Twospotted spider mites are occasional pests of soybeans in Nebraska, typically during hot, dry periods in mid to late summer. However, it has been an uncommonly warm spring, and reports of spider mite problems in Richardson County and parts of Iowa are already coming in.
Biology
Severe spider mite damage in a soybean field in Richardson County in early June. (Photos by Gary Lesoing)Early stages of spider mite damage in soybean in Richardson County.
Twospotted spider mites typically overwinter in alfalfa and other broadleaved plants along field borders. In the spring or summer, mites either crawl or are carried by wind to soybean fields where they deposit small, round, pearly white eggs on the underside of leaves. Early infestation and injury often appear first on the south and west edges of fields because of the prevailing wind direction, but infestations also may occur in "hot" spots scattered throughout the field. The eggs usually hatch in about three or four days. Young mites resemble the adults, and increase in size by periodically shedding their skins. It takes about five to 10 days after hatching before mites are mature and begin to produce eggs. All stages of mites may be present at the same time, and there may be seven to 10 generations during the growing season.
Soybean Injury
Mites injure soybean by piercing plant cells with their mouthparts and sucking the juices. Feeding often begins on the lower part of the plant, moving to the top as the plant condition deteriorates. The first evidence of mite feeding is usually seen on the top of the leaf, appearing as yellow or whitish spotting in areas where the mites are feeding on the lower leaf surface. Many other things can cause similar symptoms, so it is important to check leaves to make sure that mites are actually causing the damage. Leaf discoloration caused by mite feeding can be easily identified by checking the undersurface of leaves for mites, eggs, and webbing.
Twospotted spider mites produce webbing, and a network of fine, silken webs will likely be associated with mite colonies. A magnifying glass or 10X hand lens is helpful in examining plants for mites. As mite infestations develop, leaves may be severely damaged and the food-manufacturing ability of the plant is progressively reduced. Damage includes leaf spotting or stippling, leaf drop, accelerated senescence and pod shattering, as well as yield loss. Leaves may have a sand-blasted appearance and under severe infestation, may die. Early and severe mite injury left untreated can completely eliminate yields. More commonly, mite injury during the late vegetative and early reproductive growth stages will reduce soybean yields 40-60 percent. Spider mites can cause yield reductions as long as green pods are present.
Factors Contributing to Mite Infestations
Mites do not cause major economic damage every year. Several factors, such as weather and natural enemies, strongly influence spider mite numbers. Overwintering sites that are close to soybean fields, especially grasses, wheat, and perhaps alfalfa, also may increase the possibility of mite invasion.
Dry, hot weather favors mite reproduction and survival, especially if accompanied by drought stress. When the weather in June, July, and August is especially hot and dry, mites can reach damaging numbers in most soybean-growing areas of Nebraska. Major mite infestations are more likely to occur in central and western counties that normally experience less rainfall. Sandy soil types may also contribute to spider mite problems because crops grown on these soils are more likely to experience drought stress even when irrigated.
Twospotted spider mites. (Photo by Jim Kalisch) Spider mites have been reported earlier than normal in southeast Nebraska and Iowa. Dry, hot weather favors their reproduction and survival, especially in drought conditions.
Natural Enemies. Several species of insects and mites prey on spider mites. The most important of these are the predatory spider mite, the mite destroyer beetle, six-spotted thrips, and the minute pirate bug. In addition to these predators, a fungal disease may reduce spider mite populations.
In many years these natural enemies may help keep spider mite populations below damaging levels. They are particularly effective during cool, moist periods in early and mid summer when mite reproduction is slowed. For this reason, their presence and abundance should be noted and considered when evaluating spider mite populations. Low numbers of spider mites may allow predators to build up and prevent the spider mites from reaching damaging levels. Nearly all synthetic insecticides have severe, detrimental effects on spider mite predators.
Treatment
No research has been conducted that would allow calculation of economic injury levels or thresholds for twospotted spider mites on soybeans. Iowa State University Extension specialists have suggested that control may be warranted when infested plants have live mites and substantial spotting or leaf yellowing, but before there is browning and leaf drop. Injury from mites may be confused with that caused by drought and several foliar diseases, so be sure to base treatment decisions on the presence of mites, rather than just apparent injury symptoms. Fields may be spot-treated if the infestation is localized, but check other areas for mites (especially downwind of infestation) and extend treatments into these areas if you find large numbers of mites. Although late season infestations may accelerate soybean senescence and increase pod shattering, take caution when deciding to treat with miticides. Some products registered for mite control in soybeans have up to a 45-day preharvest interval.
For effective control, spider mites must come into contact with the miticide. Since mites are found primarily on the underside of the leaves, they are difficult to reach with low volume applications. Using three or more gallons of water per acre to carry pesticides may increase effectiveness. Aerial applications are generally more effective if applied very early in the morning or in the late evening. Applications made at these times avoid the upward movement of sprays away from the plants on hot rising air. Eggs are difficult to kill with soybean miticides, so reinfestation is likely to occur seven to 10 days after treatment due to subsequent egg hatching. The reinfestation is frequently heavy because natural enemies have been reduced or eliminated. A second application may be necessary to kill newly hatched mites before they mature and deposit more eggs. In many cases, slowing the rate of population increase is all that can be accomplished with a miticide application.
Several pesticides are labeled for spider mite management in soybean. These can be found on the page Insecticides for Control of Spider Mites in Soybean. Note that several entries indicate “Suppression only.” If managing spider mites is the goal, do not use “suppression” products, as the chance for spider mite flare-up is high. Organophosphates are the preferred chemistry for twospotted spider mites in soybean, as pyrethroids appear to be less effective and mite flare-ups are a possibility. Because spider mites are appearing early this year, and treatment may be within several days of an herbicide application, make sure to check the label for any phytotoxic herbicide/miticide interactions before making a miticide application.
Resource
For more information, see the UNL NebGuide, Spider Mite Management in Corn and Soybeans (G1167).
Tom Hunt Extension Entomologist, Haskell Ag Lab, Northeast REC Bob Wright Extension Entomologist, Lincoln
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