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Issue No. 19, Article 4/August 1, 2008

Other Insect Observations Worth Noting

As attention on soybean aphids, western bean cutworms, and corn rootworms intensifies, a few other occurrences are worth noting. One observation probably overstates the obvious, but the others should help inspire more dedicated scouting efforts.

Japanese beetle. This insect continues to be the recipient of a lot of management activity in several areas of Illinois; many corn and soybean fields have been treated with insecticides. As a reminder, corn is out of the woods after pollination is complete, but soybean defoliation may occur for some time to come, so monitoring for Japanese beetles in soybeans, as scrutiny for soybean aphids intensifies, is still important.

Corn leaf aphid. On July 29, I received a report of a noticeable infestation of corn leaf aphids in V10-stage corn in east-central Illinois. The grower and his advisers were contemplating an insecticide application. The broadly accepted guidelines for corn leaf aphids suggest that an insecticide application may be warranted during late whorl to early tassel stages when 50% of the plants have light to moderate infestations (50 to 400 aphids per plant) and plants are under drought stress. If soil moisture is adequate, treatment may be warranted if there are more than 400 aphids per plant. Also keep in mind that unlike soybean aphids, corn leaf aphids tend to thrive in hot weather, although wet and humid conditions increase the likelihood of epizootics caused by fungal pathogens. We strongly encourage growers who have applied fungicides to cornfields to be particularly alert for corn leaf aphids. Circumstantial evidence from 2007 suggested that the use of fungicides may have enabled a late-season surge in populations of corn leaf aphids and bird cherry-oat aphids. Fungicides applied to manage plant pathogens may also kill off aphid-pathogenic fungi, enabling aphid populations to build up in their absence.

Whiteflies. Jim Donnelly, with Monsanto in northern Illinois, reported that he is beginning to observe whiteflies in soybeans. Numbers of these insects have been very low, but their presence is worth noting. You may recall that we had a rash of reports of whiteflies in soybeans in 2007; the species involved was likely the greenhouse whitefly, Trialeurodes vaporariorum, not believed to cause economic damage to soybeans. However, we are interested in accurate identification of whiteflies this year, too, so we will make the necessary attempts to document the species. The appearance of some other species of whiteflies would be cause for some concern.

Corn borers. As noted in "The Hines Report," numbers of southwestern corn borer adults laying eggs for the second generation were noticeable, and likely have peaked, in traps in Pulaski and St. Clair counties. Although numbers of European corn borers were low in all of the traps in southern Illinois, the recognition of noticeable injury caused by first-generation larvae in western Illinois is reason enough to keep the scouting efforts going. Look for infestations of both pests in non-Bt cornfields, and be prepared to apply an insecticide, if necessary, before the larvae grow large enough to tunnel into corn stalks.

Please keep us apprised of the insect situation in field crops in your area, and we will share such reports with our readers.--Kevin Steffey

Kevin Steffey

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