Explanations for the Low Susceptibility of Leptinotarsa decemlineata to Steinernema carpocapsae

Leptinotarsa decemlineata is less susceptible to entompathogenic nematodes than many other insect species. The roles of host-finding ability by the nematode Steinernema carpocapsae and host nonself response (immunity) toward the nematodes were determined to partially explain this low susceptibility. An agar-based assay was used to assess the chemotactic responses of S. carpocapsae infective juveniles to host-derived cues. The nematodes were attracted to CO2 and feces of Galleria mellonella and Tenebrio molitor but were repelled by L. decemlineata feces. Examination of L. decemlineata larvae that survived exposure to S. carpocapsae revealed that the nematodes were often enclosed in hemocytic capsules in the hemocoel. When nematodes were injected directly into the hemocoel, an individual L. decemlineata larva could encapsulate up to 21 nematodes, but at loads above nine nematodes per larva at least one always escaped encapsulation resulting in insect death. Thus, the low susceptibility of L. decemlineata to S. carpocapsae is attributed, in part, to repellence of the host feces and to hemocytic encapsulation of penetrating nematodes. This paper adds to the understanding of the interactions between entomopathogenic nematodes and their insect hosts. This understanding is critical for the effective use of these pathogens as biological control agents.