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Issue No. 12, Article 1/June 22, 2012

Japanese Beetles, Corn Rootworm Beetles, and Twospotted Spider Mites: Key Pests to Monitor in the Coming Weeks

As we approach late June and early July, it will become increasingly important to monitor corn and soybean fields for some important pests. For many areas of Illinois, the dry and hot weather intensifies this recommendation, because crops are increasingly vulnerable to yield loss under these stressful conditions.

Japanese beetles. Japanese beetles can now be found throughout Illinois, and both corn and soybeans are susceptible to injury and subsequent yield loss. In soybeans, Japanese beetles are one of many defoliators to monitor. Rescue treatments may be warranted if defoliation levels reach 30% prior to bloom and 20% between bloom and pod fill.

Densities of Japanese beetles in both corn and soybean fields tend to be greatest along field margins. Treatment decisions should be made only after scouting field interiors and border rows. When Japanese beetles are numerous in corn, the key concern for producers is the potential for excessive silk clipping. A rescue treatment may be needed during the reproductive phase of development when three or more beetles per ear are present and pollination is not finished. Plants under severe moisture stress are particularly vulnerable to this injury because they cannot grow sufficient silk tissue to keep up with the beetles' clipping activity.

Western corn rootworm beetles. As I reported in issue 11, western corn rootworm adult emergence is nearly a month ahead of a more typical growing season. For many fields, plants have not yet begun to tassel and shed pollen. Consequently, beetles are feeding on the epidermis of corn leaves, injury that will continue until pollen and silks become available. Leaf injury reduces plants' photosynthetic efficiency, and some yield loss should be anticipated for those fields, particularly any under moisture stress. As silks become available, consider a rescue treatment if there are five or more beetles per plant, silks have been clipped to less than ½ inch of the ear tip, and the pollination process is incomplete. While scouting for silk clipping, also look for lodged or goose-necked plants as evidence of larval injury to root systems. If you find lodged plants, dig some of them up, wash the soil from the roots, and look for signs of feeding or pruning. If there is excessive injury, contact your seed company representative.

Twospotted spider mites. Hot, dry weather and twospotted spider mites are typically found in tandem. Many producers are still trying to forget the drought of 1988 and the severe outbreak of mites that occurred across many areas of the Corn Belt. Mite infestations have been reported in some soybean fields that have missed the widely scattered rain showers. Generally, soybean plants along field margins begin to show some of the characteristic bronzing and mottling of leaves. By tapping the leaves over a sheet of white paper, you can knock mites off the plant and see them moving on the paper. Webbing also is often present on the lower surfaces of leaves.

Research conducted in 1988 indicated that as spider mite injury to soybean leaves intensified, photosynthetic efficiency decreased, stomatal resistance increased, transpiration rate decreased, and the total chlorophyll content of leaves decreased. Some of these adverse physiological reactions were evident when leaves became paler green and some yellow mottling was evident. If mite injury to plants is evident along field margins and mites are found on plants with relative ease throughout a field, a rescue treatment should be considered, especially if hot and dry weather is expected to continue.

The most common insecticides used as rescue treatments include chlorpyrifos (Lorsban 4E and generics) and dimethoate. In 1988, many fields were treated multiple times due to the continuing drought and residual activity of the products, which lasted approximately a week. In addition, many of the fields in which only border rows were treated ultimately required sprays of the full field. At this point, it's difficult to predict where this summer is headed with respect to twospotted spider mites. Let's hope we begin to see more widespread and abundant precipitation across the state, in which case infestations will abate.--Mike Gray

Mike Gray

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