The western corn rootworm "season" is underway at a pace earlier than I have experienced since I began studying this versatile insect as a graduate student in the late 1970s, says entomologist Mike Gray. In response to a request by a seed industry representative, he traveled to western Cass County on June 7 to verify a report of severe injury to Bt corn that expresses the Cry3Bb1 protein targeted against corn rootworms.
Last year Gray observed severe root injury to some producers' fields in northwestern Illinois (Henry and Whiteside counties; issue 20, August 26, 2011) and north-central Illinois (La Salle County; issue 22, September 23, 2011) that had been planted to Bt hybrids expressing the Cry3Bb1 protein. In 2011, similar reports of injury to Bt hybrids expressing this protein surfaced in some other north-central states, most notably Iowa, where Dr. Aaron Gassmann, an Iowa State entomologist, published a journal article confirming the development of resistance to the Cry3Bb1 protein in some areas of the state by western corn rootworms (issue 18, August 5, 2011). Thus far, we have not confirmed resistance to this protein in Illinois. We are cooperating with Dr. Gassmann's laboratory to determine if the Illinois fields in 2011 were infested with a resistant population. Results should be forthcoming in August of this year.
Western corn rootworm damage to corn leaves in Cass County
In light of the problems that began to surface last season in Illinois and some other midwestern states, the report concerning performance issues with a Bt hybrid in Cass County warranted attention. On arriving at the first field, Gray was amazed at the number of western corn rootworm adults in the whorls of plants. The seed industry representatives indicated that beetles had been observed earlier in the week.
This is the earliest that I have observed adult western corn rootworms, nearly one full month ahead of when I typically begin to receive reports--around the 4th of July. The plants were under intense moisture stress, and the leaves were tightly rolled. In addition, 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. The plants in the fields that I visited were not at this reproductive stage of development, even though planting had occurred between March 30 and April 2.
Root pruning seen on Bt corn in Cass County
Thanks to Mike Gray for the article.