Scouting for spider mites in field crops is encouraged with prolonged dry conditions. Twospotted spider mites can increase whenever temperatures are greater than 85°F, humidity is less than 90 percent, and moisture levels are low. These are ideal conditions for the twospotted spider mite and populations are capable of increasing very rapidly.
Egg: Eggs of twospotted spider mites are laid singly on the underside of leaves. They are 0.14 mm long and become white as they age. Just before hatch, the red eyespots of the larvae are visible.
Immatures: Twospotted spider mites are translucent during the larval stage, where they have six legs. They go through two nymphal stages, where they have eight legs.
Adult: Adult twospotted spider mites are extremely small (< 1/60 inch long) and difficult to see. A hand magnifying lens may be required to see them in the field, and they can be mistaken for specks of dirt to the naked eye. Twospotted spider mites are typically cream or green in color when feeding on corn or soybeans, but they can be orange or red when conditions are unfavorable for growth. As the name suggests, these mites have two spots on the body near the head (Photo 1).
Twospotted spider mites overwinter in field margins and sheltered areas where permanent hosts are available. Twospotted spider mites attack a wide variety of plants, including corn, soybeans, alfalfa, vegetables, and trees. Briefly, the life cycle of twospotted spider mite includes a spherical, white egg that hatches into a 6-legged larva. The nymphs and adults both have 8 legs. Immatures (larvae and nymphs) stop moving prior to molting. A generation typically takes 5-20 days but is dependent on temperature. Development is more rapid when temperatures are over 90°F. Females produce approximately 100 eggs each, and populations can increase 70-fold in as little as 6 days during the summer.
Twospotted spider mites typically reach crops either by walking short distances from overwintering hosts or arriving from longer distances using silk threads to move with the wind. Most often, twospotted spider mites aggregate at the field edges, especially if there are weeds surrounding the border. Eventually, they may disperse with the wind to develop a field-wide infestation.
Plant injury occurs when twospotted spider mites use a piercing-sucking mouthpart to injure leaf cells and extract the contents, leading to loss of photosynthetic area and uncontrollable water loss. Feeding injury produces white to yellow spots (called stippling, Photo 2 and Photo 3), typically on the underside of leaves. Prolonged feeding will cause infested leaves to turn completely yellow, then brown, and eventually the leaf will die and fall from the plant. This is often mistaken for “firing” of the lower leaves during drought stress in corn. Webbing often is visible on the edges and underside of leaves and is an indication of prolonged colony feeding (Photo 4). Twospotted spider mites begin feeding on the bottom of the plant and move to the top as the plant’s health deteriorates. This pest is capable of reducing soybean yield by 40-60 percent when left untreated. Data from Colorado suggests corn yields can be impacted 6-48 percent for grain and up to 40 percent for silage.
Infestations of twospotted spider mites are most common in hot, dry years. Drought-stressed crops, high temperatures, and low rainfall and humidity all promote high levels of twospotted spider mites in crop fields. Other conditions, such as compaction, that limit water uptake can mimic drought conditions; these fields may experience spider mite injury in an otherwise unfavorable year. Drought conditions tend to accelerate movement and development of spider mites and provide an unsuitable environment for a fungal disease that attacks mites (see Biological Control section). Additionally, insecticide applications can contribute to twospotted spider mite population growth: natural enemies are killed with broad-spectrum insecticide applications, and pyrethroids, in particular, can cause flare-ups of spider mites.
A hand magnifying lens is recommended to scout for twospotted spider mites as they can be mistaken for specks of dirt on the leaves. Mites can be collected by shaking leaves onto a white piece of paper and then looking for moving mites.
Because aggregations typically begin at the field edge, look at edge rows to see if mites can be found. Look at the undersides of leaves, beginning at the bottom of the plant and working up. Look for mites, stippling, or webbing. If their presence is confirmed, estimate populations throughout the field by walking in a “Z” or “W” pattern and randomly sampling plants. Check drought-stressed fields every 5 days while conditions persist because populations can grow rapidly.
