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Late-July Insect Management Opportunities

July 26, 2002
Calls and e-mails into our office at this point in the summer are dominated by questions concerning the following pests: Japanese beetles, twospotted spider mites, corn leaf aphids, soybean aphids, corn rootworm larval damage, and corn rootworm adults and silk clipping in late-planted corn. A few comments follow about each of these pests.

Japanese beetles--Densities of Japanese beetles appear to be less concentrated than a few weeks ago. Ron Hines, senior research specialist, Dixon Springs Agricultural Center, reports that Japanese beetle trap captures are down from approximately 600 beetles per day (2 weeks ago) to only 1 to 9 adults per day most recently. Ron's weekly insect monitoring report can be viewed at the following Web site: http://ipm.uiuc.edu/publications/hines-report/. Producers should continue to monitor their cornfields, especially the late-planted fields, for silk clipping activity caused by Japanese beetles. In addition, growers will need to continue monitoring their soybean fields for defoliation caused by these voracious insects. A treatment should be considered if defoliation reaches 20% between bloom and pod-fill stages of development.


Japanese beetle densities are declining in many fields.

Twospotted spider mites--With the recent and welcome rains in many areas of the state during the past week, reports of twospotted spider mites are beginning to wane. If hot and dry weather returns and persists throughout August, producers should anticipate mite "flare-ups" in areas of infested fields. A treatment should be considered if 20 to 25% discoloration is noted before pod set or 10 to 15% of the plants become discolored after pod set. During the past week, I've observed some large discolored (premature yellowing) areas in soybean fields, especially in east-central Illinois. Don't automatically assume that these spots are infestations of twospotted spider mites. They may actually be infestations of soybean cyst nematodes. A quick check on the undersurface of soybean leaves should confirm the presence or absence of spider mites.


Twospotted spider mites.


Twospotted spider mites.


Twospotted spider mite injury to soybean foliage. (Photo courtesy of Dale Baird, Crop Systems Extension Educator.)


Twospotted spider mite injury to a soybean field in northern LaSalle County. (Photo courtesy of Dale Baird, Crop Systems Extension Educator.)

Corn leaf aphids--As we move beyond the reproductive phase of corn development in many areas of the state, we generally become less interested in corn leaf aphid infestations. However, in some fields under severe drought stress, aphids will continue to remove moisture from plants. Thresholds have been developed primarily to aid in management decisions concerning the protection of the pollination process. Our threshold related to protecting pollination is as follows: a treatment should be considered during late whorl to early tassel when 50% of plants have light to moderate infestations (50 to 400 aphids per plant) and plants are under drought stress. If soil moisture is not an issue, more than 400 aphids per plant may be needed to interfere with pollination. Less clear information is available with respect to the effect that aphid densities have on yields due to moisture removal from plants under drought stress. With poor yield prospects in many drought-stressed fields, treatment decisions should be considered very carefully because of the poor prospect for a return on an insecticide application.


Corn leaf aphids.


Corn leaf aphids may interfere with pollination.

Soybean aphids--Damaging infestations of soybean aphids have not been widely reported for Illinois this season. Yes, we've had pockets of aphids show up in some soybean fields. However, to date, we remain uncertain what the month of August will bring with respect to soybean aphid densities. So far, the very hot and dry weather across many regions of the state may have helped suppress infestations of this pest. The best management advice for this insect is to keep monitoring your fields and be prepared to respond.

Corn rootworms--Corn rootworm larval injury appears to be very significant across the northern half of Illinois. Large densities last season and dry soil conditions this summer are most likely contributing to these significant problems in many fields. Recent storm activity across east-central Illinois has created impressive lodging in fields that have experienced severe root pruning. Producers also continue to report large densities of corn rootworm adults feeding on silk tissue, especially in late-planted fields. To protect the pollination process, a treatment should be considered if there are five or more beetles per plant, pollination is not complete, and silk clipping is continuing. Fields under moisture stress should especially be monitored for silk-clipping activities by corn rootworm adults.


Corn rootworm silk clipping.


Corn rootworm leaf feeding.

As we progress through August, we'll provide more information about the management of insect pests in soybean fields and their potential threat to pod development. Stay tuned.--Mike Gray

Author: Mike Gray


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

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