Two-Spotted Spider Mite

Mites are prevalent in plants and crops across the world. Within any given plant group, whether in the wild, in a greenhouse, or out in the field, there are various mite species that can infect and damage the plant in question. In field crops--particularly in soybeans--the two-spotted spider mite attracts the most attention from entomologists, crop advisors, and farmers.

The two-spotted spider mite, Tetranychus urticae, is only an occasional pest of soybeans across the northeast and upper Midwest. It is most often associated with drought years in these regions. In drier regions of soybean production areas like Western Kansas, Nebraska, Texas, and Oklahoma, the mites are considered a significant pest and pose a potential economic problem every year. You can scout virtually any given field with a magnifying lens and find populations of spider mites. Finding them at economic levels is another matter, other than in regions where they proliferate every year.

Also notable are the biological controls in the environment in the form of predatory insects, other mite species, and fungi that exist in the background keeping the two-spotted spider mites in check. In fact, there is research that shows  early season pyrethroid insecticides applied for control of other insects can increase mite populations and flare-ups significantly because the beneficial controls are killed by the insecticide applied earlier.

Two-Spotted Spider Mite Identification

Spider mites are technically closer relatives to the spider than the mite. Two spotted mites are minute in size, requiring a hand lens of at least 10X magnification to see. They are between 1/60th and 1/80th of an inch in length.

Mites colonize on lower leaves of the target plant, creating a thin web across the underside surface of the leaf. Symptoms include a speckled appearance on the leaf, followed by curling, yellowing, senescence, browning, and death when found in severe infestations.

Stages of the Two-Spotted Spider Mite

Egg – shiny, straw colored, and very small. Webbing keeps them attached to the leaf and makes them difficult to see. They incubate for a period of 3-20 days (according to weather) then hatch.

Larva – six-legged, colorless. The larva resembles the body form of both the nymph and the adult. Slightly larger than egg stage. Larva consume very little leaf tissue.

Nymph – nymphs are 8-legged, and resemble small adults, but smaller. There are two stages as a nymph before moving on to the adult stage.

Adult – 8-legs, reaching mature size of 0.4 mm – 0.3 mm in length. Color will vary from yellow to green to orange to brown with 2 dorsal spots, which are actually transparent ‘windows’ to the contents of the gut.

An adult female can have up to 300 offspring throughout her lifecycle. Of the eggs laid, unfertilized eggs develop as males, fertilized as females. Typical populations are mostly female.

Two-spotted spider mites are wingless but do move through the field. The adults prefer overwintering in non-cropland, weedy patches, grass waterways, edges, idle land, and perennial sods like pastures. The overwintering adult will come out of dormancy to feed on a wide range of grasses, clovers, and other broadleaf winter annuals like chickweed. In heavy infestations, this means damage is first observed along the edges of fields, lending a degree of effectiveness toward perimeter spot treatments. The assurance that all four stages might exist in a field during the season requires that if discovered, a thorough scouting regimen to evaluate the entire field should be conducted before presuming only a spot treatment is needed.

Control of the Two-Spotted Spider Mite

Before deciding to treat a given field, many factors must be taken into account. There are limited economic threshold data available to fine tune the treatment decision.

The advisor or farmer must first have a thorough understanding of the pest’s life cycle, and tendencies during different climate and weather trends for a given season. Combining this knowledge with a practical but thorough scouting regimen can assist greatly in determining whether or not to treat. The mite’s ability to reduce yield is directly related to duration and intensity. Duration is driven by when initial infestations are detected, and intensity will be driven by populations building through the season, which will be further determined by the weather patterns and subsequent presence of biological pathogens.

However, when found in heavy populations at the time the setting is setting, two-spotted spider mites can cause seed abortion, affect pod fill, and impact other yield factors. If the two-spotted spider mite is detected and dry weather is imminent, a labeled treatment may be useful. When spot treating, be sure to also treat a perimeter of the infested area, usually up to 200 feet around the actual area identified. Spot treats are most effective before further generations develop and movement has progressed across a field. If scouting shows moderate populations across the entire field with several ‘hot spots’ of a field, it may be best to treat the entire field.

Finally, selecting the proper material is important. When treating for two-spotted spider mites alone, using a targeted miticide would be recommended. Using a broader spectrum insecticide that also has two-spotted spider mite on the label can reduce populations of predators and re-infestation of uncontrolled eggs and tolerant portions of the population could cause a flush later.

Mites do not like wet conditions, and their predatory pathogens prefer them. One might then assume that irrigating will reduce mite surges. This is only true if the canopy of the crop remains at a high humidity for an extended period of time (minimum of 48 hours). Sometimes, irrigating in a drought can actually increase mite damage because the plants take up the water, but the leaves dry out on the surface more rapidly due to the drought conditions. It’s more about relative humidity and temperature than just moisture.

Always consult with your Certified Crop Advisor or Extension Specialist regarding controls of the Two Spotted Spider Mite.

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The information provided above was authored by John Diebel and provided by Farmers Business Network, Inc. for informational purposes only. It does not constitute an endorsement or recommendation of a particular course of action or product. Please conduct your own due diligence prior to selecting a particular course of action or product.
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