Comparison of Entomopathogenic Nematode Infectivity from Infected Hosts Versus Aqueous Suspension

Abstract

Infectivity is an important measure of virulence in insect pathogens. The proportion of entomopathogenic nematodes that invaded a host, Galleria mellonella (L.), was used as a measure of infectivity in 2 species of entomopathogenic nematodes: S teinernema carpocapsae (Weiser) and Heterorhabditis bacteriophora Poinar. We compared infectivity between nematodes emerging directly from cadavers into sand with nematodes that were applied to sand in aqueous suspensions after collection in a White trap. Assay arenas consisted of 10 G. mellonella larvae in petri dishes filled with moist sand. After 20 or 44 h the larvae were removed and the number of nematodes that had invaded was counted after dissection. Infectivity of H. bacteriophora was significantly greater when nematodes emerged directly into sand compared with the aqueous application. After 24 h, a11% of H. bacteriophora from cadaver treatments had penetrated into a host, whereas infectivity of aqueous treatments was 4–10 times lower. A similar trend was observed after 48 h. The infectivity of S. carpocapsae from cadaver treatments was a24% in both treatments. An additional experiment was conducted for H. bacteriophora, which consisted of 3 treatments: nematodes directly from cadavers, nematodes applied in aqueous suspension, and nematodes in aqueous suspension that were exposed to an extract from macerated host cadavers. Higher infectivity was observed in the amended aqueous suspension compared with aqueous nematodes without the extract. This study emphasizes that differences in fitness and behavior must exist between nematodes reared under standard laboratory procedures and those in nature.