The saprophytic fungus Fusarium solani increases the insecticidal efficacy of the entomopathogenic nematode Steinernema diaprepesi.

In two field surveys, high proportions of Galleria mellonella L. (Lepidoptera: Pyralidae) sentinel larval cadavers were infected by Fusarium solani without evidence of concomitant entomopathogenic nematode (EPN) or entomopathogenic fungus (EPF) reproduction. Because F. solani is not considered entomopathogenic, the survey suggested the possibility that F. solani competes with EPNs. We tested the hypotheses that F. solani attracts the EPN, Steinernema diaprepesi, to facilitate infection of Diaprepes root weevils (Diaprepes abbreviatus L.) and thereafter competes with the nematode in the insect cadaver. In two-choice olfactometer assays where one side was treated with F. solani mycelia and conidia, juvenile S. diaprepesi were attracted to the fungus, in either raw soil, or in autoclaved soil in the presence or absence of insects. However, this attraction was attenuated as the habitat became more complex, by using raw soil in combination with insect larvae. Fusarium oxysporum did not recruit the nematode. When soil microcosms were tested with F. solani conidia and S. diaprepesi, the concomitant infection increased the mortality of the insect (P = 0.02) to 83%, compared to 58% and 0% mortality when nematodes or fungi were individually applied, respectively. Concomitant inoculation also increased the number of cadavers that supported nematode reproduction and increased the population density of fungus in soil. The number of IJs entering the host insect was not affected by F. solani. These results support the possibility that F. solani can facilitate the insecticidal efficiency of S. diaprepesi in order to exploit the resources in the cadaver.

Responses of Anastrepha suspensa, Diachasmimorpha longicaudata, and Sensitivity of Guava Production to Heterorhabditis bacteriophora in Fruit Fly Integrated Pest Management.

Caribbean fruit fly, also known as Caribfly or Anastrepha suspensa , is a major tephritid pest of guavas. A virulent entomopathogenic nematode (EPN) species was investigated to suppress the fruit-to-soil stages of Caribflies, which are also attacked by the koinobiont parasitoid Diachasmimorpha longicaudata in south Florida. The main objective was to develop a feasible and cost-effective EPN-application method for integrated pest management (IPM) of Caribfly to improve guava production. Naturally infested guavas were treated with increasing Heterorhabditis bacteriophora infective juvenile (IJ) concentration or rate (0, 25, 50, …, 1,600 IJs cm -2 ) in field trials to measure the optimum IJ rate and then examine sensitivity of producing guavas to inclusion of Heterorhabditis bacteriophora in Caribfly IPM plans. Relative survival of Caribfly in treatments significantly decreased with increasing IJ rate from 0 to 100 IJs cm -2 . Similarly, probability of observing large numbers of parasitoid wasps ( Diachasmimorpha longicaudata ) in EPN treatments significantly declined with increasing IJ rate (0-100 IJs cm -2 ), even though the non-target effects of Heterorhabditis bacteriophora on relative survival of Diachasmimorpha longicaudata could not be determined because of few emerging parasitoid wasps. Optimum suppression (⩾ 60%) of Caribfly was consistently achieved at 100 IJs cm -2 or 17,500 IJs fruit -1 . Profitability analysis showed that Heterorhabditis bacteriophora can be included in Caribfly IPM tactics to produce guavas. Costs of EPNs in Caribfly IPM are minimized if Heterorhabditis bacteriophora is strategically applied by spot treatment of fruit. Repayment of costs of EPN-augmentation by spot treatments appears achievable by recovering 5.71% of the annual yield losses (⩾1,963 kg ha -1 ≈ US$ 8,650 ha -1 ), which are largely due to Caribfly infestation. Hectare-wide EPN-augmentation (or broadcasting) method requires more fruit recovery than the total annual yield losses to repay its high costs. Profitability of guava production in south Florida will not be very sensitive to marginal costs of the spot treatment method, when compared to the field-wide broadcasting of Heterorhabditis bacteriophora .

Spatial relationships between entomopathogenic nematodes and nematophagous fungi in Florida citrus orchards.

