: Spider mites beginning to appear on soybeans, can corn be far behind?

Doug Johnson, Extension Entomologist

I warned of this pest some time back (See KPN No. 1308 June 26, 2012. I Hate Crying Wolf…). In fact, I am mildly surprised that it has taken this long for field scouts to come across the pest. Reports of spider mites have alerted us of their activity and samples sent to the Plant Diagnostic Lab have confirmed that it is the two-spotted spider mite causing the problems. At present the damage appears to be on a small scale and very localized in fields. Somewhat surprisingly some of the infestations were noted in irrigated fields.

For as long as we have drought stressed plants, and in particular LOW humidity, we are increasingly likely to be plagued by this pest. In soybeans, the most important time is during the reproductive stages of R1 (beginning bloom) - R5 (beginning seed) and it is even more likely in fields where a synthetic pyrethroid insecticide has been used. In our current corn production situation, it may very well be too late to worry about mites. In either case, one of the first and perhaps most important questions is: Am I likely to be able to make a crop? If you are or are planning to convert your corn to silage, then pay attention to the “days to harvest” restrictions on the product you choose to use.

To sample in soybeans, shake plants over a piece of white paper and look for tiny, black, moving specks. You will need a hand lens to determine if the specks are actually mites, but if they are crawling across the paper you probably know the answer. In corn, scouting is much more difficult and less is known about making a control decision. The mites are no harder to find, simply follow the instructions for soybean but use corn leaves. One simply wishes to determine if the infestation is only on the outer edges or occurs across the field. You will need to sample several areas of the field. Mites are notorious for being spotty in their distribution. They also have a very strong “edge effect” which would include waterways, etc., that may cross the field. If a treatable infestation is found, it may very well be controlled by a border application.

Scouting should start well before R1 (soybean) and in younger corn. Cooler temperatures and high humidity allow a natural fungus to control spider mites. Rainfall will help the plant tolerate the infestation but will not reduce the mite population. Application of synthetic pyrethroid insecticides and fungicides may worsen the infestation because these insecticides don’t work well on mites and the fungicide may hamper the natural occurring fungus that kills the mites.

In soybeans, if direct control is required, consider using an organophosphate insecticide like chlorpyrifos or dimethoate. If you must use a pyrethroid, consider bifenthrin. An application is warranted when most plants are infested with spider mites and leaf speckling and discoloration are apparent. Treating after R7 is not recommended

In corn, chlorpyrifos is not labeled for spider mites; dimethoate may not be applied during pollen shed. Bifenthrin may be used. See labels for rates and specific directions.

Information on insecticides is supplied for the readers convenience. Use of a trade name does not constitute a recommendation. These products are not ranked in preference or efficacy. Always read and follow the label.

Photo: Frank Peairs, Colorado State Univ., bugwood.org