Incidence of Endemic Entomopathogenic Nematodes Following Application of Steinernema riobrave for Control of Diaprepes abbreviatus

Control of Diaprepes abbreviatus by endemic and exotic entomopathogenic nematodes (EPN) was monitored during 2000-2001 in two citrus orchards in central Florida (Bartow and Poinciana). Caged sentinel insect larvae were buried beneath citrus trees for 7 days at 1 to 2-month intervals from April to October each year. At Bartow, the survey occurred in experimental plots that were (i) not treated with commercial EPN, (ii) treated twice annually since 1998 with commercially formulated Steinernema riobrave, or (iii) treated twice annually with S. riobrave and liquid fertilization (15 times/year) occurred in place of dry fertilizer (3 times/year) used in the other treatments. Four endemic EPN species, in addition to S. riobrave, were recovered from the sandy soil at Bartow: S. diaprepesi, Heterorhabditis zealandica, H. indica, and H. bacteriophora. Mean insect mortality in control plots was 39.4% (range = 13% to 74%), with seasonal maxima in May to July each year. Endemic EPN were recovered from 55% (range = 22% to 81%) of the cadavers each month. Total numbers of endemic EPN recovered in all plots during 2 years were directly related to the numbers of adult weevils (D. abbreviatus and Pachnaeus litus) captured in modified Tedder''s traps and inversely related to recovery of S. riobrave. Insect mortality was higher and cadavers containing endemic EPN were more numerous in untreated control plots than in S. riobrave-treated plots, except during months in which S. riobrave was applied. In treated plots, endemic EPN were recovered from cadavers at twice the rate of S. riobrave. Suppression of endemic EPN in plots treated with S. riobrave, combined with inferior persistence by the introduced species, may have attenuated the net efficacy of S. riobrave against D. abbreviatus. In contrast, H. indica was the only endemic nematode recovered from the sandy clay loam soil at Poinciana, where the average mortality of D. abbreviatus was 12% (range 3% to 20%) and incidence of H. indica did not exceed 8%. Results of these surveys suggest that the regional patterns in the abundance and damage to citrus caused by D. abbreviatus in Florida are regulated by endemic EPN and other soilborne enemies of the weevil.     

Estimating Sample Size and Persistence of Entomogenous Nematodes in Sandy Soils and Their Efficacy Against the Larvae of Diaprepes abbreviatus in Florida

In two studies to estimate sampling requirements for entomogenous nematodes in the field, highest persistence of Heterorhabditis bacteriophora after application occurred beneath the canopies of mature citrus trees. Nematode persistence declined with distance from the center-line of the tree row toward the row-middles. Immediately after nematode application to soil, 32 samples (15 cm deep, 2.5-cm diameter) beneath a single tree were required to derive 95% confidence intervals that were within 40% of mean nematode population density. The estimated probability of measuring the mean density within 40%, using 32 samples, declined to 88% at 2 days post-application and to 76% at 7 days. The persistence in soil of Steinernema carpocapsae, S. riobravis, and two formulations containing H. bacteriophora and their efficacy against the larvae of Diaprepes abbreviatus were compared in a grove of 4-year-old citrus trees. Within 6 days, the recovered population densities of all nematodes declined to <5% of levels on day 0. The recovery of H. bacteriophora during the first 2 weeks was lower than that of the other two species. Steinemema riobravis and both formulations of H. bacteriophora reduced recovery of D. abbreviatus by more than 90% and 50%, respectively. Steinernema carpocapsae did not affect population levels of the insect.     

Bionomics of a Phoretic Association Between Paenibacillus sp. and the Entomopathogenic Nematode Steinernema diaprepesi

