The potential for rootworm damage to corn following corn is greatest in the northern two-thirds of the state. However, moderate to severe damage to corn roots may occur in any field where corn is planted after corn or where corn is planted after soybeans in east-central Illinois. The abundance of rootworm beetles in a cornfield during the summer of 1997 will largely determine the potential for rootworm larval injury if corn is planted in the same field in 1998. Generally, if beetle numbers reached or exceeded 0.75 per plant at any time during late July, August, or September 1997, rootworm larvae may cause economic damage in that field in 1998. The ratio of female to male beetles in first-year corn is usually higher than in continuous corn.
Females migrate more than males, so most of the beetles found in first-year corn are females.
As a consequence, the threshold for determining whether to rotate away from corn or to use a soil insecticide in 1998 may be as low as 0.5 beetle per plant. Fields of corn planted in June 1997 may have exten-sive rootworm damage if they are planted to corn again in 1998. Seeking fresh pollen and silks to feed on, the beetles lay millions of eggs in these fields. Planting the fields to a crop other than corn in 1998 is suggested. Other management tactics are application of a soil insecticide at planting or during cultivation.