At least one soybean field in the Jackson Purchase area has suffered from mite damage. The weather that we are now experiencing is what I would expect for mites to become a problem. There is no guarantee that mites will be a problem in any particular field, but dry (low humidity) weather and drought-stressed plants increase the probability of a problem occurring. Mites will infest both corn and soybeans, but in Kentucky the latter is more likely to result in an economic problem. Double crop beans could be at a greater risk than full season beans just because they are in a “younger” plant stage at the time of infestation.
The most common mite problem in Kentucky grain crops is the two spotted spider mite (Figure 1). This is a tiny yellowish-reddish pest. The mature females have two well-developed, black spots located on either side of their body. These are not insects, but their eight-legged relatives.
In soybeans, the most important infestation window is the reproductive stages of R1 (beginning bloom) to 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. Certainly no control is warranted after the dent stage. In either case, one of the first and most important questions is: am I likely to be able to make a crop? Or, is the crop worth treating?
Scouting for Mites
To sample for mites in soybeans, shake plants over a piece of white paper and look for tiny, 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 soybeans 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.
Deciding on the need for mite control is very difficult. Suffice it to say that the field needs to be heavily infested; so mites should not be hard to find. Also, two-spotted spider mite is difficult to control with pesticides, especially if a large population is present before they are detected.
In corn, if direct control is required, consider using an organophosphate active ingredient like dimethoate (e.g. Dimethoate). If you must use a pyrethroid, consider a product containing bifenthrin. These are two active ingredients that happen to have some activity on mites. There are many products containing these active ingredients. In addition, there are a few acaracide active ingredients such as propargite (e.g. Comite), spiromesifen (e.g. Oberon), hexythiazox (e.g. Onager), and etoxazole (e.g. Zeal). These latter products are sometimes difficult to find and may be quite expensive. If you are planning to convert your corn to silage, then pay special attention to the “days to harvest” restrictions on the product you choose to use.
In soybeans, if direct control is required, most plants will be infested with live spider mites and leaf speckling and discoloration will be apparent. Treating after R7 is not recommended. The product choices are less diverse than with corn. Bifenthrin alone or in combination with other products (of which there are many) are the most common. The only true acaracide available is bifenazate (e.g. Acramite).
Cooler temperatures and high humidity allow a natural fungus to control spider mites. Rainfall will help the crop 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. They kill mite predators and the fungicide may hamper the natural occurring fungus that kills the mites.
Information on insecticides is supplied for the reader’s convenience, and is NOT a substitute for the label. 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.
Doug Johnson Extension Entomologist, University of Kentucky