Freezing and Desiccation Tolerance in Entomopathogenic Nematodes: Diversity and Correlation of Traits

The ability of entomopathogenic nematodes to tolerate environmental stress such as desiccating or freezing conditions, can contribute significantly to biocontrol efficacy. Thus, in selecting which nematode to use in a particular biocontrol program, it is important to be able to predict which strain or species to use in target areas where environmental stress is expected. Our objectives were to (i) compare inter- and intraspecific variation in freeze and desiccation tolerance among a broad array of entomopathogenic nematodes, and (ii) determine if freeze and desiccation tolerance are correlated. In laboratory studies we compared nematodes at two levels of relative humidity (RH) (97% and 85%) and exposure periods (24 and 48 h), and nematodes were exposed to freezing temperatures (-2°C) for 6 or 24 h. To assess interspecific variation, we compared ten species including seven that are of current or recent commercial interest: Heterorhabditis bacteriophora (VS), H. floridensis, H. georgiana, (Kesha), H. indica (HOM1), H. megidis (UK211), Steinernema carpocapsae (All), S. feltiae (SN), S. glaseri (VS), S. rarum (17C&E), and S. riobrave (355). To assess intraspecific variation we compared five strains of H. bacteriophora (Baine, Fl1-1, Hb, Oswego, and VS) and four strains of S. carpocapsae (All, Cxrd, DD136, and Sal), and S. riobrave (355, 38b, 7-12, and TP). S. carpocapsae exhibited the highest level of desiccation tolerance among species followed by S. feltiae and S. rarum; the heterorhabditid species exhibited the least desiccation tolerance and S. riobrave and S. glaseri were intermediate. No intraspecific variation was observed in desiccation tolerance; S. carpocapsae strains showed higher tolerance than all H. bacteriophora or S. riobrave strains yet there was no difference detected within species. In interspecies comparisons, poor freeze tolerance was observed in H. indica, and S. glaseri, S. rarum, and S. riobrave whereas H. georgiana and S. feltiae exhibited the highest freeze tolerance, particularly in the 24-h exposure period. Unlike desiccation tolerance, substantial intraspecies variation in freeze tolerance was observed among H. bacteriophora and S. riobrave strains, yet within species variation was not detected among S. carpocapsae strains. Correlation analysis did not detect a relationship between freezing and desiccation tolerance.     

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.     

Characterization of Biocontrol Traits in Heterorhabditis floridensis: A Species with Broad Temperature Tolerance

Biological characteristics of two strains of the entomopathogenic nematode, Heterorhabditis floridensis (332 isolated in Florida and K22 isolated in Georgia) were described. The identity of the nematode’s symbiotic bacteria was elucidated and found to be Photorhabdus luminescens subsp. luminescens. Beneficial traits pertinent to biocontrol (environmental tolerance and virulence) were characterized. The range of temperature tolerance in the H. floridensis strains was broad and showed a high level of heat tolerance. The H. floridensis strains caused higher mortality or infection in G. mellonella at 30°C and 35°C compared with S. riobrave (355), a strain widely known to be heat tolerant, and the H. floridensis strains were also capable of infecting at 17°C whereas S. riobrave (355) was not. However, at higher temperatures (37°C and 39°C), though H. floridensis readily infected G. mellonella, S. riobrave strains caused higher levels of mortality. Desiccation tolerance in H. floridensis was similar to Heterorhabditis indica (Hom1) and S. riobrave (355) and superior to S. feltiae (SN). H. bacteriophora (Oswego) and S. carpocapsae (All) exhibited higher desiccation tolerance than the H. floridensis strains. The virulence of H. floridensis to four insect pests (Aethina tumida, Conotrachelus nenuphar, Diaprepes abbreviatus, and Tenebrio molitor) was determined relative to seven other nematodes: H. bacteriophora (Oswego), H. indica (Hom1), S. carpocapsae (All), S. feltiae (SN), S. glaseri (4-8 and Vs strains), and S. riobrave (355). Virulence to A. tumida was similar among the H. floridensis strains and other nematodes except S. glaseri (Vs), S. feltiae, and S. riobrave failed to cause higher mortality than the control. Only H. bacteriophora, H. indica, S. feltiae, S. riobrave, and S. glaseri (4-8) caused higher mortality than the control in C. nenuphar. All nematodes were pathogenic to D. abbreviatus though S. glaseri (4-8) and S. riobrave (355) were the most virulent. S. carpocapsae was the most virulent to T. molitor. In summary, the H. floridensis strains possess a wide niche breadth in temperature tolerance and have virulence and desiccation levels that are similar to a number of other entomopathogenic nematodes. The strains may be useful for biocontrol purposes in environments where temperature extremes occur within short durations.     