Spores of an unidentified bacterium were discovered adhering to cuticles of third-stage infective juvenile (IJ) Steinernema diaprepesi endemic in a central Florida citrus orchard. The spores were cup-shaped, 5 to 6 mm in length, and contained a central endospore. Based on 16S rDNA gene sequencing, the bacterium is closely related to the insect pathogens Paenibacillus popilliae and P. lentimorbus. However, unlike the latter bacteria, the Paenibacillus sp. is non-fastidious and grew readily on several standard media. The bacterium did not attach to cuticles of several entomopathogenic or plant-parasitic nematodes tested, suggesting host specificity to S. diaprepesi. Attachment of Paenibacillus sp. to the third-stage cuticle of S. diaprepesi differed from Paenibacillus spp. associated with heterorhabditid entomopathogenic nematodes, which attach to the IJ sheath (second-stage cuticle). The inability to detect endospores within the body of S. diaprepesi indicates that the bacterial association with the nematode is phoretic. The Paenibacillus sp. showed limited virulence to Diaprepes abbreviatus, requiring inoculation of larvae with 108 spores to achieve death of the insect and reproduction of the bacterium. The effect of the bacterium on the nematode population biology was studied in 25-cm-long vertical sand columns. A single D. abbreviatus larva was confined below 15-cm depth, and the soil surface was inoculated with either spore-free or spore-encumbered IJ nematodes. After 7 days, the proportion of IJ below 5-cm depth was seven-fold greater for spore-free IJ than for spore-encumbered nematodes. Mortality of D. abbreviatus larvae was 72% greater (P <= 0.01) for spore-free compared to spore-encumbered S. diaprepesi. More than 5 times as many progeny IJs (P <= 0.01) were produced by spore-free compared to spore-encumbered nematodes. These data suggest that the bacterium is a component of the D. abbreviatus food web with some potential to regulate a natural enemy of the insect.     

Effects of host nutrition on virulence and fitness of entomopathogenic nematodes: Lipid- and protein-based supplements in Tenebrio molitor diets

Entomopathogenic nematodes, Heterorhabditis indica and Steinernema riobrave, were tested for virulence and reproductive yield in Tenebrio molitor that were fed wheat bran diets with varying lipid- and protein-based supplements. Lipid supplements were based on 20% canola oil, peanut, pork or salmon, or a low lipid control (5% canola). Protein treatments consisted of basic supplement ingredients plus 0, 10, or 20% egg white; a bran-only control was also included. Some diet supplements had positive effects on nematode quality, whereas others had negative or neutral effects. All supplements with 20% lipids except canola oil caused increased T. molitor susceptibility to H. indica, whereas susceptibility to S. riobrave was not affected. Protein supplements did not affect host susceptibility, and neither lipid nor protein diet supplements affected reproductive capacity of either nematode species. Subsequently, we determined the pest control efficacy of progeny of nematodes that had been reared through T. molitor from different diets against Diaprepes abbreviatus and Otiorhynchus sulcatus. All nematode treatments reduced insect survival relative to the control (water only). Nematodes originating from T. molitor diets with the 0% or 20% protein exhibited lower efficacy versus D. abbreviatus than the intermediate level of protein (10%) or bran-only treatments. Nematodes originating from T. molitor lipid or control diets did not differ in virulence. Our research indicates that nutritional content of an insect host diet can affect host susceptibility to entomopathogenic nematodes and nematode fitness; therefore, host media could conceivably be optimized to increase in vivo nematode production efficiency.     

Competition Between Entomopathogenic and Free-Living Bactivorous Nematodes in Larvae of the Weevil Diaprepes abbreviatus

Field and laboratory experiments were conducted to determine the degree to which free-living, bactivorous nematodes (FLBN) are able to competitively displace entomopathogenic nematodes (EPN) from insect cadavers. Two hundred larvae of the insect Diaprepes abbreviatus were buried at regular intervals during 2 years in experimental plots that were untreated or treated twice annually with Steinernema riobrave. Larvae were recovered after 7 days, and nematodes emerging from cadavers during the next 30 days were identified. The monthly prevalence of FLBN was directly related to that of S. riobrave (r = 0.38; P = 0.001) but was not related to the prevalence of the endemic EPN, S. diaprepesi, Heterorhabditis zealandica, H. indica, or H. bacteriophora (r = 0.02; P = 0.80). In a second experiment, treatment of small field plots with S. riobrave increased the prevalence of insect cadavers in which only FLBN were detected compared to untreated controls (30% vs. 14%; P = 0.052), and increased numbers of FLBN per buried insect by more than 10-fold. In the laboratory, sand microcosms containing one D. abbreviatus larva were treated with (i) the FLBN, Pellioditis sp.; (ii) S. riobrave; (iii) S. riobrave + Pellioditis; or (iv) neither nematode. Insect mortality was higher in the presence of both nematodes (57%) than when S. riobrave was alone (42%) (P = 0.01). An average of 59.2 Pellioditis sp. g^-1 insect body weight emerged in the presence of S. riobrave, whereas 6.2 nematodes g^-1 insect were recovered in the absence of the EPN (P = 0.01). Pellioditis sp. reduced the number of S. riobrave per cadaver by 84%; (P = 0.03), and per available insect by 82% (P = 0.001), compared to S. riobrave alone. Population size of S. diaprepesi was not affected by Pellioditis sp. in experiments of the same design. Faster development (P = 0.05) and nutrient appropriation within the insect cadaver by S. diaprepesi compared to S. riobrave may increase the fitness of the former species to compete with Pellioditis sp. The results of these studies demonstrate the potential of FLBN to regulate population densities of EPN and to dampen estimates of EPN-induced mortality of insect pests in the field.     

