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No. 18/July 28, 2000

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Entomological Observations from the Road
Earlier this week, our crew evaluated corn rootworm larval injury in our experimental insecticide trials in Monmouth and DeKalb. The remainder of this week will be devoted to washing roots and rating them for larval injury. Root injury was evident at both locations, with the potential for lodging most likely in DeKalb.

As I returned from DeKalb, I was a bit surprised, and disappointed, that more yellow sticky traps were not evident in soybean fields. We recommend that trapping for western corn rootworm adults in soybean fields should begin during the last week of July and continue through the third week in August.

Time to Watch for Defoliators in Soybeans
Although we are not aware of excessive defoliation being caused by any one of the insects discussed in this article (with the possible exception of the Japanese beetle), when soybeans bloom and pods begin to fill out, defoliation by insects can result in noticeable yield loss. Keep in mind that a static threshold may not be appropriate during a year when soybean prices are low. Adjust the percentage defoliation threshold upward to accommodate low prices.

Observations reported of bean leaf beetles, grape colaspis adults, and a few northern corn rootworm adults. Other potential defoliators that may be common in soybean fields in Illinois during this time of year include grasshoppers, green cloverworm, and yellow woollybear caterpillar.

Gives guidelines for scouting a soybean field for evidence of defoliation caused by any of the insects discussed in this article. Although defoliation thresholds accommodate virtually all defoliators, it's important to identify the pests accurately to choose the right insecticide, if warranted. Different insecticides may be labeled for different insects, and the rates of application vary as well.

Gray Leaf Spot of Corn
Gray leaf spot of corn is being reported across the state. Gray leaf spot in a susceptible variety has easily identifiable symptoms: The disease probably would have been more appropriately named tan rectangle blight.

The most severe gray leaf spot epidemics usually occur in continuous-corn-production fields, where there is also a substantial amount of corn debris that was infected in the previous season remaining on the soil surface.

Progress has certainly been made in the past several years in developing hybrids with some level of resistance to the disease. Remember, though, that resistance does not mean the hybrid will not be infected by the disease. Resistance in a resistant hybrid can be expressed in a number of ways: smaller lesions produced, fewer spores subsequently produced from those lesions, a longer time for lesions to develop, or fewer lesions produced overall

Water, Canopy, and the Corn Crop
The main determining factor for yield in most Illinois cornfields will be the water supply for the rest of the season. Although there are few signs that dryness has hurt the crop, there are areas of the state where rainfall during July has been much below normal.

Make no mistake, the crop is in excellent condition in the majority of fields in Illinois. In parts of the state, however, July rainfall has been heavy, and as a result the crop has continued to suffer from lack of root growth and nutrient uptake. Overall, the crop is well positioned for "typical" August weather.

Discusses the measurement called "open pan evaporation," or OPE, and the "crop coefficient," the percentage of OPE that the crop uses; also, assessing the canopy.

Regional Reports
Extension Center educators, Unit educators, and Unit assistants in northern, west-central, east-central, and southern Illinois prepare regional reports to provide more localized insight into pest situations and crop conditions in Illinois. The reports will keep you up to date on situations in field and forage crops as they develop throughout the season. This week's issue includes reports from northern and west-central Illinois.

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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