Viability and Virulence of Entomopathogenic Nematodes Exposed to Ultraviolet Radiation

Entomopathogenic nematodes (EPNs) can be highly effective biocontrol agents, but their efficacy can be reduced due to exposure to environmental stress such as from ultraviolet (UV) radiation. Our objectives were to 1) compare UV tolerance among a broad array of EPN species, and 2) investigate the relationship between reduced nematode viability (after exposure to UV) and virulence. Nematodes exposed to a UV radiation (254 nm) for 10 or 20 min were assessed separately for viability (survival) and virulence to Galleria mellonella. We compared 9 different EPN species and 15 strains: Heterorhabditis bacteriophora (Baine, fl11, Oswego, and Vs strains), H. floridensis (332), H. georgiana (Kesha), H. indica (HOM1), H. megidis (UK211), Steinernema carpocapsae (All, Cxrd, DD136, and Sal strains), S. feltiae (SN), S. rarum (17C&E), and S. riobrave (355). In viability assessments, steinernematids, particularly strains of S. carpocapsae, generally exhibited superior UV tolerance compared with the heterorhabditids. However, some heterorhabditids tended to be more tolerant than others, e.g., H. megidis and H. bacteriophora (Baine) were most susceptible and H. bacteriophora (Vs) was the only heterorhabditid that did not exhibit a significant effect after 10 min of exposure. All heterorhabditids experienced reduced viability after 20 min exposure though several S. carpocapsae strains did not. In total, after 10 or 20 min exposure, the viability of seven nematode strains did not differ from their non-UV exposed controls. In virulence assays, steinernematids (particularly S. carpocapsae strains) also tended to exhibit higher UV tolerance. However, in contrast to the viability measurements, all nematodes experienced a reduction in virulence relative to their controls. Correlation analysis revealed that viability among nematode strains is not necessarily related to virulence. In conclusion, our results indicate that the impact of UV varies substantially among EPNs, and viability alone is not a sufficient measure for potential impact on biocontrol efficacy as other characters such as virulence may be severely affected even when viability remains high.     

Control of the Oriental Fruit Moth, Grapholita molesta, Using Entomopathogenic Nematodes in Laboratory and Fruit Bin Assays

The oriental fruit moth (OFM), Grapholita molesta (Busck), which is among the most important insect pests of peaches and nectarines, has developed resistance to a wide range of insecticides. We investigated the ability of the entomopathogenic nematodes (EPN) Steinernema carpocapsae (Weiser), S. feltiae (Filipjev), S. riobrave (Cabanillas et al.), and Heterorhabditis marelatus (Liu and Berry) to control OFM under laboratory and fruit bin conditions. At a dosage of 10 infective juveniles (IJ)/cm² in the laboratory, S. carpocapsae caused 63%, S. feltiae 87.8%, S. riobrave 75.6%, and H. marelatus 67.1% OFM mortality. All four nematode species caused significant OFM larval mortality in comparison to the nontreated controls. Steinernema feltiae was used for the bin assays due to the higher OFM mortality it caused than the other tested EPN species and to its ability to find OFM under cryptic environments. Diapausing cocooned OFM larvae in miniature fruit bins were susceptible to IJ of S. feltiae in infested corner supports and cardboard strips. Treatment of bins with suspensions of 10 or 25 S. feltiae IJ/ml water with wetting agent (Silwet L77) resulted in 33.3 to 59% and 77.7 to 81.6% OFM mortality in corner supports and cardboard strips, respectively. This paper presents new information on the use of EPN, specifically S. feltiae, as nonchemical means of OFM control.     

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

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

Effectiveness of Steinernema spp. and Heterorhabditis bacteriophora against Popillia japonica in the Azores

Steinernema carpocapsae (Breton strain), S. glaseri, and Heterorhabditis bacteriophora were evaluated for their potential to control immature stages of the Japanese beetle, Popillia japonica, on Terceira Island (the Azores). In bioassays carried out at temperatures higher than 15 C, S. glaseri and H. bacteriophora caused 100% mortality of larvae, whereas S. carpocapsae caused 56% larval mortality. At temperatures slightly below 15 C, only S. glaseri remained effective. In field plots, in September, S. glaseri and S. carpocapsae reduced larval populations by 91% and 44%, respectively, when applied at the rate of 10⁶ nematodes/m². In April, S. glaseri caused 31% reduction in numbers of larvae, but S. carpocapsae was ineffective. In colder months (November-February) neither steinernematids nor H. bacteriophora reduced larval populations. Increasing the application rate from 10⁶ to 5 x 10⁶ infective stage S. glaseri per m² increased efficacy from 63% to 79% mortality.     

