Conditions optimal for spider mite development in corn, soybean

Twospotted spider mites (left) and Banks grass mites (right). Notice the difference in the pigment patterns between the two insects.
Banks grass mites, Oligonychus pratensis.
Twospotted spider mites, Tetranychus urticae.

We are reaching the time of year when spider mites may start damaging corn and soybeans. I have seen low numbers of spider mites in corn and soybeans in southeastern and south central Nebraska. While we haven’t received reports of any economically damaging infestations, hot, dry conditions will favor an increase in populations. Producers should be scouting fields and keeping a close eye out for spider mites.

Identifying the species

Two species of spider mites, the Banks grass mite and twospotted spider mite, commonly feed on Nebraska corn. Banks grass mites feed almost exclusively on grasses, including corn and sorghum. Twospotted spider mites feed on many species of grasses as well as on soybeans, fruit trees and a variety of vegetables and ornamental plants. Although these two species are somewhat similar in appearance, they differ in several biological characteristics and in their susceptibility to pesticides.

Banks grass mites usually appear earlier in the season, feed mostly on the lower leaves of the corn plant, and in Nebraska are moderately susceptible to many of the commonly used miticides. On the other hand, twospotted spider mites tend to appear in mid to late season, increase rapidly, feed over the entire plant, and often are not consistently controlled by available pesticides. The most useful characteristics for identification are the overall shape of the body and the pattern of pigmentation spots on the back (see Figure 1). The dark green spots on both species are caused by food particles that accumulate in their gut. Because of differences in gut structure, these pigment spots accumulate in slightly different patterns. In Banks grass mites the pigments accumulate along both edges of the body near the rear and along the sides of the body. In twospotted spider mites, the pigments accumulate along the sides of the body in two distinct spots and do not extend back more than halfway on the body. The Banks grass mite is also slightly less robust than the twospotted spider mite and is slightly flatter from top to bottom.

Mites damage crops by piercing plant cells with their mouthparts and sucking the plant juices. The first evidence of mite feeding, which can usually be seen on the top of the leaf, is a yellow or whitish spotting of the leaf tissues in areas where the mites are feeding on the lower leaf surface. Because many other things can cause similar discoloration, it is important to check leaves closely to make sure mites are actually causing the damage. Leaf discoloration caused by mite feeding can be easily identified by checking the undersurface of leaves for the presence of mites, eggs and webbing. Both Banks grass mites and twospotted spider mites produce webbing, and a fine network of silken webs will likely be associated with mite colonies. A magnifying glass or 10X hand lens is helpful in examining plants for mites.

Management

The economic injury level indicated in the table provides a method for deciding when to treat, taking into account the value of corn. This table works for both twospotted spider mites and Banks grass mites. The first row refers to the expected value of the crop ($/acre), determined by multiplying the expected yield (bu/acre), by the expected crop price ($/bu). For example, if the expected yield is 200 bu/acre and the expected price is $1.50/bu, then the value per acre is $300.

Deciding whether to treat involves two steps. First, determine the percentage of leaves infested with mites (an infested leaf has one or more live mites). Compare that number with the first number in the table. If the field average is less than the table value, you don’t need to treat, but do continue to monitor the field. If the field average exceeds the table value, then estimate the percentage of total leaf area damaged by mites. If the field average exceeds the table value, it is likely that treating for spider mites will increase yield above the cost of treatment.

Also, note that control costs are a factor in this table. Depending on the product chosen, the critical values may change greatly. For example, under the column $300 market value, the critical value for percent infested leaves varies from 20%, if control costs are $10, to 49% if control costs are $25.

Products labeled for spider mite control on corn include dimethoate (several formulations), Comite 6.55EC and Capture 2EC, and Oberon 4SC. Dimethoate has performed reasonably well in Nebraska against Banks grass mites, but not twospotted spider mites. If twospotted spider mites are present, either Comite, Oberon, or Capture would provide better control. See product labels or the UNL Department of Entomology Web site on spider mite control for specific rates and restrictions. With the exception of Comite, pesticides do not kill mite eggs, and there is a possibility of re-infestation of the fields as eggs hatch out. Since many of their natural enemies were probably killed by the initial pesticide application, these populations may build up rather quickly and should be monitored carefully. Corn is unlikely to benefit from treatment for spider mites after the dent stage.

Twospotted spider mites also may develop on soybeans. No research has been conducted that would allow calculation of an economic injury level for twospotted spider mites on soybeans. Iowa State University Extension specialists have suggested that control may be warranted when infested plants have substantial spotting or leaf yellowing and live mites, but before mites cause browning and leaf drop. Damage 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. Especially watch soybean fields treated for soybean aphids or other insects, as foliar insecticides reduce the population of natural enemies that help reduce spider mite numbers.

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 large numbers of mites are found. Although late season infestations may accelerate soybean senescence and increase pod shattering, use caution when evaluating whether to treat with pesticides. Many of the pesticides used for mite control have 21-28 day preharvest intervals.