Issue No. 10, Article 1/May 27, 2005
Waiting for Insects
There's not much happening in the agricultural insect world in Illinois right now, nor elsewhere in most of the north-central states. I quickly reviewed the newsletters from land-grant universities throughout the Midwest, and most entomologists are writing the "watch-out-for" types of articles. Reports of agricultural insect problems are few and far between. Most notable are armyworms in pastures in Missouri (reported by entomologist Wayne Bailey) and a gang of secondary insect pests of corn in Kentucky (reported by entomologist Ric Bessin). The entomologists in Ohio have reported signs of slug injury in corn.
Attention thus far in 2005 has been focused, justifiably, on the effects of weather and soil conditions on both corn and soybeans. The less-than-ideal growing conditions in many areas (temperatures too cool/cold, soil too dry) have resulted in uneven emergence and slow growth of both corn and soybeans. Inadequate corn stands have prompted significant replanting and numerous articles by specialists throughout the Midwest. In the meantime, insects have been keeping a low profile, at least for the most part. Is this the calm before the storm, or is the general lack of insect activity early in this growing season a portent for the rest of the growing season? Obviously, we do not have the predictive powers to answer these questions adequately, so let's keep watching while we are waiting. A reduction in vigilance could result in unwanted surprises.
The list of insects that can cause injury to corn early in the season is lengthy; we often refer to these insects as secondary insect pests. If the generally cool weather this spring has simply delayed the onset of some of these pests, then continued awareness is essential. If we take cues from our neighbor to the south (Kentucky), we should be watching for white grubs, wireworms, cutworms, stink bugs, southern corn leaf beetles, and flea beetles in cornfields. I reiterate that accurately assigning an effect to a cause is very important. We all know that one of the symptoms of injury from feeding by grape colaspis larvae and white grubs is purple stems. However, many other factors can cause purpling, not the least of which is environmental conditions, and much has been written about purple corn this spring. So don't assume that grubs are responsible unless the grubs are found.
We have written several articles about white grubs and wireworms this spring, but we have not said much about southern corn leaf beetles. This insect caused a significant stir in the mid- to late 1990s after seeming to be almost absent for the previous 40 years or so. Although the southern corn leaf beetle has caused few problems the past couple of years, it's important for everyone to know what this insect looks like and how to diagnose the injury caused by its feeding.
"Adult southern corn leaf beetles are 3/16 inch long, dark brown, and often covered with bits of soil, rendering them difficult to find in the field. The prothoracic shield just behind the head has three 'teeth' on each lateral edge. The adults feed on the stems and chew out notches on the edges of leaves of corn seedlings; injured plants appear ragged. Sometimes the beetles feed in such large numbers that injured plants die" (from "Southern Corn Leaf Beetle," page 100, in Handbook of Corn Insects, Entomological Society of America, Lanham, MD).
Southern corn leaf beetle adult (photo courtesy of Mike Roegge).
Southern corn leaf beetle injury to corn seedling (photo courtesy of Mike Roegge).
With soybeans just emerging in many areas of Illinois, the watch for bean leaf beetles is on. However, we have received no reports of bean leaf beetles causing economic damage anywhere in Illinois thus far. Actually, based on the low densities of bean leaf beetles that occurred late last summer, the potential for large densities of the pest this spring (the same beetles that overwintered from last fall) is low. If we ever get some wet weather conditions, watch for slugs.
Our telephone lines have been relatively silent, and our e-mail in-boxes relatively empty. But please don't forget to let us know what's going on in your neck of the woods should the insect situation begin to change.--Kevin Steffey