleae added to the substrate before and after nematode application were infected at similar levels. (3) Effect of three temperature regimes (min–max: 10–27, 6–18, and 3–12 °C) corresponding to October through December in Davis, California on S. feltiae survival and infectivity was determined. After 8 weeks, the IJs at the 3–12 °C treatment showed the highest survival rate. However, the cold temperature significantly limited S. feltiae infectivity. Our results demonstrate that B. oleae mature larvae are susceptible to EPN infection both in the soil and within infested olives. Being the most effective species, S. feltiae may have the potential to suppress overwintering populations of B. oleae. We suggest that November is the optimal time for S. feltiae field application in Northern California.

Harmful effects of mustard bio-fumigants on entomopathogenic nematodes

Mustard ( Brassica and Sinapis spp.) green manures tilled into the soil preceding potato crops act as bio-fumigants that are toxic to plant–parasitic nematodes, providing an alternative to synthetic soil fumigants. However, it is not known whether mustard green manures also kill beneficial entomopathogenic nematodes (EPNs) that contribute to the control of pest insects. We used sentinel insect prey ( Galleria mellonella larvae) to measure EPN infectivity in Washington State (USA) potato fields that did or did not utilize mustard green manures. We found a trend toward lower rates of EPN infection in fields, where mustard green manures were applied, compared to those not receiving this cultural control method. In a series of bioassays we then tested whether the application of two mustard ( Brassica juncea) cultivars, differing in glucosinolate levels, disrupted the abilities of a diverse group of EPN species to infect insect hosts. Mustard-exposure trials were conducted first in laboratory arenas where EPNs were exposed to mustard extracts suspended in water, and then in larger microcosms in the greenhouse where EPNs were exposed to green manure grown, chopped, and incorporated into field soil. In all trials we used G. mellonella larvae as hosts and included multiple EPN species in the genera Steinernema ( Steinernema carpocapsae, Steinernema feltiae, Steinernema glaseri, and Steinernema riobrave) and Heterorhabditis ( Heterorhabditis bacteriophora, Heterorhabditis marelatus, and Heterorhabditis megidis). In the laboratory, EPN infection rates were lower in arenas receiving mustard extracts than the control (water), and lower still when EPNs were exposed to extracts from plants with high versus low glucosinolate levels. Results were nearly identical when mustard foliage was soil-incorporated into greenhouse microcosms, except that the negative effects of mustards on EPNs developed more slowly in soil. Significantly, in arenas of both types one EPN species, S. feltiae, appeared to be relatively unaffected by mustard exposure. Together, our results suggest that the use of mustard bio-fumigants for the control of plant–parasitic nematodes has the potential to interfere with the biocontrol of insect pests using EPNs. Thus, it may be difficult to combine these two approaches in integrated pest management programs.

Investigations on the efficacy of entomopathogenic nematodes against Spodoptera littoralis (Biosd.) and Galleria mellonella (L.)

The infectivity of seven entomopathogenic nematode strains of the families Steinernematidae and Heterorhabditidae were tested against the last instar larvae of the cotton leafworm, Spodoptera littoralis (Biosd.) and the greater wax moth, Galleria mellonella (L.) in a Petri dish and sand column assay. Exposure of the insect species was done at a dose of 100 IJ/ ml. Heterorhabditis sp. ELG., H. indica, Heterorhabditis sp. ELB. were the highest in activity, they gave a 100% mortality to S. littoralis larvae in a Petri dish assay 24 h postexposure. The rest of the strains tested gave mortalities ranged between 50 and 90% at 24 h then 100% after 48 h postexposure . In the sand-column assay strains, Heterorhabditis sp. ELB., S. riobravae and S. carpocapsae were the highest in activity against S. littoralis with 100% mortality within 24 h postexposure. The results of infectivity against G. mellonella showed that H. indica was the highest in the Petri dish assay and H. egyptii was the highest in the sand column assay with 100% mortality after 24 h of exposure. The penetration rate as an indicator of entomopathogenic nematode infection was also evaluated. The highest value was recorded for Heterorhabditis sp. ELB. (62.3%) and S. carpocapsae (61.4%) against S. littoralis in both the Petri dish and sand-column assay, respectively. In the case of G. mellonella, the highest value of penetration was recorded for Heterorhabditis sp. ELG. (68.3%) and S. riobravae (65.00%) in both the Petri dish and sand-column assay, respectively.

