The western corn rootworm season is progressing at an unprecedented pace, professor of entomology and crop sciences Extension coordinator Mike Gray reports.
Western Cass County, Illinois, has reports of severe injury to Bt corn that expresses the Cry3Bb1 protein targeted against rootworms. Gray notes that resistance to this protein in Illinois has not yet been confirmed, but says large numbers of western corn rootworm adults are visible on the whorls of plants in the county. The beetles were observed early last week -- nearly a month ahead of normal.
The plants were under intense moisture stress, the leaves were rolled tightly, and beetles were feeding on the epidermis of corn leaves. "This type of injury often occurs when beetle emergence is early and plants have not yet begun to shed pollen or produce silks," Gray said. The plants in the fields that were visited were not at this reproductive stage of development.
No lodging was observed, likely because the plants were short and the soil was hard and dry. "However, as root feeding continues and plants become taller, lodging should be expected, especially if storms with rain and winds materialize," said Gray.
The fields in Cass County had been in continuous corn for many years (at least 10 consecutive years). Moreover, the same trait (Cry3Bb1) had been used since 2007 (six growing seasons). Under these conditions, the selection pressure for resistance development is increased markedly.