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Monitor Corn, Soybean Crops for Japanese Beetles, Western Corn Rootworm, Two-Spotted Spider Mite

japanese beetle

During the later stages of June and early July, it will become increasingly important to monitor corn and soybean fields for some important pests warns University of Illinois Professor of Entomology and Crop Sciences Extension Coordinator Mike Gray. "For many areas of Illinois, the dry and hot weather makes this recommendation even more important because crops are increasingly vulnerable to yield loss under these stressful conditions," he says.


Japanese beetles

Photo by USDA ARS,

Japanese beetles can now be found throughout the state of Illinois. They can cause injury and subsequent yield loss in both corn and soybeans. 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.

In corn, the key concern is the potential for excessive silk clipping. Plants that are under severe moisture stress are vulnerable to this type of injury because they cannot grow sufficient silk tissue to keep up with the beetles' clipping activity. During the reproductive phase of plant development, a rescue treatment may be needed if there are three or more beetles per ear and pollination is not finished.

Densities for this pest tend to be greatest along field margins in both corn and soybean fields. "Treatment decisions should be made only after scouting field interiors and border rows," Gray warns.


Western corn rootworm

Photo by Tom Hlavity, USDA ARS,

The adult western corn rootworm has emerged nearly one month early. In the fields where plants have not begun to tassel and shed pollen, beetles are feeding on corn-leaf epidermal tissue and will continue to do so until pollen and silks become available. "Leaf injury reduces the plants' photosynthetic efficiency, so some yield loss should be anticipated for those fields, particularly those that are under moisture stress," Gray says.

As silks become available, a rescue treatment should be considered if there are five or more beetles per plant, silks have been clipped to less than 1/2 in. of the ear tip and the pollination process is not complete. While scouting for silk clipping, look for lodged or goose-necked plants. This is evidence of larval injury to root systems.

"If lodged plants are observed, dig up some of the plants, wash the soil from the root systems, and look for signs of feeding or pruning," Gray advises. "If excessive injury is found, contact your seed company representative."


Two-spotted spider mites

Photo by Eugene E. Nelson,

Hot, dry weather and two-spotted spider mites are typically found in tandem. Many farmers are still trying to forget the drought of 1988 and the severe outbreak of mites that occurred across much of the Corn Belt. Mite infestations have been reported in soybean fields that have missed the widely scattered rain showers.

Generally, soybean plants along field margins are the first to show the characteristic bronzing and mottling of leaves. "By tapping the leaves over a sheet of white paper, you can observe mites moving about the surface of the paper," Gray says. "Often, webbing is present on the lower surface of leaves."

A 1988 study found 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. Symptoms included pale green leaves and some yellow mottling.

If mite injury to plants is evident along field margins, and mites are found on plants with relative ease throughout a field, consider a rescue treatment, 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 the residual activity of the products lasting approximately one week. Many of the fields in which only border rows were treated ultimately required full-field sprays.

"At this point, it's difficult to predict where this summer is headed with respect to this pest," Gray says. "Let's hope we begin to see more widespread and abundant precipitation across the state. If this occurs, two-spotted spider mite infestations will abate."

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