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A Summary of the Insect Situation

May 31, 2002
The persistent wet, cool weather has slowed down crop growth (and production in some areas) so dramatically that we find ourselves at a virtual standstill regarding insect problems. Although I continue to receive insect reports from here and there, the weather is far more attention-grabbing than insects are. Consequently, I see no reason to provide a lot of additional detail about scouting for insects while crops in many areas

· are very small,

· have just been planted, or

· have not been planted.

Following is an overview of the insect situation in Illinois, such as it is, by crop. Where appropriate, maps of accumulated degree-days are provided. When corn and soybeans begin to grow more rapidly, we'll resume more in-depth articles about the insect situation and management suggestions.


Corn rootworms. No one has confirmed hatch of corn rootworm larvae, so we don't know whether larvae have begun to feed or are just hatching. Rootworm development models predicted hatch in central Illinois as early as the week of May 20, but the abnormal weather conditions may render development models less reliable. Figure 1 shows the accumulated soil degree-days (base 52F), at the 4-inch level, from January 1 through May 27, 2002. Only 50 (central and northern counties) to 100 (southern counties) degree-days accumulated from May 20 through May 27. Approximately 380 to 426 accumulated degree-days are required for 50% of the larvae to hatch. Hatch should be well under way in the southern two-thirds of the state. However, rootworm survival is unknown.

Cutworms. Reports of black cutworm damage have increased a little, primarily from western Illinois, but the reports are not numerous. Tom Leezer, a consultant in Stark County, reported that a few fields around Peoria and Galesburg have cutworm damage at or slightly above the 3%-cutting threshold. In general, however, reports of black cutworm damage are few and far between.

European corn borer. Ron Hines, senior research specialist at the University of Illinois Dixon Springs Agricultural Center, has been capturing European corn borer moths throughout May. I also recently received a report of some moths in grassy areas near cornfields in central Illinois. However, as most of you know, the odds of European corn borer larvae surviving are quite diminished on small corn plants. The following paragraph, extracted from European Corn Borer: Ecology and Management (North Central Regional Extension Publication No. 327, 1996), explains the interaction of small corn plants and European corn borer larvae:

"If the corn plant is small (usually before the 6th-leaf stage) when eggs hatch, most of the larvae fail to become established. They wander off and die due to several factors, including natural feeding deterrents. In many corn hybrids, a primary factor for behavioral deterrence is a plant aglucone, 2-4 dihydroxy-7-methoxy-1, 4-benzoxazin-3-one (DIMBOA). The concentration of DIMBOA in a given corn hybrid usually decreases proportionally with plant growth. A greater proportion of larvae survive on corn in mid-to-late whorl stage, which is when plants are in the 8th- to 12th-leaf stage."

So, judge for yourself. In counties on the western side of Illinois, some corn may be large enough to allow for survival of European corn borer larvae. But the overall prospects for corn borers are not good right now.

Southwestern corn borer. During the week ending May 28, Ron Hines also captured the largest number of first-flight southwestern corn borer moths since 1998, indicating that survival of these insects over winter, at least in some areas, was better than had been anticipated. However, the lack of corn or the small size of emerged corn probably will reduce the potential for larval survival.

Stalk borer. I received one unconfirmed report of stalk borers in central Illinois. Based on degree-day accumulations (base 41 deg F, Figure 2), initial movement of stalk borers from weed hosts into corn (~1,100 degree-days) may have begun as far north as Quincy in western Illinois, Springfield in south-central Illinois, and Marshall in eastern Illinois. We recommend scouting when 1,300 to 1,400 degree-days have accumulated, and a decision to treat with an insecticide should be made when 1,400 to 1,700 degree-days have accumulated. Corn in the southern one-third of the state should be monitored right now for stalk borers.

Economic thresholds for stalk borers have been developed and published by Iowa State University. These thresholds (Table 1) are based on six corn-leaf stages, three corn prices (admittedly at least one of these prices is a little optimistic), control costs of $13 per acre, and a control level of 80%. The information in the table reveals that, as corn price increases, the economic thresholds decline. The economic thresholds for smaller plants are lower than they are for taller plants. Also, recognize that these are guidelines, not carved in stone. If you can treat for a lower price or if you can achieve better control than 80%, feel free to manipulate the thresholds accordingly. Economic thresholds decline as cost of control decreases and effectiveness of control increases. Insecticides for control of stalk borers are presented in Table 2.

White grubs, wireworms. Although people continue to find white grubs (primarily Japanese beetle grubs), these larvae will complete their feeding soon, at which time injury will cease. In the meantime, I have received some reports of insecticides not providing acceptable control of white grubs. If you have any information about the performance of different insecticides, I'd be interested in learning more.

The cool soil temperatures have prolonged wireworm activity, and reports of wireworm damage have increased slightly. Unfortunately, replanting corn as a consequence of wireworm (or white grub) damage is becoming an unattractive option as the season progresses. Farmers who are encountering damage caused by these pests may have to accept the decreased stands.


Bean leaf beetle. Early-planted soybean fields are attracting large numbers of bean leaf beetles. However, early-planted soybean fields are not that common. I believe it's only a matter of time before many bean leaf beetles begin to perish due to the lack of an adequate food source. Although bean leaf beetles will feed on alfalfa and clover, and even a little on corn, these crops cannot sustain them.

Watch early-planted soybeans carefully. Bean leaf beetles are quite mobile, and their search for food will bring them into fields in which soybean seedlings are present.

Seedcorn maggot. Injury caused by seedcorn maggots is more prevalent during cool, wet springs. Fields most likely to harbor seedcorn maggots are

· fields to which manure was applied,

· fields with high organic matter content, and

· fields with vegetation decaying in the soil (e.g., plowed-under cover crops).

Farmers concerned about soybean maggots in soybeans may consider the use of an insecticidal seed treatment to prevent seedcorn maggot injury. Seed treatments that contain lindane as an active ingredient are effective, as is Kernel Guard Supreme (active insecticide is permethrin).


Alfalfa weevil. Alfalfa weevil activity, for the most part, is over with in 2002. Some larvae hangers-on can be found in alfalfa fields in northern counties, but adults are present throughout most of the state. They will leave alfalfa fields soon to "oversummer" in noncrop areas.

Potato leafhopper. We are beginning to receive reports of the presence of potato leafhoppers in alfalfa fields throughout Illinois. Although the numbers are low right now, alfalfa farmers should be alerted to the presence of this important insect pest. When temperatures increase, the development of potato leafhoppers will accelerate, and injurious levels could be encountered soon.

Send any reports of insect activity you have. We want to know what's happening around the state, and so do our readers.--Kevin Steffey

Author: Kevin Steffey

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

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