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

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

Effects of a novel entomopathogenic nematode-infected host formulation on cadaver integrity, nematode yield, and suppression of Diaprepes abbreviatus and Aethina tumida

An alternative approach to applying entomopathogenic nematodes entails the distribution of nematodes in their infected insect hosts. Protection of the infected host from rupturing, and improving ease of handling, may be necessary to facilitate application. In this study our objective was to test the potential of a new method of formulating the infected hosts, i.e., enclosing the infected host in masking tape. Tenebrio molitor L. cadavers infected with Heterorhabditis indica Poinar, Karunakar and David or Steinernema carpocapsae (Weiser) were wrapped in tape using an automatic packaging machine; the machine was developed to reduce labor and to standardize the final product. The effects of the tape formulation on the ability to protect the cadavers from mechanical damage, nematode yield, and pest control efficacy were tested. After exposure to mechanical agitation at 7-d-post-infection, S. carpocapsae cadavers in tape were more resistant to rupture than cadavers without tape, yet H. indica cadavers 7-d-post-infection were not affected by mechanical agitation (with or without tape), nor was either nematode affected when 4-d-old cadavers were tested. Experiments indicated that infective juvenile yield was not affected by the tape formulation. Laboratory experiments were conducted measuring survival of the root weevil, Diaprepes abbreviatus (L.), or the small hive beetle, Aethina tumida Murray, after the application of two H. indica-infected hosts with or without tape per 15 cm pot (filled with soil). A greenhouse experiment was also conducted in a similar manner measuring survival of D. abbreviatus. In all experiments, both the tape and no-tape treatments caused significant reductions in insect survival relative to the control, and no differences were detected between the nematode treatments. Fifteen days post-application, the infected host treatments caused up to 78% control in A. tumida, 91% control in D. abbreviatus in the lab, and 75% in the greenhouse. These results indicate potential for using the tape-formulation approach for applying nematode infected hosts.     

Impact of Surfactants on Control of the Root Weevil Diaprepes abbreviatus Larvae with Steinernema riobravis

A filter paper bioassay was developed for testing the efficacy of nematodes and other agents for control of Diaprepes abbreviatus larvae. Surfactants, with and without Steinernema riobravis, were screened first in the filter paper bioassay and then in a potted citrus seedling study. The results of the two assays were in agreement on the relative merits of the compounds tested. Surfactants increased larval mortality at 4 days for the filter paper bioassay and 1 week for the potted plant bioassay. At 8 days for the filter paper bioassay and 2 weeks for the potted plant bioassay, nematodes alone were equal to treatments of nematodes plus surfactants.     

From Augmentation to Conservation of Entomopathogenic Nematodes: Trophic Cascades, Habitat Manipulation and Enhanced Biological Control of Diaprepes abbreviatus Root Weevils in Florida Citrus Groves

The use of entomopathogenic nematodes (EPN) for management of the root weevil, Diaprepes abbreviatus, in Florida citrus groves is considered a biological control success story and typically involves augmentation in which EPN are applied inundatively as biopesticides to quickly kill the pest. However, recent evidence indicates that efficacy of EPN applications in Florida citrus depends on soil type. They are very effective in the well drained coarse sands of the Central Ridge but often less so in poorly drained fine-textured soils of the Flatwoods. Moreover, groves on the Central Ridge can harbor rich communities of endemic EPN that might often suppress weevil populations below economic thresholds, whereas Flatwoods groves tend to have few endemic EPN and frequent weevil problems. Current research is examining the ecological dynamics of EPN in Florida citrus groves, the potential impact of EPN augmentation on soil food webs, especially endemic EPN, and whether habitat manipulation and inoculation strategies might be effective for conserving and enhancing EPN communities to achieve long-term control in problem areas. Conservation biological control could extend the usefulness of EPN in Florida citrus and be especially appropriate for groves with persistent weevil problems.     

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.     

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.     

