Effect of Dietary Cucurbitacin C on Southern Corn Rootworm (Coleoptera: Chrysomelidae) Egg Survival

Laboratory, greenhouse, and field studies were conducted to determine whether the eggs of southern corn rootworm, Diabrotica undecimpunctata howardi Barber, from adults that had fed on a bitter (cucurbitacin C-containing) cucumber variety experience less predation than eggs from beetles that had fed on a nonbitter (noncucurbitacin C-containing) cucumber variety. Human taste tests and insect bioassays indicated the presence of cucurbitacin in the “bitter” eggs. Eggs from adults that had fed upon bitter and nonbitter cucumber cotyledons and early foliage were placed into soil at three different distances from one another and at four different density combinations of bitter:nonbitter eggs. There were no significant differences in predation of eggs by any of the four arthropod predators tested caused by cucumber variety or by egg distance and density. In a laboratory experiment, eggs from adults feeding on bitter and nonbitter cucumber varieties were compared for percentage egg hatch without moisture stress (held at 100% RH) or stressed for 6, 12, or 18 h at 94% RH before being held at 100% RH. Bitter eggs in both unstressed and stressed for 6 h treatments hatched significantly earlier than nonbitter eggs. There was no significant difference in hatched rate between bitter and nonbitter eggs stressed for 12 or 18 h. The presence of cucurbitacin C does not appear to protect southern com rootworm eggs from predation by the arthropod predators tested compared with nonbitter eggs. However, more rapid development and earlier eclosion of bitter eggs relative to nonbitter eggs may help bitter eggs survive in soils that fluctuate between dry and moist conditions.