Scouting For Corn Rootworm Injury

From now through June, corn growers should be scouting their fields for corn rootworm. Newly hatched larvae feed primarily on root hairs and outer root tissue. As larvae grow and their food requirements increase, they burrow into the roots to feed. This interferes with water and nutrient uptake, makes the plant unstable, and cuts down on yield.

Anthony Ohmes is an Extension agronomy specialist at the University of Missouri. He says one of the signs of rootworm activity is root tips that appear brown, tunneled into, and chewed back to the base of the plant.

"One root average per plant pruned back to 1.5 inches of a stalk, or an entire node of roots destroyed. Somewhere in that range is where economic loss is going to be potentially possible," says Ohmes. "One of the things you’d see is that root pruning, and then the next obvious symptom would be if there’s a wind occurring after excessive root pruning, you’ll get some plants that lay over, then you’ll get this goose-necking where the plant’s trying to upright itself again."

Scouting doesn’t end with the larvae. In late July and August, the adult beetles will show up on the corn plant.

"The beetles feed on silks, they’re typically not an economic threat because we’ve already got pollination occurred, and so silk pruning’s not a big deal at that time frame," he says. "But, if we get an average of about one beetle per plant in that time frame, then that would probably be enough because the beetles just fall back to the ground and deposit the eggs."

The first management strategy is rotating out of corn. If you have continuous corn, the only option is to use Bt or insecticide treatments.