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.     

Use of Entomopathogenic Nematodes to Suppress Meloidogyne incognita on Greenhouse Tomatoes

Tomato seedlings in a growth chamber were inoculated with 150 Meloidogyne incognita eggs and 25 infective juveniles (IJ)/cm² of Steinernema feltiae, S. riobrave, or Heterorhabditis bacteriophora. With the exception of seedling roots treated with H. bacteriophora, all seedlings treated with entomopathogenic nematodes had fewer M. incognita juveniles inside roots and produced fewer eggs than the control seedlings. Tomato plants in the greenhouse were infested with 4,000 M. incognita eggs and treated 2 weeks before, 1 week before, at the same time, 1 week after, or 2 weeks after with 25 or 125 IJ/cm² of S. feltiae, S. riobrave, or H. bacteriophora. Plants with pre- and post-infestation applications of S. feltiae or S. riobrave suppressed M. incognita. Plants treated with H. bacteriophora 1 week before and at the time of infestation suppressed M. incognita. Increasing the rate of H. bacteriophora and S. feltiae from 25 to 125 IJ/cm² improved M. incognita suppression.     

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

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

Effects of Earthworms on the Dispersal of Steinernema spp.

Previous studies indicated that dispersal of S. carpocapsae may be enhanced in soil with earthworms. The objective of this research was to determine and compare the effects of earthworms on dispersal of other Steinernema spp. Vertical dispersal of Steinernema carpocapsae, S. feltiae, and S. glaseri was tested in soil columns in the presence and absence of earthworms (Lumbricus terrestris). Dispersal was evaluated by a bioassay and by direct extraction of nematodes from soil. Upward dispersal of S. carpocapsae and S. feltiae increased in the presence of earthworms, whereas upward dispersal of S. glaseri was not affected by earthworms. No significant differences were detected in downward dispersal of S. carpocapsae and S. feltiae in soil with earthworms compared to soil without earthworms. Downward dispersal of S. glaseri, however, was greater in soil without earthworms relative to soil with earthworms. In soil void of earthworms, dispersal of S. glaseri was greatest followed by dispersal of S. carpocapsae. The presence of earthworm burrows in soil did not influence nematode dispersal. Nematodes were recovered from the surface, interior, and casts of earthworms. Therefore, nematodes may have a phoretic association with earthworms.     

Impact of a Nematode-parasitic Fungus on the Effectiveness of Entomopathogenic Nematodes

The impact of the nematode-parasitic fungus Hirsutella rhossiliensis on the effectiveness of Steinernema carpocapsae, S. glaseri, and Heterorhabditis bacteriophora against Galleria mellonella larvae was assessed in the laboratory. The presence of Hirsutella conidia on the third-stage (J3) cuticle of S. carpocapsae and H. bacteriophora interfered with infection of insect larvae. Conidia on the J3 cuticle of S. glaseri and on the ensheathing second-stage cuticle of H. bacteriophora did not reduce the nematodes'' ability to infect larvae. The LD₅₀ values for S. carpocapsae, S. glaseri, and H. bacteriophora in sand containing H. rhossiliensis were not different from those in sterilized sand when Galleria larvae were added at the same time as the nematodes. However, when Galleria larvae were added 3 days after the nematodes, the LD₅₀ of S. glaseri was higher in Hirsutella-infested sand than in sterilized sand, whereas the LD₅₀ of H. bacteriophora was the same in infested and sterilized sand. Although the LD₅₀ of S. carpocapsae was much higher in Hirsutella-infested sand than in sterilized sand, the data were too variable to detect a significant difference. These data suggest that H. bacteriophora may be more effective than Steinernema species at reducing insect pests in habitats with abundant nematode-parasitic fungi.     

