Influence of a Native Weed, Acalypha ostryaefolia (Euphorbiaceae), on Coleomegilla maculata (Coleoptera: Coccinellidae) Population Density, Predation, and Cannibalism in Sweet Corn

Abstract

We investigated the density of Coleomegilla maculata (DeGeer), predation by C. maculata on Helicoverpa zea (Boddie) eggs, and egg cannibalism by C. maculata when the weed Acalypha ostryaefolia Riddell was grown in stands of sweet corn ( Zea mays L.). Densities of C. maculata eggs and larvae were significantly higher in weedy plots than in weed-free plots during each of the 3 yr the study was done. Pupal density was significantly higher in weedy plots in 1995 and 1996, but not in 1997. Adult C. maculata density was significantly higher in weedy plots in 1995, but not in 1996 or 1997. Egg cannibalism by adult and larval C. maculata is common on sweet corn plants in Kentucky. C. maculata frequently oviposits on A. ostryaefolia in and around sweet corn plots, and our preliminary observations suggested egg cannibalism was less frequent on A. ostryaefolia than on sweet corn plants. There was a trend for higher abundance of C. maculata eggs on corn plants in weed-free plots than on corn plants in weedy plots; in weedy plots, >85% of eggs were on A. ostryaefolia plants rather than on corn plants. However, larvae were always more abundant on corn plants in weedy plots than on corn plants in weed-free plots and significantly so in 1995 and 1997. Predation of H. zea eggs was significantly higher on corn plants in weedy plots than on corn plants in weed-free plots, whereas cannibalism of C. maculata eggs was significantly higher on corn plants in weedy plots only during 1 of the 3 yr. Thus, presence of A. ostryaefolia in sweet corn plots increased C. maculata egg and larval population densities and predation of H. zea eggs. Direct observations of H. zea egg predation and egg cannibalism by C. maculata revealed that adults predominantly were diurnal but larvae fed day and night.