Entomopathogenic Nematodes for Biological Control of Pryeria sinica Moore (Lepidoptera: Zygaenidae) and Persistence on Euonymus japonica Thunberg Foliage

Korean entomopathogenic nematodes (EPN), Steinernema carpocapsae Pocheon strain, Steinernema sp. GSNUS-4 strain, Steinernema sp. GSNUS-14 strain, Steinernema sp. GSNUS-16 strain, Heterorhabditis sp. GSNUH-2 strain, and Heterorhabditis sp. GSNUH-3 strain were evaluated for the biological control of pellucid zygaenid, Pryeria sinica Moore (Lepidoptera: Zygaenidae), a leaf-feeding insect pest of Euonymus japonica Thunberg. In addition, persistence of S. carpocapsae Pocheon strain on E. japonica foliage was checked. Nematode species, strain, and concentration influenced infectivity of Korean EPNs against 3rd instar of P. sinica larvae. S. carpocapsae Pocheon strain was the most effective nematode. Larval mortality by S. carpocapsae Pocheon strain in the exposure experiments was low in both light and dark conditions representing 36.7% at 100 infective juveniles/larva in each dark and light condition. Mortality by fenitrothion, however, was 100% 3 days later. Infectivity of S. carpocapsae Pocheon strain was not different between non-exposed foliage and exposed foliage to sunlight for 30 min. Mortality of 4th instar of P. sinica larvae was very low for both S. carpocapsae Pocheon alone and S carpocapsae Pocheon with spreading agent. Application time of nematodes, shading, and leaf position on the tree influenced persistence of S. carpocapsae Pocheon strain on E. japonica foliage. Nematode survival on foliage was positively correlated with shading level and higher at the lowest level than the middle or upper level of the tree.

Efficacy of entomopathogenic nematodes against neonate larvae of Capnodis tenebrionis (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%).

Impacts of fluctuating temperature on the development and infectivity of entomopathogenic nematode Steinernema carpocapsae A10

Three different laboratory conditions were used to examine the impacts of fluctuating temperature on the development and infectivity of entomopathogenic nematode (EPN) Steinernema carpocaposae A10. Set I experiments focused on the impact of cold stress early in the development cycle. In these studies Galleria mellonella hosts were infected and incubated for 2 days at the control temperature of 23 °C and then subjected to lower temperatures of −10, 4, 10 or 14 °C, respectively, from days 3 to 36 post-infection (PI). Dissections of infected cadavers indicated arrested development at the adult stage at all lower temperatures tested. Set II experiments examined the impacts of cold stress early in the development followed by a return to 23 °C. Hosts were infected and incubated as in Set I and subjected to the same temperatures as above for 7 days, followed by incubation at 23 °C until 23 days PI. A limited number of EPN populations were able to complete development at 10 and 14 °C though emergent population numbers were significantly lower than those of control infections incubated continuously at 23 °C. In Set III experiments, infected hosts were subjected to cold stress later during development starting at day 4 post-infection followed by incubation at the control temperature. Population survival past first and second stage juveniles was reduced by at least 95% or more at the lower temperatures compared with controls. Emergent populations from the Set III cold-stressed hosts were not infectious. These studies may provide insights as to how EPN survive seasonal temperature fluctuations under natural environmental conditions.

