Two-spotted spider mites, Tetranychus urticae, are serious pests of many crops throughout the United States. Whenever hot, dry weather persists, large spider mite populations are possible on both soybean and corn.

Two-spotted spider mites were reported in a Story County soybean field by the Roland Heart of Iowa Cooperative. The mites had invaded the field from a grassy border and caused significant leaf stippling in soybean edge rows. After scouting plants near the center of the field, mites were found but not in high numbers. Spider mites also have been reported on soybean in eastern Iowa, and some fields have been treated.

Spider mites are small animals more closely related to chiggers, ticks, and spiders than to insects. Although many species of mites can injure plants, it is the two-spotted spider mite that attacks soybean and is the primary mite on corn in Iowa. Most spider mites are greenish-white to yellow in color, although some can be orange or reddish.

The two-spotted spider mite is named for the two dark spots on the sides of the abdomen, which are digested food visible through the insect's translucent body. Three or four spots may be apparent and are most prominent on adult mites.

Spider mite injury to soybean can resemble herbicide injury or a foliar disease, however, characteristic signs are tiny yellow spots, or stipples, on leaves. As the injury becomes more severe, leaves turn yellow, then brown or bronze, and finally die and drop off. Soybean plants injured by mites mature early, have increased shattering, produce smaller seeds, and may produce wrinkled seeds. Early and severe mite injury left untreated can completely eliminate yields. More typical mite injury, occurring during late vegetative and early reproductive growth, will reduce soybean yields 40-60 percent. Spider mites can cause yield reductions as long as green pods are present. Soybean plants can recover from substantial amounts of mite injury after treatment, although less compensation is possible in later developmental stages.

Before treating, check that living mites are present. Examine other areas of the field, especially downwind, to identify the extent of the infestation in healthy plants. Because they are small and stay on the undersides of leaves, spider mites can be difficult to see. One way to spot them is to tap leaves against a white piece of paper and look for moving yellow dots on the paper.

Under normal weather patterns (with locally dry conditions) as we're having this year, treatment is recommended when plants in infested areas have substantial stippling or leaf-yellowing and live mites. Treatment may be delayed if cooler temperatures and high humidity are expected, however, scattered thunderstorms and rain alone will not reduce mite infestations. Closely monitor infested fields if treatments are delayed, and treat before mites cause browning and leaf drop. Spot treat if the infestation is localized, but check other areas for mites.

Under drought conditions, treatment is recommended if leaves in infested areas are stippled and live mites are present. Before treating, check the entire field (and adjacent fields) for mites. Under very dry conditions, mites usually will occur throughout the field and spot treatments are unlikely to prevent the infestation from spreading. If mites are found throughout the field (even in low numbers) in addition to the more heavily infested areas, treat the entire field. Closely monitor treated fields for reinfestations. Avoid unnecessary sprays, but treat before injury becomes severe and leaves drop.

Late infestations can be difficult to control because mites will accelerate soybean maturity and increase shattering. Chemicals labeled for mite control have 21- to 28-day harvest intervals. Consequently, if infested fields still have green pods but seeds are filling, it may be better to accept some yield loss from mites and not treat, rather than treat and have shattering but be unable to harvest. Scouting for mites earlier in the season will help avoid problems with harvest intervals.

If an insecticide is appropriate, two compounds should be considered for soybean: Cygon 400 at 1 pint per acre (21-day harvest interval), and Lorsban 4E at 0.5-1 pint per acre (28-day harvest interval). Both materials were tested during the 1988 mite outbreak, and there were no detectable differences in performance.