Food Web Responses to Augmenting the Entomopathogenic Nematodes in Bare and Animal Manure-Mulched Soil

Factorial treatments of entomopathogenic nematodes (EPN) and composted, manure mulches were evaluated for two years in a central Florida citrus orchard to study the post-application biology of EPN used to manage the root weevil, Diaprepes abbreviatus. Mulch treatments were applied once each year to study the effects of altering the community of EPN competitors (free-living bactivorous nematodes) and antagonists (nematophagous fungi (NF), predaceous nematodes and some microarthro-pods). EPN were augmented once with Steinernema riobrave in 2004 and twice in 2005. Adding EPN to soil affected the prevalence of organisms at several trophic levels, but the effects were often ephemeral and sometimes inconsistent. EPN augmentation always increased the mortality of sentinel weevil larvae, the prevalence of free-living nematodes in sentinel cadavers and the prevalence of trapping NF. Subsequent to the insecticidal effects of EPN augmentation in 2004, but not 2005, EPN became temporarily less prevalent, and fewer sentinel weevil larvae died in EPN-augmented compared to non-augmented plots. Manure mulch had variable effects on endoparasitic NF, but consistently decreased the prevalence of trapping NF and increased the prevalence of EPN and the sentinel mortality. Both temporal and spatial abundance of NF were inversely related to the prevalence of Steinernema diaprepesi, whereas Heterorhabditis zealandica prevalence was positively correlated with NF over time. The number of weevil larvae killed by EPN was likely greatest in 2005, due in part to non-target effects of augmentation on the endemic EPN community in 2004 that occurred during a period of peak weevil recruitment into the soil.     

New citriculture system suppresses native and augmented entomopathogenic nematodes

Since its first detection in 2005, the bacterial disease huanglongbing (HLB or citrus greening) has emerged as a critical threat to the citrus industry in Florida. An “Advanced Production System” (APS) could mitigate the impact of HLB by bringing citrus trees into production more quickly and economically than conventional citriculture methods. However, unlike conventional practices, APS fertigates plants daily, thereby changing the soil properties in ways that might impact soil biota. We tested the hypothesis that changes to soil properties caused by APS would affect the abundance of native entomopathogenic nematodes (EPNs) and/or the survival of augmented EPNs. The densities of organisms at different trophic levels were measured by real-time qPCR in three experiments conducted in an ongoing field experiment. Target organisms included 6 entompathogenic nematodes, 5 nematophagous fungi (NF) and a phoretic bacterium, Paenibacillus sp. Soil properties, free-living nematodes and citrus fibrous roots were also measured. Compared to soil under conventional citriculture (CC), APS increased soil pH and Mg content, while reducing the electrical conductivity, and content of K, Mn and Fe. The naturally occurring EPN Steinernema diaprepesi was 5 times less abundant in APS plots where these nematodes were more heavily encumbered by the phoretic bacterium Paenibacillus sp., which limits the foraging success of EPNs. In general, when EPNs were augmented in either treatment, fewer Steinernema riobrave than Heterorhabditis indica were recovered and recovery of both species declined rapidly over time. As seen with native S. diaprepesi, fewer augmented S. riobrave were recovered from APS plots in two of the three experiments, whereas the management system did not affect the recovery of H. indica. More of some endoparasitic and trapping NF were recovered from soil augmented with S. riobrave than with H. indica. However, variation in the responses of NF to the management systems suggests that these NF species were not primarily responsible for the steinernematid responses to APS. Although APS has the potential to reduce EPN populations and exacerbate herbivory by subterranean pests such as the root weevil Diaprepes abbreviatus, additional study of the physical causes of this effect may reveal ways to avoid the problem.     

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.     

Efficacy of indigenous entomopathogenic nematodes (Rhabditida: Heterorhabditidae, Steinernematidae), from Rio Grande do Sul Brazil, against Anastrephafraterculus (Wied.) (Diptera: Tephritidae) in peach orchards

