The Mexican corn rootworm may not pose a threat to grain sorghum or corn every year, but the occasional damage it causes warrants a little mid-winter shovel work.
“Growers can't afford to build their pest management program around the Mexican corn root worm,” says Texas A&M entomologist Allan Knutson. “But they can provide themselves a little peace of mind by digging samples out of fields in February, just to see if any grubs are present.”
He said one grub per square foot is enough to cause concern.
Knutson, speaking to farmers during the Stiles Farm Field Day, said the Mexican corn root worm larvae is one of several white grubs found in the soil. These grubs emerge into the green June bugs folks see flying around in early summer. The grubs that cause trouble in corn and grain sorghum, however, are not the same ones that attack turfgrass but are a bit larger.
“We saw a big outbreak of Mexican corn rootworm last year,” Knutson said, “It causes problems only about two years out of 20, but it can damage seedling sorghum.”
Growers must take control measures before crop emergence. “Once the plant is up, we have no resources to control the pest,” he said.
Control options include: Lorsban, either in-furrow or banded; Aztec, in-furrow or banded; Force, in furrow; and Counter, in-furrow. Knutson said Counter is labeled for suppression only. Growers may see some damage but in a normal year Counter will be adequate to protect the crop.
Seed treatments include Cruiser, Prescribe and Poncho (approved for 2003).
Heavy root damage
Knutson said Mexican corn rootworm infestations may cause heavy damage to root systems. “Wind will blow the plants over.”
He also recommends growers check rates of seed treatments to make certain its effective.
Genetic engineering also holds promise for control. The Bt toxin is effective against the pest and “will protect against the Mexican corn rootworm but not other pests. Growers will still need a seed treatment or other pesticide option.”
A&M entomologist Dale Mott said research plots indicate transgenic varieties fare significantly better against Mexican corn rootworm than check plots.