Location: Fruit and Tree Nut ResearchTitle: Directional movement of entomopathogenic nematodes in response to electrical fields: Effects of species, magnitude of voltage, and infective juvenile age Author
Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2011
Publication Date: 1/1/2012
Citation: Shapiro Ilan, D.I., Campbell, J.F., Lewis, E.E., Kim-Shapiro, D.B. 2012. Directional movement of entomopathogenic nematodes in response to electrical fields: Effects of species, magnitude of voltage, and infective juvenile age. Journal of Invertebrate Pathology. 109:34-40. Interpretive Summary: Entomopathogenic (insect-killing) nematodes are small round worms that are used as environmentally safe bio-insecticides. In order to improve the use of these nematodes in sustainable pest control approaches, we want to understand their basic behavior, e.g., how do they find insect hosts to attack. Previously, we showed that two species (kinds) of nematodes respond to electrical fields. From this previous work, we hypothesized that entomopathogenic nematode response to electrical fields might help them find insect hosts in the soil. Also, we hypothesized that the response of nematodes varies by species, and might be related to their foraging ("hunting") strategy. Additionally, we determined the lowest intensity of current that would elicit a directional response (tested in S. feltiae and S. carpocapsae), and we investigated the impact of nematode age on response to electrical current. Our results supported our hypothesis. Nematode species that tend to take an ambush type of foraging strategy tended to move toward electrical fields, whereas other types of foragers (cruisers) moved away from the electrical fields. Also, we observed that nematode response to electrical fields diminishes with age, and we discovered that the sensitivity of nematodes in detecting electrical fields is similar to the level of voltage produced on some insect surfaces, which supports our hypothesis that the nematodes might use the electrical fields in navigating to find their food source. These results contribute to our understanding of nematode foraging behavior and support the idea that electrical fields may be a novel cue in host-finding for these nematodes.
Technical Abstract: Entomopathogenic nematodes respond to a variety of stimuli when foraging. Previously, we reported a directional response to electrical fields for two entomopathogenic nematode species; specifically, when electrical fields were generated on agar plates Steinernema glaseri (a nematode that utilizes a cruiser-type foraging strategy) moved to a higher electric potential, whereas S. carpocapsae, an ambush-type forager, moved to a lower potential. Thus, we hypothesized that entomopathogenic nematode directional response to electrical fields varies among species, and may be based on foraging strategy. In this study, we tested the hypothesis by comparing directional response among seven additional nematode species: Heterorhabditis bacteriophora, H. georgiana, H. indica, H. megidis, S. feltiae, S. riobrave, and S. siamkayai. S. carpocapsae and S. glaseri were also included as positive controls. Heterorhabditids tend toward cruiser foraging approaches whereas S. siamkayai is an ambusher and S. feltiae and S. riobrave are intermediate. Additionally, we determined the lowest intensity of current that would elicit a directional response (tested in S. feltiae and S. carpocapsae), and we investigated the impact of nematode age on response to electrical current in S. carpocapsae. In the experiment measuring diversity of response among species, we did not detect any response to electrical fields among the heterorhabditids except for H. georgiana, which moved to a higher electrical potential; S. glaseri and S. riobrave also moved to a higher potential, whereas S. carpocapsae, S. feltiae, and S. siamkayai moved to a lower potential. Overall our hypothesis that foraging strategy can predict directional response was supported. The lowest level of current that elicited was generated 0.1 volts, which is comparable to electrical potential associated with some insects and plant roots. The response to electrical current diminished with nematode age. These results expand our knowledge of electrical fields as cues that may be used by entomopathogenic nematodes for host-finding or other aspects of fitness support.