Damage Potential of Adult and Larval Southern Corn Billbugs (Coleoptera: Curculionidae) on Field Corn in Eastern North Carolina

Adult and larval damage potential of the southern corn billbug (SCB), Sphenophorus callosus (Olivier), was studied in small-plot enclosures and in commercial cornfields in eastern North Carolina from 1978 to 1980. In small-plot studies, corn at the two- and four-leaf stages showed damage by adult SCB at several insect densities, but more mature corn was not visibly damaged. Female SCB caused more damage than male SCB on three- to four-leaf stage corn. Damage by females tended to result in dead terminals, whereas feeding by males more often resulted in holes in the leaves. In small-plot studies, average plant height per plot was negatively correlated with average damage ratings per plot on three dates in 1980 on whorl stage corn. However, grain yield per plot at harvest was positively correlated with average damage ratings per plot on 9- to lO-leaf stage corn. In commercial cornfields, larval SCB infestations were often associated with early death in corn plants. Of a total of 506 plants sampled at two sites in late July and early August 1980, 85.2% of plants dying early were infested with larval SCB, whereas nondying plants had 10.0% SCB infestations. This early death was associated with a 42.4% yield decrease in paired comparisons of adjacent dying and nondying plants. The southern cornstalk borer, Diatraea crambidoides (Grote), was the only other insect found in large numbers in the lower cornstalk region. However, its presence was not well related to the incidence of early death