Wireworms Rear Ugly Heads

Early reports question insecticide effectiveness. Tom J. Bechman

Published on: May 19, 2004

Seedsmen report that in some areas, up to 60% of their corn seed went out the door this spring with a low-level of seed-coated insecticide on it. Several products are available. Seed-coated insecticides at low levels won't stop corn rootworms, but they are billed to fight off other pests, including wireworms.

That's why one report from extreme southern Indiana is especially curious. A well-respected agronomist happened across a field in the Kentuckiana area where wireworms were feasting on corn roots, even though the seed was reportedly treated with a seed-coated insecticide.

What might have went wrong could take time to figure out. Even if this report is verified, it doesn't mean that this relatively new concept bombed out. Factors beyond the product control could have been involved.

For example, one factor affecting any pesticide is severity of the infestation. The same seed-coated insecticides are typically offered at higher rates for rootworm control. But for those applications, manufacturers make sure farmers understand that the product won't control every rootworm. Agronomists typically say that the seed-applied insecticides available so far are fair to good, depending upon the product, at rootworm rates, on rootworm control. However, they break down in control at higher infestations.

It's possible that severity of infestation may have factored into this case of a seed-coated insecticide at the lower rate being overpowered by wireworms, even though it normally controls wireworms.

"I was shocked when I opened up the file and saw the picture my cohort had sent me," said a second agronomist, who received an email from the person who actually found the damage in the field. "I expected to see one or two of the wiry, copper-colored insects chewing on roots. But instead, there were gobs of them in there, eating away."

Wireworms are difficult to diagnose because they do their damage below the soil surface. Plants tend to wilt and often appear stunted. That's because wireworms are chewing away under ground.

These insects are far less predictable than rootworms as to where they will appear. They're usually attracted to higher organic matter content, often where residue or other material has been incorporated into the soil. Since they have a long life-cycle, they can reoccur in the same spot for up to 7 years.

Don't expect wireworms to wipe out an entire field. They are more typically a pest that eats away in patches, although the patches can cover big areas. Once wireworms are detected, it's too late to help that crop. Replanting is the only option, if the stand is so severely damaged that replanting is warranted.

Once soil temperatures warm up to summer levels, you won't likely see these pests again this year. They retreat deeper into the soil.