The Western corn rootworm season is progressing at an unprecedented pace, reports professor of entomology and crop sciences Extension coordinator Mike Gray of Illinois. Reports of severe injury to Bt corn that expresses the Cry3Bb1 protein targeted against corn rootworms have come in from western Cass County. In 2011, similar reports of injury to Bt hybrids expressing this protein surfaced in other North Central states, particularly in Iowa.
“Thus far, resistance to this protein in Illinois has not been confirmed,” said Gray. However large numbers of Western corn rootworm adults are visible in the whorls of plants in Cass County. Seed industry representatives confirmed that beetles were observed early last week, nearly a full month earlier than when they are usually first reported, around the Fourth of July.
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.
Roots were removed randomly from this field and from a nearby field that also had been planted to a Bt hybrid expressing the Cry3Bb1 protein. Roots from both fields exhibited feeding injury with more injury in the second field, although fewer beetles were observed there. Because adults were not as noticeable in the second field and emergence was just beginning in the first field, root feeding is expected to continue in both fields.
No lodging was observed, probably 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.
At this point, it is not clear why performance challenges of some Bt hybrids expressing this protein are continuing. However, producers should remain vigilant during this growing season and report any performance issues with corn rootworm injury on their Bt hybrids.