Effects of storage temperature on survival and infectivity of three indigenous entomopathogenic nematodes strains (Steinernematidae and Heterorhabditidae) from Meghalaya, India

Natural enemies of herbivores maintain their biological control potential under short-term exposure to future CO2, temperature, and precipitation patterns

Climate change will profoundly alter the physiology and ecology of plants, insect herbivores, and their natural enemies, resulting in strong effects on multitrophic interactions. Yet, manipulative studies that investigate the direct combined impacts of changes in CO2, temperature, and precipitation on the third trophic level remain rare. Here, we assessed how exposure to elevated CO2, increased temperature, and decreased precipitation directly affect the performance and predation success of species from four major groups of herbivore natural enemies: an entomopathogenic nematode, a wolf spider, a ladybug, and a parasitoid wasp. A four-day exposure to future climatic conditions (RCP 8.5), entailing a 28% decrease in precipitation, a 3.4°C raise in temperature, and a 400 ppm increase in CO2 levels, slightly reduced the survival of entomopathogenic nematodes, but had no effect on the survival of other species. Predation success was not negatively affected in any of the tested species, but it was even increased for wolf spiders and entomopathogenic nematodes. Factorial manipulation of climate variables revealed a positive effect of reduced soil moisture on nematode infectivity, but not of increased temperature or elevated CO2. These results suggest that natural enemies of herbivores may be well adapted to short-term changes in climatic conditions. These findings provide mechanistic insights that will inform future efforts to disentangle the complex interplay of biotic and abiotic factors that drive climate-dependent changes in multitrophic interaction networks.

EPNs Exhibit Repulsion to Prenol in Pluronic Gel Assays

Entomopathogenic nematodes (EPNs) are lethal parasites of insects that have become valuable in biological control and as a model system for studying host-parasite interactions, behavioral ecology, neurobiology, and genomics, among other fields. Their ability to locate hosts is paramount to successful infection and host seeking has been extensively studied in many species in the lab. Here, we explored the usefulness of pluronic gel as a medium to assess EPN host seeking in the lab by characterizing the response of Steinernema carpocapsae, S. feltiae, S. glaseri, S. riobrave, Heterorhabditis bacteriophora, and H. indica to the odor prenol. We found that the infective juveniles (IJs) of these species were repelled by prenol in pluronic gel. We then evaluated how storing the IJs of S. carpocapsae, S. feltiae, and S. glaseri for different amounts of time affected their behavioral responses to prenol. The response of S. carpocapsae was significantly affected by the storage time, while the responses of S. feltiae and S. glaseri were unaffected. Our data support the notion that pluronic gel is a useful medium for studying EPN behavior and that the response of S. carpocapsae to informative odors is significantly affected by long-term storage.

Bioactive Excreted/Secreted Products of Entomopathogenic Nematode Heterorhabditis bacteriophora Inhibit the Phenoloxidase Activity during the Infection

Entomopathogenic nematodes (EPNs) are efficient insect parasites, that are known for their mutualistic relationship with entomopathogenic bacteria and their use in biocontrol. EPNs produce bioactive molecules referred to as excreted/secreted products (ESPs), which have come to the forefront in recent years because of their role in the process of host invasion and the modulation of its immune response. In the present study, we confirmed the production of ESPs in the EPN Heterorhabditis bacteriophora, and investigated their role in the modulation of the phenoloxidase cascade, one of the key components of the insect immune system. ESPs were isolated from 14- and 21-day-old infective juveniles of H. bacteriophora, which were found to be more virulent than newly emerged nematodes, as was confirmed by mortality assays using Galleria mellonella larvae. The isolated ESPs were further purified and screened for the phenoloxidase-inhibiting activity. In these products, a 38 kDa fraction of peptides was identified as the main candidate source of phenoloxidase-inhibiting compounds. This fraction was further analyzed by mass spectrometry and the de novo sequencing approach. Six peptide sequences were identified in this active ESP fraction, including proteins involved in ubiquitination and the regulation of a Toll pathway, for which a role in the regulation of insect immune response has been proposed in previous studies.

Temperature-dependent behaviors of parasitic helminths

Parasitic helminth infections are the most common source of neglected tropical disease among impoverished global communities. Many helminths infect their hosts via an active, sensory-driven process in which environmentally motile infective larvae position themselves near potential hosts. For these helminths, host seeking and host invasion can be divided into several discrete behaviors that are regulated by both host-emitted and environmental sensory cues, including heat. Thermosensation is a critical sensory modality for helminths that infect warm-blooded hosts, driving multiple behaviors necessary for host seeking and host invasion. Furthermore, thermosensory cues influence the host-seeking behaviors of both helminths that parasitize endothermic hosts and helminths that parasitize insect hosts. Here, we discuss the role of thermosensation in guiding the host-seeking and host-infection behaviors of a diverse group of helminths, including mammalian-parasitic nematodes, entomopathogenic nematodes, and schistosomes. We also discuss the neural circuitry and molecular pathways that underlie thermosensory responses in these species.

Prolonged Storage Increases Virulence of Steinernema Entomopathogenic Nematodes Toward Drosophila Larvae

Entomopathogenic nematodes are excellent organisms for dissecting the molecular basis of parasitism and probing the insect innate immune system. The nematode parasite Steinernema carpocapsae is a potent pathogen of insects that has emerged recently as a model for parasitic infection and anti-nematode immune signaling and response. The nematodes are mutualistically associated with the bacteria Xenorhabdus nematophila, which are also pathogenic to insects. Separation of nematodes from their associated bacteria facilitates mechanistic studies focusing on the impact of the parasites without considering the contribution of their bacterial partners. An important aspect in insect infection experiments with entomopathogenic nematodes includes the storage duration of the parasites. Here we have infected larvae of the model insect Drosophila melanogaster with S. carpocapsae nematodes that had been stored for 3 wk or 3 mo. Survival data consistently revealed that infective juveniles with prolonged storage exhibit substantially increased virulence toward D. melanogaster larvae compared with those that had been stored for a shorter time, and the presence of mutualistic X. nematophila in the nematodes does not influence this result. Although the basis for this effect is currently unknown, these surprising findings indicate that prolonged nematode storage can markedly alter virulence. This is significant knowledge that should be taken into account in functional assays involving infection with parasitic nematodes. Future efforts will focus on the identification and characterization of the factors that might determine the interrelationship between prolonged storage and virulence in nematode parasites.

Characterization of a new isolate of entomopathogenic nematode, Steinernema sangi (Rhabditida, Steinernematidae), and its symbiotic bacteria Xenorhabdus vietnamensis (γ-Proteobacteria) from Mizoram, northeastern India

A survey was conducted to isolate indigenous EPN, specifically from the northeastern part of India, a biodiversity hotspot region, to record the occurrence and their further use as biological control agent. The morphological and molecular analysis (ITS rDNA for Steinernema and 16S rRNA for Xenorhabdus) revealed that the entomopathogenic nematodes isolated from four different habitats and its symbiotic bacteria are conspecific with Steinernema sangi and Xenorhabdus vietnamensis respectively. The phylogenetic analysis based on maximum parsimony (MP) revealed that Steinernema sangi belongs to feltiae-kraussei-oregonense group. The study constitutes the first report of Steinernema sangi and its symbiotic bacteria Xenorhabdus vietnamensis outside the type locality, Vietnam, and in particular from India.