Effects of host desiccation on development, survival, and infectivity of entomopathogenic nematode Steinernema carpocapsae

This study investigates the effect of host desiccation on entomopathogenic nematode (EPN) development, emergence, infectivity, and cross-protection against secondary environmental stress. Galleria mellonella hosts infected with the EPN Steinernema carpocapsae A10 were allowed to dehydrate in an environmental chamber for up to 56 days at 23 °C achieving a weight loss of approximately 86% by day 44 post-infection. Host carcasses were rehydrated on water-saturated filter paper in White traps to collect emergent infective juveniles (IJ) at specific time intervals. Populations were counted with an apparent peak coinciding with desiccated hosts rehydrated at 24-day post-infection. Desiccation-stressed IJ populations from each time interval were tested for infectivity, and cross-resistance to secondary temperature and pH stresses and were found to have significant increases in both infectivity and protection from extremes of temperature and pH compared with controls. Total aqueous soluble protein profiles from control and desiccation-stressed IJs were analyzed using 10% SDS Laemmli gels. Several novel proteins were over-expressed in EPN from hosts subjected to desiccation suggesting the induction and expression of stress response genes.

Infectivity of entomopathogenic nematodes to soil‐dwelling developmental stages of the tree leaf beetles Altica quercetorum and Agelastica alni

AbstractThe infectivity and biocontrol potential of entomopathogenic nematodes against two common urban tree leaf beetles (Altica quercetorum and Agelastica alni) pupating in the soil were examined under laboratory and semi‐field conditions. In the laboratory experiments, pre‐pupae and pupae of both insect species were shown to be highly susceptible to nematode infection when challenged in soil pre‐treated with the parasites’ infective juveniles. In general, Heterorhabditis megidis was more effective than Steinernema feltiae. However, significant differences were observed between individual isolates within the latter species. Nematodes developed and reproduced in cadavers of both insect species.A semi‐field experiment studying the biocontrol potential of selected nematode strains, conducted under the canopy of urban trees, confirmed the preliminary laboratory findings and revealed that H. megidis could eliminate most of the insects pupating in the soil, when applied at a relatively low dose of 105 IJs m−2. The potential of entomopathogenic nematodes as environmentally safe, effective, and economically viable agents for the biological control of tree leaf beetles in urban green areas is discussed.

Effects of pressure differentials on the viability and infectivity of entomopathogenic nematodes

During passage through the different components of a spray application system, a nematode suspension will undergo pressure changes. The extent of damage to three species of entomopathogenic nematodes (EPNs) (Steinernema carpocapsae, Heterorhabditis bacteriophora, and Heterorhabditis megidis) in suspension due to the effects of a pressure differential was studied. A French pressure cell and press was used to subject the newly emerged EPN suspensions to a series of pressure differentials ranging from 1283 kPa (186 psi) to 10,690 kPa (1550 psi). Aged suspensions (3 weeks) of H. bacteriophora and H. megidis were also evaluated. Damage was quantified by counting living and dead (whole and pieces) EPNs and by bioassay techniques. As the pressure differential increased, the relative viability of the EPNs decreased. Entomopathogenic nematodes that survived the pressure differential were, in general, able to survive for at least 1 week and maintain infectivity to Galleria mellonella at rates equivalent to EPNs that had not been pressure treated. In general, the relative viabilities of fresh and aged EPNs were equivalent after pressure differential treatments. The relative viability of the treated EPNs remained above 85% for pressure differentials less than or equal to 1283 kPa for H. megidis and 2138 kPa (310 psi) for S. carpocapsae and H. bacteriophora, but decreased rapidly for higher pressure differentials. Greater reductions in relative viability were experienced by Heterorhabditis spp. than S. carpocapsae, indicating that nematode species is an important factor to consider when defining spray operating conditions. We recommend a maximum operating pressure of 1380 kPa (200 psi) for H. megidis and 2000 kPa (290 psi) for S. carpocapsae and H. bacteriophora.

