WESTERN CORN-ROOTWORM (COLEOPTERA, CHRYSOMELIDAE) LARVAL INJURY TO CORN GROWN FOR SEED PRODUCTION FOLLOWING SOYBEANS GROWN FOR SEED PRODUCTION

In late June 1987, severe western corn rootworm, Diabrotica virgifera virgifera LeConte, larval injury to corn grown for seed production (in bred corn) was observed within a 3-km(2) area near Piper City (Ford Co unty), IL. The rootworm injury occurred in 6 fields that in the prior year had been planted to weed-free soybeans grown for seed production. The severe rootworm injury problem reoccurred in 1st-yr seed corn in the same area in 1988 and to varying degrees in the years since (throu gh 1994). Laboratory and field studies were initiated in 1987 to inves tigate several possible causes of the problem. Under simulated field s oil temperature conditions, western corn rootworm eggs from the Piper City population did not show evidence of the prolonged diapause trait. Although higher than expected oviposition took place in Piper City so ybean fields, a large field study with staggered plantings of soybeans at Urbana, IL, < 75 km away, confirmed earlier studies that few weste rn corn rootworm eggs are laid in weed-free soybean fields. Pyrethroid insecticides are routinely used in seed corn for corn earworm, Helico verpa zea (Boddie), control. In laboratory bioassays, the pyrethroid i nsecticide, permethrin, repelled western corn rootworm female beetles from treated corn to lay eggs in untreated soybeans and demonstrated t hat western corn rootworm damage to 1st-yr seed corn could have been c aused by pyrethroid use the previous summer in adjacent cornfields.