No. 16 Article 2/July 22, 2011

Twospotted Spider Mites Seen in Champaign County Soybeans

On July 20, I received a report that twospotted spider mites had been observed in a Champaign County soybean field. Hot and dry weather often encourages the development of economic infestations of this pest. The forecast for continuing hot and dry conditions across Illinois suggests that careful scouting of soybean fields for mites is a good investment of time. Under current conditions, mite densities can increase rapidly. In many instances, mites are first noticed along field margins. Their development rates increase when temperatures climb over 91°F, and a generation can be completed in as few as 4 days under the very hot temperatures we are experiencing.

Use a handheld lens (magnifying glass) to examine the undersurface of soybean leaves for the presence of mites. Tapping leaves over a sheet of white paper may also reveal dislodged mites. Mites injure plants by inserting their needlelike mouthparts into leaf cells and sucking out the contents. This type of feeding injury contrasts sharply with that of most other pests, which remove plant fluids by inserting their mouthparts into vascular tissue. Early signs of injury on soybean plants include leaf stippling (very small yellow or white spots). As mites continue to feed, chlorophyll is reduced and the plant's photosynthetic efficiency is curtailed. If hot and dry conditions persist and mite densities in a field increase, plants may take on a bronzed appearance. Severely injured leaves eventually drop off of plants.

A twospotted spider mite infestation within the border rows of a soybean field.

Twospotted spider mite adults, nymphs, and eggs.

Twospotted spider mites on a soybean leaf tip.

If we return to cooler temperatures and get some rain in the next week to 10 days, the observation of twospotted spider mites may be just an interesting footnote to the season. However, I believe careful examination of plants is warranted, especially in areas of Illinois that have not been blessed with rainfall the past several weeks. Making treatment decisions for this pest is not easy. A rescue treatment should be considered when plants within field margins are showing leaf discoloration and mites are present. Growers should consider a treatment when 20% to 25% discoloration is found before pod set or 10% to 15% discoloration occurs after pod set. In 1988, a severe mite outbreak occurred across Illinois and many other north-central states. The two most common insecticides used to limit mite injury were products with the active ingredients chlorpyrifos and dimethoate. In many instances, repeat applications were required.

Ultimately, drought and spider mites were dominant that year, and yields were greatly reduced. If you begin to find mites in soybean fields, please pass along the observations to me so I can share them with readers of the Bulletin.--Mike Gray

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