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The Insect Situation--An Overview

August 9, 2002
By this time of year, we have discussed, in some detail, a lot of what is occurring in the insect world in Illinois field crops. We have provided insect situation reports, scouting tips, economic thresholds, and suggested insecticides if such applications are necessary. Now, we just sort of sit back and watch the insects and crops develop toward the end of another rather unusual season.

With some exceptions, most of the insect problems we report at this time of year are (or have become) familiar refrains. For this issue of the Bulletin, I'll provide an overview of some of our familiar foes, with a separate article on an insect not frequently encountered.

Corn rootworms. In many areas of Illinois, corn rootworms have been the "insects of the summer." Significant silk clipping was observed throughout July, and root damage has been severe in many fields, for reasons we have explained in detail. Most important, the variant of western corn rootworms that deposits eggs in soybean fields has showed up in areas in which farmers have not observed rootworm problems in corn planted after soybeans in the past.

Consequently, we have embarked on a survey of fields of corn planted after soybeans to assess the level of root injury, from western to eastern Illinois. As of August 5, we had completed obtaining samples from fields in the following 15 counties: Brown, Ford, Fulton, Iroquois, Livingston, Macon, Marshall, Mason, McDonough, McLean, Peoria, Sangamon, Schuyler, Stark, and Tazewell. We extracted five roots in each of 10 fields scattered throughout each county.

Within the next week and a half, Extension educators will have completed sampling another 15 counties: Adams, Champaign, Christian, Grundy, Hancock, Kankakee, Knox, LaSalle, Logan, Mercer, Pike, Vermilion, Warren, Will, and Woodford. After all of the roots are washed and rated for rootworm larval injury, we should have a reasonable assessment of the extent of rootworm problems in corn after soybeans throughout the central one-third of the state. We'll provide preliminary results from these efforts in near-future issues of the Bulletin.

European corn borer. Indicators earlier this summer suggested we might encounter some heavy infestations of second-generation European corn borers this year. However, we have received very few reports about corn borers within the past few weeks, so it seems that threatening populations did not materialize. It's entirely possible that the dry weather had a negative impact on survival of European corn borers. Adults require water for successful egg production. If they had a difficult time locating water, the numbers of eggs deposited may have been small.

During our survey of cornfields for rootworm damage, several of us observed tunneling by second-generation European corn borers in stalks that we cut off. However, the level of injury in any given field was slight. Apparently the potential threat of infestations of second-generation borers has dissipated.

Grasshoppers. Although problems with grasshoppers in Illinois pale by comparison with the current problems being experienced in Nebaraska, numbers of grasshoppers in certain locations are quite high. During the aforementioned survey for rootworm damage, we walked through many grassy and weedy roadsides to get to cornfields, and we occasionally encountered very large numbers of grasshoppers around the field margins. However, in most situations very little damage had been done to the adjacent crops.

Such was not the case in a couple of fields in Jefferson County. Dennis Epplin, Crop Systems Educator in Mt. Vernon, submitted a couple of photographs of severe defoliation of cornfields he visited. Fortunately, this level of damage has been rare, restricted to areas where severe drought conditions prevail. However, light to moderate grasshopper damage is common throughout southern Illinois. Dennis also noted that grasshoppers destroyed some fields earlier this year that had been planted late. The level of damage encouraged some producers to replant, and the concern exists that these replanted fields will produce little, if any, yield.

Grasshoppers and severe damage to corn in Jefferson County. (Photo courtesy of Dennis Epplin.)

Severe grasshopper damage to corn in Jefferson County. (Photo courtesy of Dennis Epplin.)

Soybean aphid. We are still awaiting developments of soybean aphid populations in northern Illinois, but these recently invasive pests seem to have been limited in their scope of infestations this year. The same seems to be true for most of the rest of the Midwest, too. Nevertheless, scouting is still strongly encouraged. As we have repeated often, the numbers of aphids can increase dramatically in a very short time.

Stephen Doench, with Pioneer Hi-Bred International, has submitted the most thorough report of soybean aphids in Illinois. On August 1 he indicated that numbers of aphids had increased in all of the counties he works in: Bureau, Fulton, Henderson, Henry, Knox, Lee, Marshall, Mercer, Peoria, Rock Island, Stark, Warren, and Whiteside. In one field in Henry County, Stephen counted 40 to 50 aphids per plant on the lower leaves. In one field in Lee County, Stephen observed some plants with more than 1,000 aphids near field edges, with an average of 40 or more aphids per plant in the rest of the 40 acres. He found aphids on leaves, stems, and pods. Keep watching and reporting.

Twospotted spider mite. Twospotted spider mites can be found at the edges of soybean fields in almost any area of the state where rainfall has been limited. These pests will continue to move into the field if their numbers increase during the hot, dry weather we have experienced. Keep watching the development of their populations, and treat them along field edges to prevent the injury from spreading to the interior of fields. However, remember to scout the interiors of fields, too, even if symptoms of injury are not apparent beyond field edges. If mites are present throughout the field, the symptoms of injury will show up soon.

Potato leafhopper. Numbers of potato leafhoppers have been extremely high this year. Injury in alfalfa fields is not uncommon. After driving through the countryside at night, many drivers have to scrape tiny, green little bodies from their windshields, headlights, radiator grills, and bumpers. Leafhopper population densities will begin to decline in August, but they're still numerous enough in some fields that treatments still are warranted. Don't let the damage get too severe, because alfalfa injured by potato leafhoppers often enters the fall and winter with fewer reserves and may not survive well over the winter.--Kevin Steffey

Author: Kevin Steffey

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

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