Aggregative group behavior in insect parasitic nematode dispersal

Movement behavior of foraging animals is critical to the determination of their spatial ecology and success in exploiting resources. Individuals sometimes gain advantages by foraging in groups to increase their efficiency in garnering these resources. Group movement behavior has been studied in various vertebrates. In this study we explored the propensity for innate group movement behavior among insect parasitic nematodes. Given that entomopathogenic nematodes benefit from group attack and infection, we hypothesised that the populations would tend to move in aggregate in the absence of extrinsic cues. Movement patterns of entomopathogenic nematodes in sand were investigated when nematodes were applied to a specific locus or when the nematodes emerged naturally from infected insect hosts; six nematode species in two genera were tested (Heterorhabditis bacteriophora, Heterorhabditis indica, Steinernema carpocapsae, Steinernema feltiae, Steinernema glaseri and Steinernema riobrave). Nematodes were applied in aqueous suspension via filter paper discs or in infected insect host cadavers (to mimic emergence in nature). We discovered that nematode dispersal resulted in an aggregated pattern rather than a random or uniform distribution; the only exception was S. glaseri when emerging directly from infected hosts. The group movement may have been continuous from the point of origin, or it may have been triggered by a propensity to aggregate after a short period of random movement. To our knowledge, this is the first report of group movement behavior in parasitic nematodes in the absence of external stimuli (e.g., without an insect or other apparent biotic or abiotic cue). These findings have implications for nematode spatial distribution and suggest that group behavior is involved in nematode foraging.     

Effect of Entomopathogenic Nematodes on Mesocriconema xenoplax Populations in Peach and Pecan

The effect of Steinernema riobrave and Heterorhabditis bacteriophora on population density of Mesocriconema xenoplax in peach was studied in the greenhouse. Twenty-one days after adding 112 M. xenoplax adults and juveniles/1,500 cm³ soil to the soil surface of each pot, 50 infective juveniles/cm² soil surface of either S. riobrave or H. bacteriophora were applied. Another entomopathogenic nematode application of the same density was administered 3 months later. The experiment was repeated once. Mesocriconema xenoplax populations were not suppressed (P ≤ 0.05) in the presence of either S. riobrave or H. bacteriophora 180 days following ring nematode inoculation. On pecan, 200 S. riobrave infective-stage juveniles/cm² were applied to the soil surface of 2-year-old established M. xenoplax populations in field microplots. Additional applications of S. riobrave were administered 2 and 4 months later. This study was terminated 150 days following the initial application of S. riobrave. Populations of M. xenoplax were not suppressed in the presence of S. riobrave.     

Effects of Etomopathiogenic Nematodes on Meloidogyne javanica on Tomatoes and Soybeans

Two Hawaiian isolates of Steinernema feltiae MG-14 and Heterohabditis indica MG-13, a French isolate of S. feltiae SN, and a Texan isolate of S. riobrave TX were tested for their efficacy against the root-knot nematode, Meloidogyne javanica, in the laboratory and greenhouse. Experiments were conducted to investigate the effects of treatment application time and dose on M. javanica penetration in soybean, and egg production and plant development in tomato. Two experiments conducted to assess the effects of entomopathogenic nematode application time on M. javanica penetration demonstrated that a single application of 10⁴ S. feltiae MG-14 or SN infective juveniles per 100 cm³ of sterile soil, together with 500 (MG-14) or 1,500 (SN) second-stage juveniles of M. javanica, reduced root penetration 3 days after M. javanica inoculation compared to that of a water treatment. Entomopathogenic nematode infective juveniles applied to assess the effects on M. javanica egg production did not demonstrate a significant reduction compared to that of the water control treatment. There was no dose response effect by Steinernema spp. On M. javanica root penetration or egg production. Steinernema spp. did not affect the growth or development of M. javanica-infected plants, but H. indica MG-13-treated plants had lower biomass than untreated plants infected with M. javanica. Infective juveniles of S. riobrave TX, S. feltiae SN, and MG-14 but not those of H. indica MG-13 were found inside root cortical tissues of M. javanica-infected plants. Entomopathogenic nematode antagonism to M. javanica on soybean or tomato was insufficient in the present study to provide a consistent level of nematode suppression at the concentrations of infective juveniles applied.     

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.     

Effect of Soil Moisture and a Surfactant on Entomopathogenic Nematode Suppression of the Pecan Weevil, Curculio caryae