Relationships between entomopathogenic nematodes (EPNs), nematophagous fungi (NF) and soil physical and chemical properties were studied in a survey of 53 citrus orchards in central ridge and flatwoods ecoregions of Florida. Seven species of NF associated with nematodes were quantified directly using a real time qPCR assay. All nematophagous fungi studied except Arthrobotrys musiformis and Hirsutella rhossiliensis were frequently detected (24-56%) in both regions. Paecilomyces lilacinus and Gamsylella gephyropagumwere encountered more frequently in the flatwoods (P=0.03) and on the ridge (P=0.02), respectively. Redundancy analysis revealed seven abiotic and biotic factors as significantly related to the NF occurrence. Multiple regression of fungi on these variables explained 78%, 66%, 48%, 36%, 23% and 4% of the variation in Catenaria sp., A. musiformis, A. dactyloides, P. lilacinus, A. oligospora and G. gepharopagum, respectively. When the data from citrus were pooled with those reported previously from natural areas and subjected to principle component analysis, the first two principle components explained 43% of the variation in NF communities. The surveys (citrus vs natural areas) were discriminated by PC2 (P<0.001) and the ecoregion by PC1 (P<0.002), and all but one NF species were related (P<0.01) to one or both components. NF communities tended to have more species and greater diversity in the flatwoods, where EPN richness and diversity were the least. However, the strength of associations between individual EPN and NF species as measured by SADIE reflected the associations between each species and ground water depth, suggesting that ecoregion preferences affected the species associations. Within each ecoregion, significant relationships between the individual NF and EPN species measured by stepwise regression tended to be positive. The results did not support the hypothesis that NF modulate the spatial patterns of EPN species between or within these two ecoregions.

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.

Wide interguild relationships among entomopathogenic and free-living nematodes in soil as measured by real time qPCR.

Entomopathogenic nematodes (EPNs) are promising biological control agents of soil-dwelling insect pests of many crops. These nematodes are ubiquitous in both natural and agricultural areas. Their efficacy against arthropods is affected directly and indirectly by food webs and edaphic conditions. It has long been suggested that a greater understanding of EPN ecology is needed to achieve consistent biological control by these nematodes and the development of molecular tools is helping to overcome obstacles to the study of cryptic organisms and complex interactions. Here we extend the repertoire of molecular tools to characterize soil food webs by describing primers/probe set to quantify certain free-living, bactivorous nematodes (FLBNs) that interact with EPNs in soil. Three FLBN isolates were recovered from soil baited with insect larvae. Morphological and molecular characterization confirmed their identities as Acrobeloides maximum (RT-1-R15C and RT-2-R25A) and Rhabditis rainai (PT-R14B). Laboratory experiments demonstrated the ability of these FLBNs to interfere with the development of Steinernema diaprepesi, Steinernema riobrave and Heterorhabditis indica parasitizing the weevil Diaprepes abbreviatus (P<0.001), perhaps due to resource competition. A molecular probe was developed for the strongest competitor, A. maximum. We selected the highly conserved SSU rDNA sequence to design the primers/probe, because these sequences are more abundantly available for free-living nematodes than ITS sequences that can likely provide better taxonomic resolution. Our molecular probe can identify organisms that share ⩾98% similarity at this locus. The use of this molecular probe to characterize soil communities from samples of nematode DNA collected within a citrus orchard revealed positive correlations (P<0.01) between Acrobeloides-group nematodes and total numbers of EPNs (S. diaprepesi, H. indica and Heterorhabditis zealandica) as well as a complex of nematophagous fungi comprising Catenaria sp. and Monachrosporium gephyropagum that are natural enemies of EPNs. These relationships can be broadly interpreted as supporting Linford's hypothesis, i.e., decomposition of organic matter (here, insect cadavers) greatly increases bactivorous nematodes and their natural enemies.

Entomopathogenic nematodes, root weevil larvae, and dynamic interactions among soil texture, plant growth, herbivory, and predation.

