How entomopathogenic nematodes recruit for mass invasion

Multiple studies have explored communication systems in entomopathogenic nematodes (EPNs), including the use of pheromones for dispersal and host finding as well as learning of host related volatile cues. However, few studies have investigated the compounds that cause aggregations of EPNs. In petri dish assays, infective juveniles of Heterorhabditis bacteriophora, Steinernema diaprepesi, and Steinernema riobrave would commonly aggregate around entry points on wax worms (Galleria mellonella). While all available insects would be infected and killed, visible aggregations would only form on a subset of hosts. This suggests nematodes could be utilizing a pheromone to recruit nearby conspecifics for mass invasion. To study this behavior, wax worms with aggregated IJ’s were gently solvent washed. The extract was, after filtering, used for behavioral studies and chemical analysis. Larvae without aggregations were treated in the same way to produce a control extract. When the extracts were applied to agar plates, conspecific IJs exhibited positive chemotaxis towards the aggregation extract. However, when nematodes were exposed to the aggregation extract of other nematode species, reduced or no chemotaxis was observed. In sand column assays, the infectivity of the IJs was increased when a wax worm was dosed with conspecific aggregation extract compared to dosing with the control extract. These results indicate that EPNs produce species-specific recruitment pheromones during the process of host infection leading to increased infectivity. By semipreparative liquid chromatography, biological activity of each extract was narrowed to a limited number of fractions, some of which showing species specific activity.