Populations Spike During Times of Drought
Two-spotted spider mite (Tetranychus urticae) is a pest of soybeans that proliferates during extended periods of drought. Drought conditions accelerate spider mite movement and reproduction and inhibit fungal pathogens that normally help keep spider mite populations in check. Economically damaging outbreaks of spider mites are relatively rare, but populations can grow rapidly when conditions are favorable.
Photo from Pioneer
Two-spotted Spider Mites Lifecycle
Two-spotted spider mites have four stages of development: egg, larva, nymph, and adult. Spider mites overwinter as adults in field edges and roadsides bordering fields, feeding on weeds until spring. After early spring mating, female spider mites lay eggs on weeds that usually hatch to the larval stage in 3 to 5 days. Unlike most damaging insects in soybeans, spider mites do little feeding during the larval stage of development.
Nymphs are young eight-legged mites that resemble full-size adults but do not yet have reproduction capability. Adults are tiny at only 1/60 (female) to 1/80 (male) inch in size when fully developed, with females laying an average of 50 to 100 eggs during their lifetime.
The entire life cycle of this pest can be completed within 5 to 14 days, depending on environmental conditions. The fastest reproduction occurs when temperatures are over 85 °F (29 °C) and dry weather conditions. During heavy outbreak years, all stages of mites may be present in the field at one time. Two-spotted spider mites have the potential for up to 10 generations per year during the growing season.
Photo by Jim Kalisch
Two-spotted spider mites damage crops by piercing plant leaves and feeding on the plant juices with their mouth parts. Mites suck on the bottom sides of soybean leaves and remove moisture and nutrient contents from plant cells, resulting in a yellow or whitish spotting on the top side of the leaf surface. In heavy infestations, a common visual symptom of spider mite feeding is leaf burning and stippling.
Hot spots are typically noticed first on field margins as infested plants appear wilted. Drought-prone fields or field areas that contain lighter soils or sands are often affected first by spider mites. As populations increase, spider mites will move out across the entire field if left unchecked. Fields heavily infested by mites can cause premature leaf drop and significant reductions in yield.
Populations of spider mites increase significantly during extended hot, dry conditions. This is due to a reduction in predators and naturally occurring pathogenic fungi that keep populations at non-economic levels in normal years.
Spider Mite Scouting and Economic Thresholds
Look on the undersides of affected soybean plants and leaves for mites, eggs, and webbing in the lower canopy. Mites are almost impossible to see with the naked eye, so doing a simple “paper test” is a quick and easy way to confirm their presence. Shaking the plant onto a white piece of paper should allow you to see the tiny orange- to yellow-colored mites slowly moving on the paper.
There is currently limited information regarding the potential economic threshold for two-spotted spider mite infestations in soybeans, which makes treatment decisions challenging. Some extension sources suggest treating spider mites if 20 to 50 percent of the leaves are discolored before the pods are set. After pod set has begun, the suggested treatment threshold is 10 to 15 percent of the leaves discolored.
Consideration for the treatment of two-spotted spider mites should take several factors into account:
- Are other insect pests that cause economic injury (such as soybean aphids, bean leaf beetles, and grasshoppers?)
- What are the weather trends? If heavy rains and moderating temperatures occur, mite populations may be reduced or contained in the short term.
- Are there thrips, pirate bugs, mite destroyer beetles, and/or naturally occurring fungi in the field? Under proper conditions, these beneficials can significantly reduce or limit populations of two-spotted spider mites.
- Is the outbreak confined to field edges or borders? If mite outbreaks are caught on outside field edges before moving across the entire field, spot treatments or treating field margins might reduce the need for whole field treatments. If scouting reveals that mites have spread across the field, then whole-field protection will be necessary.
If hot and dry weather persists, spider mites will continue to build, and it will be important to control them. Field scouting is necessary for the detection of early outbreaks and for effective early treatments and control.
Rating Scale to Assess Damages by Spider Mites
U of MN Data
No spider mite injury observed.
Minor stippling on lower leaves. No premature yellowing of leaves observed.
Stippling common on lower leaves. Small areas of yellowing of leaves on scattered plants.
Spray threshold: Heavy stippling on lower leaves, with some stippling progressing into the middle canopy. Mites are present in the middle canopy, with scattered colonies in the upper canopy. Lower leaf yellowing is common, and there’s some lower leaf loss.
Economic loss: Lower leaf yellowing is present. Leaf drop is common. Stippling in the middle canopy with webbing and mites. Mites and minor stippling are present in the upper canopy.
Lower leaf loss is common, with yellowing or browning moving up the plant into the middle canopy. Stippling and distortion of the upper leaves are common. Mites are present in high levels in the middle and lower canopy.
Treatment and Control of Spider Mites
Chemical control of spider mites is challenging. While some pyrethroid products may suppress the activity of spider mites, nearly all synthetic pyrethroid products have a detrimental effect on spider mite predators. The lack of full control by pyrethroids allows mite numbers to increase unchecked or “flare up” when conditions are favorable.
Photo by Jim Kalisch
Like other soybean insects, Spider mites are found on the undersides of soybean leaves. For optimal control of spider mite populations, use high pressure and a high carrier volume to achieve thorough crop canopy coverage and penetration. Using higher pressures (40 to 60 psi) and increased carrier volume (15 to 25 gpa) will improve overall performance.
Unfortunately, residual control of most treatments is short-lived, and applications will only control adults and nymphs. Treated fields need to be re-scouted five to ten days following application. It is possible that a second application might be necessary to pick up any newly hatched spider mites, so be sure to scout treated fields about a week after application.
Conditions can change quickly depending on environmental conditions. Heavy rainfall or changes in temperature, humidity, or crop conditions may warrant a re-evaluation of mite populations before treatments are made.
AFS Recommendations for Spider Mites
AFS recommends Agri-Mek SC for its extended residual control that can last up to 21 days in soybeans. Agri-Mek SC is easily tank mixed for broad-spectrum control of additional pests and can be mixed with Endigo ZCX for soybean aphids, Japanese beetles, and stinkbug species.
For more information on Agri-Mek SC, Endigo ZCX, and other insecticides give you trusted AFS agronomist a call.
Asmus Farm Supply, Inc.
Consulting Clients with Sustainable Recommendations to Maximize Production