Getting Grubby

June 21, 2009 07:00 PM

Pam Smith, Farm Journal Seeds & Production Editor
Crop producers earn the right to hate grubs' guts. The pests prune root hairs on corn plants and can cause damage severe enough to require replanting. White grub larvae can also feed on soybean roots. Young soybean plants can be killed, but soybeans typically compensate for stand reductions better than corn.
The bad news this year is an increasing number of the gooey critters turned up during tillage and planting. University of Illinois entomologist Mike Gray says early corn planting tends to elevate the significance of grubs. Early planting (say in early April) exposes the seedling root systems to grubs for a longer period of time. Although early planting wasn't really in the cards this year, fields with plenty of organic matter give the pests enough to munch on until the corn gets up and growing

Not all white grubs are alike. The true white grub has a three year life cycle. "During the second year of its life cycle, the true white grub has the ability to prune root hairs on corn plants all summer long,” Gray says. Annual white grubs, as the name suggests have a one-year life cycle. Consequently, these grubs feed for a shorter period of time in the spring. However, it appears they may do more than their fair share of damage—at least in the Midwest.
There are many species of annual white grubs, but the most likely culprits to damage corn are the larvae of the Japanese beetle. The adults of Japanese beetle lay eggs in the soil in mid- to late summer. Larvae hatch and feed through the fall, then descend in the soil to escape cold winter temperatures. The grubs move back toward the soil surface in the spring and feed on organic matter. However, they also feed on available corn roots, especially if organic matter is limited. When they finish feeding, the grubs pupate, and green metallic adults with bronze wing covers emerge in early summer to start the process all over again.
White grub damage typically appears as stunted, wilted, discolored or dead seedlings and/or as gaps in rows where plants fail to emerge. White grubs prune roots and can feed on the mesocotyl causing plant death. If such damage is apparent, dig up some corn plants in the affected area(s). Look for white grubs in the root zone and examine the seedling and root system for damage.
Unfortunately, once white grubs have been found in fields damaging plants, rescue treatments typically prove infective, Gray says.  "There really are no good options,” he says. "If the damage is very severe, sometimes replanting areas of the field is required, but at this late date replanting corn is a poor option.”
White grubs are generally controlled with planting-time insecticides. Gray says in-furrow treatments often are preferred vs. banded applications. Performance of seed treatments have been inconsistent, he notes.
The point to bringing this up now is keep track of the problems for next year. Gray encourages producers to consider treating fields with a history of white grub problems with a soil applied insecticide at planting next spring.. "Keep in mind that Bt corn alone have not offered consistent protection against heavy densities of secondary soil insects such as white grubs and wireworms,” he adds.

You can email Pam Smith at

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