Host plant and substrate effects on mortality of southern corn rootworm from entomopathogenic nematodes

Bioassays were conducted to determine the effects of host plant and substrate type on the susceptibility of Diabrotica undecimpunctata howardi larvae (southern corn rootworm) to entomopathogenic nematodes. Larvae were reared on corn, peanuts, and two squash species. One squash species produces bitter cucurbitacin E, whereas the second squash lacks these secondary compounds. Nematodes of the species Heterorhabditis bacteriophora (Lewiston strain) and Steinernema carpocapsae (Mexican strain) were applied at two concentrations in assays conducted in soil and sand to which rootworm larvae were transferred from different host plants. Host plants and substrate type significantly affected larval susceptibility to both nematode species. The hypothesis that ingested cucurbitacins serve as defense compounds for diabroticites was not supported by the results obtained in this study. D. undecimpunctata howardi larvae suffered higher mortality when reared on both squash species compared to larvae reared on corn or peanuts. Mortality caused by H. bacteriophora was significantly higher in soil than in sand. Generally, S. carpocapsae caused 10 times greater mortality than H. bacteriophora.