Our overall goal was to investigate several aspects of pecan weevil, Curculio caryae, suppression with entomopathogenic nematodes. Specifically, our objectives were to: 1) determine optimum moisture levels for larval suppression, 2) determine suppression of adult C. caryae under field conditions, and 3) measure the effects of a surfactant on nematode efficacy. In the laboratory, virulence of Heterorhabditis megidis (UK211) and Steinernema carpocapsae (All) were tested in a loamy sand at gravimetric water contents of negative 0.01, 0.06, 0.3, 1.0, and 15 bars. Curculio caryae larval survival decreased as moisture levels increased. The nematode effect was most pronounced at –0.06 bars. At –0.01 bars, larval survival was ≤5% regardless of nematode presence, thus indicating that intense irrigation alone might reduce C. caryae populations. Overall, our results indicated no effect of a surfactant (Kinetic) on C. caryae suppression with entomopathogenic nematodes. In a greenhouse test, C. caryae larval survival was lower in all nematode treatments compared with the control, yet survival was lower in S. carpocapsae (Italian) and S. riobrave (7–12) treatments than in S. carpocapsae (Agriotos), S. carpocapsae (Mexican), and S. riobrave (355) treatments (survival was reduced to approximately 20% in the S. riobrave [7–12] treatment). A mixture of S. riobrave strains resulted in intermediate larval survival. In field experiments conducted over two consecutive years, S. riobrave (7–12) applications resulted in no observable control, and, although S. carpocapsae (Italian) provided some suppression, treatment effects were generally only detectable one day after treatment. Nematode strains possessing both high levels of virulence and a greater ability to withstand environmental conditions in the field need to be developed and tested.     

Influence of Soil pH and Oxygen on Persistence of Steinernema spp.

Survival of infective juveniles of Steinernema carpocapsae and Steinernema glaseri gradually declined during 16 weeks of observation as the tested soil pH decreased from pH 8 to pH 4. Survival of both species of Steinernema dropped sharply after 1 week at pH 10. Survival or S. carpocapsae and S. glaseri was similar at pH 4, 6, and 8 during the first 4 weeks, but S. carpocapsae survival was significantly greater than S. glaseri at pH 10 through 16 weeks. Steinernema carpocapsae and S. glaseri that had been stored at pH 4, 6, and 8 for 16 weeks, and at pH 10 for 1 or more weeks were not infective to Galleria mellonella larvae. Steinernema carpocapsae survival was significantly greater than that of S. glaseri at oxygen:nitrogen ratios of 1:99, 5:95, and 10:90 during the first 2 weeks, and survival of both nematode species declined sharply to less than 20% after 4 weeks. Survival of both nematode species significantly decreased after 8 weeks as the tested oxygen concentrations decreased from 20 to 1%, and no nematode survival was recorded after 16 weeks. Steinernema carpocapsae pathogenicity was significantly greater than that of S. glaseri during the first 2 weeks. No nematode pathogenicity was recorded at oxygen concentrations of 1, 5, and 10% after 2 weeks and at 20% after 16 weeks.     

Influence of Salinity on Survival and Infectivity of Entomopathogenic: Nematodes

Exposure to NaC1, KCI, and CaCl₂ affected the entomopathogenic nematodes Heterorhabditis bacteriophora and Steinernema glaseri differently. Survival, virulence, and penetration efficiency of S. glaseri were not affected by these salts. At high concentrations, however, all three salts inhibited its ability to move through a soil column and locate and infect a susceptible host. Calcium chloride and KCl had no effect on H. bacteriophora survival, penetration efficiency, or movement through a soil column, but moderate concentrations of these salts enhanced H. bacteriophora virulence. NaCl, however, adversely affected each of these parameters at high salinities (>16 dS/m). Salt effects on S. glaseri are attributed solely to interference with nematode host-finding ability, whereas the NaCl effects on H. bacteriophora are attributed to its toxicity and possibly to interference with host-finding behavior.     

Susceptibility of the Colorado Potato Beetle and the Sugarbeet Wireworm to Steinernema feltiae and S. glaseri

In laboratory tests, larvae of the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), and the sugarbeet wireworm (SBW), Limonius californicus (Mannerheim), were exposed to the nematodes Steinernema feltiae Filipjev (Mexican strain) (= Neoaplectana carpocapsae) and S. glaseri Steiner in soil. S. feltiae caused significantly higher mortality in SBW larvae than did S. glaseri, but both nematode species were equally effective against CPB larvae. The minimum concentration of S. feltiae for 100% mortality of CPB larvae after 13 days was 157 nematodes/cm² of soil, and the LC₅₀ based on 6-day mortality was 47.5 nematodes/cm²; in contrast, 100% mortality of SBW larvae was not achieved with even the highest concentration tested, 393 nematodes/cm². CPB adults emerging from nematode-contaminated soil were not infected. In field cage tests, S. feltiae applied to the soil surface at the rates of 155 and 310 nematodes/cm² soil caused 59% and 71% mortality, respectively, of late-fourth-instar spring-generation CPB, and 28% and 29% mortality, respectively, of SBW. No infection was obtained when larvae of summer generation CPB and SBW were placed in the same cages approximately 6 weeks after nematodes were applied to the soil. Inundative soil applications of S. feltiae, though cost prohibitive at present, were effective in reducing caged CPB and SBW field populations.