Greenhouse experiments were conducted to assess the influence of soil texture on the persistence, efficacy and plant protection ability of entomopathogenic nematodes (EPNs) applied to control larvae of the Diaprepes root weevil (DRW), Diaprepes abbreviatus, infesting potted citrus seedlings. Seedlings were grown in pots containing either coarse sand, fine sand, or sandy loam. Three DRW larvae were added to each of 80 pots of each soil type. 24 h later, 20 pots of each soil type that had received weevil larvae were inoculated with EPN infective juveniles (IJs) of one of the following species: Steinernema diaprepesi, Steinernema riobrave and Heterorhabditis indica. Pots of each soil without EPNs were established as controls with DRW and controls without DRWs. Subsequently, pots with larvae received three additional larvae monthly, and the experiment continued for 9 months. Plant root and top weights at the end of the experiment were affected by both soil (P≤0.0001) and nematodes (P≤0.0001), and nematode species protected plants differently in different soils (interaction P≤0.0001). Soil porosity was inversely related to plant damage by DRW, whether or not EPNs were present; and porosity was directly related to the level of plant protection by EPNs. Mortality of caged sentinel weevil larvae placed in pots near the end of the experiment was highest in pots treated with S. diaprepesi. In a second, similar experiment that included an additional undescribed steinernematid of the Steinernema glaseri-group, soil type affected root damage by DRW and root protection by EPNs in the same manner as in the first experiment. Final numbers of S. diaprepesi and Steinernema sp. as measured by real-time PCR were much greater than those of S. riobrave or H. indica in all soils. Across all treatments, the number of weevil larvae in soil at the end the experiment was inversely related to soil porosity. In all soils, fewer weevil larvae survived in soil treated with S. diaprepesi or Steinernema sp. than in controls with DRW or treatments with S. riobrave or H. indica. The results of these experiments support the hypothesis that EPNs provide greater protection of seedlings against DRW larvae in coarse textured soil than in finer textured soil. However, less vigorous growth of the control without DRW seedlings in the two finer textured soils suggests that unidentified factors that stressed seedlings in those soils also impaired the ability of seedlings to tolerate weevil herbivory.

Substrate modulation, group effects and the behavioral responses of entomopathogenic nematodes to nematophagous fungi.

Laboratory experiments were conducted on the behavioral responses of five species of entomopathogenic nematodes (EPNs; Steinernema diaprepesi, Steinernema sp. glaseri-group, Steinernema riobrave, Heterorhabditis zealandica, Heterorhabditis indica) to three species of nematophagous fungi (NF; trapping fungus Arthrobotrys gephyropaga; endoparasites Myzocytium sp., Catenaria sp.). We hypothesized that EPN responses to NF and their putative semiochemicals might reflect the relative susceptibility of EPNs to particular NF species. EPN responses to "activated" NF (i.e., induced to form traps or sporangia by previous interactions with nematodes) versus controls of non-activated NF or heat-killed EPNs were compared in choice experiments on water agar in Petri dishes (dia=9 cm) and in horizontal sand columns (8 cm L×2.7 cm dia). On agar, all EPN species were attracted to all activated NF species except for S. riobrave, which was neutral. In sand, all EPN species were repelled by activated Arthrobotrys but attracted to activated Myzocytium and Catenaria, except H. indica (neutral to Myzocytium) and Steinernema sp. (neutral to Catenaria). EPN behavioral responses appeared unrelated to relative susceptibility to NF except that H. indica exhibited low susceptibility and a neutral response to Myzocytium in sand whereas the remaining EPNs were highly susceptible and attracted. These results indicate potential complexity (i.e., mixed responses, aggregation or group movement) and species specificity in the responses of EPNs to NF, demonstrate that results on agar can differ markedly from those in sand, and underline the potential importance of utilizing natural substrates to properly assess the role of semiochemicals in nematode-fungus interactions.

Entomopathogenic nematodes, phoretic Paenibacillus spp., and the use of real time quantitative PCR to explore soil food webs in Florida citrus groves.

Quantitative real-time PCR (qPCR) is a powerful tool to detect and quantify species of cryptic organisms such as bacteria, fungi and nematodes from soil samples. As such, qPCR offers new opportunities to study the ecology of soil habitats by providing a single method to characterize communities of diverse organisms from a sample of DNA. Here we describe molecular tools to detect and quantify two bacteria (Paenibacillus nematophilus and Paenibacillus sp.) phoretically associated with entomopathogenic nematodes (EPNs) in the families Heterorhabditidae and Steinernematodae. We also extend the repertoire of species specific primers and TaqMan® probes for EPNs to include Heterorhabditis bacteriophora, Steinernema carpocapsae, Steinernema feltiae and Steinernema scapterisci, all widely distributed species used commercially for biological control. Primers and probes were designed from the ITS rDNA region for the EPNs and the 16S rDNA region for the bacteria. Standard curves were established using DNA from pure cultures of EPNs and plasmid DNA from the bacteria. The use of TaqMan probes in qPCR resolved the non-specificity of EPN and some bacterial primer amplifications whereas those for Paenibacillus sp. also amplified Paenibacillus thiaminolyticus and Paenibacillus popilliae, two species that are not phoretically associated with nematodes. The primer-probe sets for EPNs were able to accurately detect three infective juvenile EPNs added to nematodes recovered from soil samples. The molecular set for Paenibacillus sp. detected the bacterium attached to Steinernema diaprepesi suspended in water or added to nematodes recovered from soil samples but its detection decreased markedly in the soil samples, even when a nested PCR protocol was employed. Using qPCR we detected S. scapterisci at low levels in a citrus grove, which suggested natural long-distance spread of this exotic species, which is applied to pastures and golf courses to manage mole crickets (Scapteriscus spp.). Paenibacillus sp. (but not P. nematophilus) was detected in low quantities in the same survey but was unrelated to the spatial pattern of S. diaprepesi. The results of this research validate several new tools for studying the ecology of EPNs and their phoretic bacteria.