Use the rating scale in Table 1 to make treatment decisions. Exact treatment thresholds for spider mites in corn and soybean do not exist. Instead, consider how long the field has been infested, mite density including eggs, mite location on the plant, moisture conditions, and plant appearance. A general guideline for soybean is to treat between R1-R5 (i.e., beginning bloom through beginning seed set) when most plants have mites, and heavy stippling and leaf discoloration is apparent on lower leaves (Photo 4). Foliar insecticides are recommended in corn from R1-R4 (i.e., silking through dough stage) when most plants have mites at or around the ear leaf and 15-20 percent leaf discoloration.
Table 1. Rating scale for making treatment decisions for spider mites in corn and soybean. Adapted from Bruce Potter and Ken Ostlie (University of Minnesota). ET = economic threshold; EIL = economic injury level
|0||No spider mites or injury observed|
|1||Minor stippling on lower leaves; no premature yellowing observed|
|2||Stippling common on lower leaves; small areas with yellowing observed|
|3 - ET||Heavy stippling on lower leaves progressing into middle canopy; leaf yellowing and leaf loss observed; mites scattered in middle and top canopy|
|4 - EIL||Lower leaf yellowing readily apparent and leaf loss common; stippling, webbing, and mites common in middle canopy; mites and minor stippling in upper canopy|
|5||Lower leaf loss common and yellowing into middle canopy; stippling and distortion of upper leaves common; mites in upper canopy|
Biological Control: In most years, twospotted spider mite is controlled well by a naturally occurring fungal pathogen. This fungus prevents outbreaks during years with adequate rainfall and performs best when temperatures are below 85°F and humidity is over 90 percent for an extended period of time (at least 12 hours). Insecticide treatment can be delayed or prevented if ideal conditions for fungal development are forecasted. Mites that are infected by the fungus will appear brown and will not move on the piece of paper used for scouting. Other natural enemies in the field include lady beetles and lacewings. Be aware that fungicides also target the beneficial fungus that attacks spider mites, and broad-spectrum insecticide will diminish natural enemy populations.
Chemical Control: Insecticide options for twospotted spider mites are rather limited. Dimethoate (organophosphate, Group 1B), bifenthrin (pyrethroid, Group 3), and abamectin (Group 6) are the only effective chemistries available for mites in corn and soybean. Insecticides target adults and immatures only. A few miticides are also available for use on twospotted spider mites in corn and soybean: hexythiazox (Group 10A), etoxazole (Group 10B), propargite (Group 12C), and spiromesifen (Group 23). These products target eggs and immatures. For a list of pesticides, current in 2021, visit this resource.
Insecticides do not kill the eggs; thus, a treated field should be scouted 7-10 days after application to determine if a second application is necessary. Keep in mind that many insecticides cause flare-ups of twospotted spider mites. Treating field edges may be a cost-effective option if heavy spider mite populations are restricted to edge rows. Usually, by the time spider mite is noticeable along the field edge, colonies are widespread throughout the field. As always, refer to the label for the appropriate rates and re-entry intervals. Remember that high temperatures shorten residuals of insecticides, and good coverage is needed to make contact with mites on the underside of leaves. To improve application coverage and contact of mites, consider increasing the water carrier volume.
Twospotted spider mite has become resistant to most insecticide/miticides in the southern and central plains states; however, this has not been documented in the Midwest. Even so, take care to rotate modes of action when applying pesticides to control twospotted spider mites. In soybean, insecticidal control decisions should take into account whether soybean aphid is also present in the field. In many areas of the Midwest, soybean aphid has documented field-evolved resistance to bifenthrin, which is one of the only insecticide active ingredients available for twospotted spider mites. See the soybean aphid encyclopedia article for more information.
Twospotted spider mites on soybean and corn (University of Minnesota)
Twospotted spider mite management in soybean and corn (University of Wisconsin)
Originally by Erin Hodgson, updated in 2022 by Erin Hodgson and Ashley Dean