Plant Health Initiative

Two-spotted spider mite

Two-spotted spider mites are tiny greenish-yellow arachnids. Photo credit: Marlin E. Rice, Iowa State University

Yellow "stippling" or speckling on green leaves are a diagnostic sign of spider mite infestation. Photo credit: Phillip Glogoza and Ian MacRae, University of Minnesota Extension Click on image to view a larger version

Two-spotted spider mites (Tetranychus urticae) are present in soybean crops every year, but they are not often a significant problem. The risk of significant two-spotted spider mite infestations increases considerably during periods of hot weather and prolonged drought. For farmers and crop advisors not familiar with spider mites, the progression of symptoms from silvering, yellowing, browning, lower leaf loss and death, may be mistaken for drought symptoms.

Two-spotted spider mites feed on a wide variety of plants, including soybeans, dry beans, alfalfa, corn, vegetables, ornamentals, and trees. Mites overwinter as adults which lay eggs in the spring. Hatching mites establish colonies on the undersides of leaves that produce the webbing over the leave surface, prompting the name “spider” mites.

How you manage aphids may increase the number of two-spotted spider mites

One factor now complicating spider mite management decisions is the soybean aphid, and vice versa. Pyrethroid insecticides commonly used to control soybean aphids do not control spider mites. and can cause mite populations to rise above untreated levels by killing off the natural enemies that help control spider mites. They also stimulate movement of the mites, spreading the infestation, and there is some data from orchard systems to indicate they may increase the reproductive rate of existing female mites and increase the number of females being born into the population.

This worsening of mite infestations can occur in as little as 7-10 days after application. Organophosphate insecticides have provided a better kill rate, and are a better choice for mite management, however they do not kill the eggs and often require two applications, further disrupting the population of mite predators.

How drought affects spider mite populations

Spidermites populations are held in balance by natural enemies, weather, and host vigor. Drought triggers spider mite outbreaks in soybean and corn by upsetting this balance in several ways:

• Drought stress accelerates spider mite movement to soybean and corn from surrounding permanent vegetation and alfalfa as it dries down or is cut for hay. Cutting initiates mass movement into adjacent soybean under drought conditions.
• Drought stress increases the amount of nutrients for spider mites in the plant sap.
• Drought inhibits or stops the activity of naturally-occurring fungal parasites attack mites and generally keep populations in check. Disease outbreaks are fostered by cool, highly humid conditions that favor spore formation and mite infection. Hot, dry weather or application of fungicides stops or kills the parasites.
• Hot temperatures increases spider mite reproduction so that predatory insects and mites can’t keep up.

Source: University of Minnesota