Biological control potential of entomopathogenic nematodes for management of Caribbean fruit fly, Anastrepha suspensa Loew (Tephritidae).

BACKGROUND: Caribbean fruit fly (Caribfly) is a serious economic insect pest because of development of larvae that hatch from eggs oviposited into fruits by female adults. This study assessed the virulence of twelve entomopathogenic nematode (EPN) isolates to Caribfly in laboratory bioassays as a starting point toward evaluation of management strategies for the fruit-to-soil-dwelling stages of A. suspensa in fields infested by Caribfly. RESULTS: Inoculation of A. suspensa with 1 mL of ca 200 IJs larva-1 killed Caribfly at either larval or pupal stage. Pupae were more resistant to EPN infections than larvae. Adult emergence from inoculated pupae in soil microcosms was significantly lower than that observed in filter paper assays. Longest or largest steinernematids suppressed emergence of more adult Caribfly from pupae in soils, whereas shorter heterorhabditids were more infectious to Caribfly larvae. The highest mortalities of A. suspensa were caused by exotic nematodes Steinernema feltiae and Heterorhabditis bacteriophora, followed by the native Heterorhabditis indica and the exotic Steinernema carpocapsae. CONCLUSION: Entomopathogenic nematodes reduced the development of Caribfly larvae and pupae to adult in our bioassays, suggesting that EPNs have potential for biological control of A. suspensa. Future work will assess management strategies, using the virulent EPNs, in orchards infested by A. suspensa. © 2016 Society of Chemical Industry.

Use of real-time PCR to discriminate parasitic and saprophagous behaviour by nematophagous fungi.

Entomopathogenic nematodes (EPNs) are important pathogens of soilborne insects and are sometimes developed commercially to manage insect pests. Numerous nematophagous fungal species (NF) prey on nematodes and are thought to be important in regulating natural or introduced EPN populations. However, nematophagy by these fungi in nature cannot be inferred using existing methods to estimate their abundance in soil because many of these fungi are saprophytes, resorting to parasitism primarily when certain nutrients are limiting. Therefore, we developed an assay to quantify NF DNA in samples of nematodes. Species-specific primers and TaqMan probes were designed from the ITS rDNA regions of Arthrobotrys dactyloides, Arthrobotrys oligospora, Arthrobotrys musiformis, Gamsylella gephyropagum and Catenaria sp. When tested against 23 non-target fungi, the TaqMan real-time PCR assay provided sensitive and target-specific quantification over a linear range. The amount of A. dactyloides or Catenaria sp. DNA in 20 infected nematodes, measured by real-time PCR, differed between fungal species (P=0.001), but not between experiments (P>0.05). However, estimates of relative NF parasitism using a bioassay with 20 nematodes infected by either species, differed greatly (P<0.001) depending on whether the fungi were alone or combined in the samples used in the assay. Tests done to simulate detection of NF DNA in environmental samples showed that, for all species, background genomic DNA and/or soil contaminants reduced the quantity of DNA detected. Nested PCR was ineffective for increasing the detection of NF in environmental samples. Indeed, real-time PCR detected higher amounts of NF DNA than did nested PCR. The spatial patterns of NF parasitism in a citrus orchard were derived using real-time PCR and samples of nematodes extracted from soil. The parasitism by Catenaria sp. was positively related to the abundance of both heterorhabditid and steinernematid EPNs. The possible significance of the associations is ambiguous because NF attack a broad range of nematode taxa whereas EPNs are a small minority of the total nematode population in a soil sample. These studies demonstrate the potential of real-time PCR to study the role of NF parasitism in soil food webs.