Laboratory, greenhouse, and field experiments were performed with the objective of selecting efficient indigenous strains of entomopathogenic nematodes (EPNs) from Rio Grande do Sul (RS) state, Brazil, for controlling the South American fruit fly, Anastrepha fraterculus (Wied.). Laboratory experiments were conducted in 24 well-plates filled with sterile sand and one insect per well. In greenhouse experiments, plastic trays filled with soil collected from the field were used, while in field experiments, holes were made in soil under the edge of peach tree canopies. Among 19 EPN strains tested, Heterorhabditis bacteriophora Poinar RS88 and Steinernema riobrave Cabanillas, Poinar, & Raulston RS59 resulted in higher A. fraterculus larval (pre-pupal) and pupal mortality, with LD₉₀ of 1630, 457 and 2851, 423 infective juveniles (IJs)/cm², respectively. Greenhouse experiments showed no differences in pupal mortality at 250 and 500 IJs/cm² of either nematode. In the field, H. bacteriophora RS88 and S. riobravae RS59 sprayed individually over natural and artificially infested fruit (250 IJs/cm²) resulted in A. fraterculus larval mortality of 51.3%, 28.1% and 20%, 24.3%, respectively. There was no significant difference in A. fraterculus pupal mortality sprayed with an aqueous suspension of either nematode; however, when using infected insect cadavers, H. bacteriophora RS88 was more efficient than S. riobrave RS59. Our results showed that H. bacteriophora RS88 was more virulent to insect larvae, with an efficient host search inside the infested fruit and control of pupae in the soil after being applied by aqueous suspension or infected cadavers.     

Management to control citrus greening alters the soil food web and severity of a pest–disease complex

Since 2005, the Florida citrus industry has faced the need to control the devasting bacterial disease huanglongbing (HLB). Advanced production systems (APS) were designed to grow citrus trees more quickly than conventional citriculture (CC) methods in order to mitigate the impact of HLB. Daily fertigation required by APS produces changes in the soil physical–chemical properties compared to those in conventionally managed orchards. We used real-time PCR in an ongoing field experiment to compare the effects of APS and CC on more than a dozen metazoan and microorganism species in soil food webs that affect larvae of a major arthropod pest of citrus, Diaprepes abbreviatus. Soil chemical properties, citrus performance, weevil occurrence, and abundance of free-living and plant-parasitic nematodes were also evaluated. The effects of polypropylene mulch that provides a barrier to soil entry by D. abbreviatus larvae were also investigated in each of the two cultural systems. Trees grew significantly larger in APS and mulching increased tree growth and reduced tree mortality, thereby increasing fruit yield per ha. APS increased the fruit yield in 2011; however, by 2013 the number of fruit per tree was not affected by any of the treatments. Root mass density increased in APS, but decreased under mulch. The numbers of plant-parasitic and free-living nematodes and some natural enemies of nematodes such as Catenaria sp. and Paecilomyces lilacinus were more abundant in the treatments with greater root mass density. Both organisms in the D. abbreviatus–Phytophthora nicotianae pest–disease complex were more abundant in APS than in CC, whereas fewer steinernematid entomopathogenic nematodes (EPNs) that prey on insect larvae occurred in APS. By contrast, heterorhabditid EPNs tended to be more numerous in APS than in CC, although they comprised <25% of the EPN communities in any treatment. Major differences between APS and CC in almost all of the measured physical and chemical soil properties provide a basis for controlled studies to understand why EPN taxa responded differently to these treatments and how APS and soils generally might be modified to conserve the beneficial activities of nematodes.     

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.     

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

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

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.     

A Comparison of Entomopathogenic Nematode Longevity in Soil under Laboratory Conditions

We compared the longevity of 29 strains representing 11 entomopathogenic nematode species in soil over 42 to 56 d. A series of five laboratory experiments were conducted with six to eight nematode strains in each and one or more nematode strains in common, so that qualitative comparisons could be made across experiments. Nematodes included Heterorhabditis bacteriophora (four strains), H. indica (Homl), H. marelatus (Point Reyes), H megidis (UK211), H. mexicana (MX4), Steinernema carpocapsae (eight strains), S. diaprepesi, S. feltiae (SN), S. glaseri (NJ43), S. rarum (17C&E), and S. riobrave (nine strains). Substantial within-species variation in longevity was observed in S. carpocapsae, with the Sal strain exhibiting the greatest survival. The Sal strain was used as a standard in all inter-species comparisons. In contrast, little intra-species variation was observed in S. riobrave. Overall, we estimated S. carpocapsae (Sal) and S. diaprepesi to have the highest survival capability. A second level of longevity was observed in H. bacteriophora (Lewiston), H. megidis, S. feltiae, and S. riobrave (3–3 and 355). Lower levels of survivability were observed in other H. bacteriophora strains (Hb, HP88, and Oswego), as well as S. glaseri and S. rarum. Relative to S. glaseri and S. rarum, a lower tier of longevity was observed in H. indica and H. marelatus, and in H. mexicana, respectively. Although nematode persistence can vary under differing soil biotic and abiotic conditions, baseline data on longevity such as those reported herein may be helpful when choosing the best match for a particular target pest.