By Janet J. Knodel
Extension Entomologist
Northern and western corn rootworm adults were observed in field corn near Page, northern Cass County. The adult northern corn rootworm is green beetle and about ¼ inch long. The adult western corn rootworm is yellow with black stripes on their wing covers and about 5/16 inch long. The wing covers of the male western corn rootworm is almost entirely black compared to the female’s wing covers. Females are often larger than males with extended abdomens (filled with eggs).
Corn rootworm beetles feed on the leaves, silk and pollen of corn. Occasionally, beetles congregate and feed on silks during early pollen shed. If silks are chewed back to the tips of ears (less than ½ inch of silks protruding) during the period of maximum pollen shed, poor pollination and grain set can occur. The action threshold is when an average of 5 or more beetles per silk mass is found during the first week of pollen shed. Another management threshold uses silk clipping. When silk clipping is occurring on 50% of the plants during pollen shed, control would be justified.