According to the U.S. Drought Monitor (<http://droughtmonitor.unl.edu/pics/in_dm.png>), as of June 12, 88% of Indiana is abnormally dry, with 39% of that in the first stages of drought. It should come as no surprise that two-spotted spider mites have just begun to move into and colonize thirsty soybeans. Iowa and Ohio have reported the same in their pest newsletters over the last week or so. Foliage damage from spider mite feeding is expressed initially as subtle stippling, which may progress to a bronzing and necrosis should dry conditions persist and mites are left unchecked. Bronzed foliage is irreversible, meaning the damage is done!
Before considering control, it is very important that spider mites are identified as the source of yellowish or bronzed plants in a field. Note that there are many other diseases, pathogens and nutrient deficiencies that cause a similar appearance of foliage. To confirm the presence of mites, shake some discolored soybean leaves over a white piece of paper. Watch for small dark specks moving about on the paper. Also look for very tiny, fine webbing on the undersides of the discolored leaves. Once spider mites have been positively identified in the damaged areas of the field, it is essential that the portions of the entire field be scouted to determine the range of infestation – spider mites are very patchy in colonizing fields – many times beginning at the borders. Sample in at least five different areas of the field and determine how far the spider mites have moved into the field from the grassy borders by using the “leaf-shake” method.
Mites are found in every field, every year. In most years they are held in check by a combination of predators, and the soybeans are healthy enough that mites are not able to build up populations to a high level. However, stressed plants actually provide a more nutritious feast for spider mites than healthy plants do, and in particular make amino acids available to the mites. This means protein, which is what mites use to make – you guessed it – more mites! Thus they thrive and quickly colonize large areas or fields where stress is more evident. Sandy, high clay, or compacted soils will exacerbate moisture stress in plants, with or without the presence of spider mites. Other stresses on soybean include pests such as soybean cyst nematode, root diseases, or nutritional imbalances, such as manganese deficiency. Obviously the best plant stress reliever under dry conditions is rain. Significant rain doesn’t control spider mites but helps the soybean plant become more vigorous and healthy. This, in turn makes the “juices” of the plant less nutritious to the mites, and makes mites less likely to reproduce as quickly.
The most severe damage occurs when the infestation starts in the early stages of plant growth and builds throughout the season (extended drought). Before applying controls carefully consider that, depending when damage is noted, multiple insecticide/miticide applications may be necessary. This is because surviving spider mites are able to repopulate a field much more quickly than their natural predators, which are usually also wiped out by these chemical applications. If leaf discoloration is apparent in multiple areas of the field, spider mites are positively identified as the culprit, and hot, dry conditions are expected to persist, it is recommended that a control be considered.
If a control is warranted, two pesticides are recommended for use. These include dimethoate (multiple generic formulations are available) and chlorpyrifos (Lorsban 4E and generics). Dimethoate is the most efficacious of these compounds for mite control. Neither of these products will control spider mite eggs, however, and each will provide a maximum of 7 days of residual activity. Proper placement of these pesticides is the key to successful control results. Nozzle pressures of 40 psi with fine to medium droplet size and 30-40 gallons of water per acre for ground application helps distribute the pesticide throughout the foliage. There are several products on the market with a combination of chlorpyrifos and synthetic pyrethroids. At this time we cannot recommend them, as field data for their efficacy for twospotted spider mites is limited. In general, do NOT use synthetic pyrethroid products when spider mites are present in fields, as they kill the natural predators and many perform poorly against mites.