Effect of Temperature and Lepidopteran Host Species on Entomopathogenic Nematode (Nematoda: Steinernematidae, Heterorhabditidae) Infection

Experiments were conducted to determine the effect of temperature on entomopathogenic nematode infection of in insect host. Late instars of Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), Heliothis virescens (F.) (Lepidoptera: Noctuidae), Trichoplusia ni (Hubner) (Lepidoptera: Noctuidae), and Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae) were incubated at 10 temperatures ranging between 9.9 and 40 degrees C, and exposed to 150 infective juvenile entomopathogenic nematodes (Nematoda: Steinernematidae; Heterorhabditidae) per larva. The number of infecting Steinernema riobrave Cabanillas, Poinar & Raulston, S. carpocapsae (Weiser) (Kapow) and Heterorhabditis bacteriophora (Poinar) (Cruiser), nematodes was recorded. Temperature, nematode species, and target insect all significantly affected, the number of infective juvenile nematodes invading host insects. The temperature supporting the highest levels of nematode infection varied depending on insect host species. Larvae of P. gossypiella were then incubated at the same range of temperatures, and exposed to 2 dose rates (100 and 200 infective juveniles per larva) of S. riobrave, S. carpocapsae (Guardian). H. bacteriophora (Cruiser), or H. bacteriophora (Lawn Patrol). Increasing nematode dosage had no effect on the temperature at which nematode infection of P. gossypiella was highest. Doubling the nematode dose rate did not result in a significant increase in the number of infective juveniles infecting larvae for the nematode species tested with the exception of S. riobrave. The optimum temperature for the control of insect targets by a nematode will vary among target species. Assuming existing nematode temperature optima and applying the same conditions to untested insect species may not result in maximum biocontrol efficacy.

Comparison of Entomopathogenic Nematode Infectivity from Infected Hosts Versus Aqueous Suspension

Infectivity is an important measure of virulence in insect pathogens. The proportion of entomopathogenic nematodes that invaded a host, Galleria mellonella (L.), was used as a measure of infectivity in 2 species of entomopathogenic nematodes: S teinernema carpocapsae (Weiser) and Heterorhabditis bacteriophora Poinar. We compared infectivity between nematodes emerging directly from cadavers into sand with nematodes that were applied to sand in aqueous suspensions after collection in a White trap. Assay arenas consisted of 10 G. mellonella larvae in petri dishes filled with moist sand. After 20 or 44 h the larvae were removed and the number of nematodes that had invaded was counted after dissection. Infectivity of H. bacteriophora was significantly greater when nematodes emerged directly into sand compared with the aqueous application. After 24 h, a11% of H. bacteriophora from cadaver treatments had penetrated into a host, whereas infectivity of aqueous treatments was 4–10 times lower. A similar trend was observed after 48 h. The infectivity of S. carpocapsae from cadaver treatments was a24% in both treatments. An additional experiment was conducted for H. bacteriophora, which consisted of 3 treatments: nematodes directly from cadavers, nematodes applied in aqueous suspension, and nematodes in aqueous suspension that were exposed to an extract from macerated host cadavers. Higher infectivity was observed in the amended aqueous suspension compared with aqueous nematodes without the extract. This study emphasizes that differences in fitness and behavior must exist between nematodes reared under standard laboratory procedures and those in nature.

Effect of recycling temperature on the infectivity of entomopathogenic nematodes

Four strains of entomopathogenic nematodes were recycled in vivo for 2 years at temperatures ranging from 10 to 25 °C, then the infectivity of their infective juveniles was compared. Infectivity was examined by measuring LC50 values for wax moth (Galleria mellonella) larvae at bioassay temperatures ranging from 5 to 25 °C. Of the four strains examined, only the Umeå and NF strains of Steinernema feltiae that had been recycled at 10 °C infected and killed the insects at a bioassay temperature of 5 °C. The Steinernema carpocapsae All and Steinernema riobravis TX strains were infective at 10 °C only when the recycling temperature was ≤ 20 °C. The infectivity of the two strains of S. feltiae at 10 or 15 °C was compromised by propagating them at higher temperatures (20–25 °C). The Umeå strain of S. feltiae displayed an impaired capacity to infect hosts at higher temperatures (20–25 °C) when recycled at lower (≤ 15 °C) temperatures. The capacity of these nematodes to adjust to different recycling temperatures is discussed in relation to their infectivity in different field situations.