No. 19 Article 3/August 4, 2006

Insects to Keep in Our Sights

Mike Gray wrote a detailed article about soybean pod-feeding insects in last week's issue (no. 18, July 28, 2006) of the Bulletin, and I urge you to continue watching for bean leaf beetles, grasshoppers, and stink bugs as soybean pods begin to fill out. Reports from different parts of the state suggest potential for significant infestations of green stink bugs and second-generation bean leaf beetles, so vigilance is due.

In corn, keep watching for two late-season insects, in particular. Some people report finding an unusual amount of injury caused by first-generation European corn borers in some areas of the state, which suggests that a larger second generation may be in store for us. Although European corn borers have become "the forgotten insect" over the past few years, we should not let our guard down. After the second-generation larvae hatch from eggs, the small larvae feed in leaf axils until they grow large enough to bore into the stalks. Control with insecticides, if necessary, is not effective after larvae have tunneled into stalks.

The other insect in corn for which timing of an insecticide application is critical is the western bean cutworm, discussed in some detail last week (no. 18, July 28, 2006). Because this insect is a relatively recent invader in Illinois, I do not believe it has reached pest status in most areas (i.e., densities large enough to cause economic yield losses). However, the numbers of western bean cutworms could be significant in some fields. Insecticide applications are effective only when applied before the larvae begin feeding in the ears, so scouting to search for egg masses and small larvae is crucial. The nominal economic threshold is 8% of the plants with eggs and/or small larvae.

Thorough scouting of both corn and soybean fields at this time of year should reveal whether any of the aforementioned insect pests are present, and also whether other pests pose any threats. As we get closer to harvest for corn and soybean, there may be a tendency to curtail scouting activities, especially in the heat we have experienced. However, always remember that unhappy surprises can be avoided through regular scouting that extends well into August.--Kevin Steffey

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