A few areas in southeast Iowa and surrounding states have already reported heavy populations of the two-spotted spider mite feeding on soybeans this year. Typically, you don’t see this insect pest reaching economically damaging levels until late July or early August when conditions (hot and dry) are favorable for their growth. Spider mites are arriving early this year.
The last time Iowa had a real issue with spider mites was four years ago, in 2012. If temperatures turn somewhat cooler and it rains, that can help take care of the problem. The U.S. Drought Monitor estimates about 70% of Iowa is abnormally dry or in severe drought as of June 28, 2016.
Scout both corn and soybean fields for spider mites this year
“I recommend scouting corn and soybean fields for mite infestations this year because this insect thrives in hot and dry conditions,” says Erin Hodgson, Iowa State University Extension entomologist.
“Spider mites generally reach economically damaging levels in late July or early August when conditions are favorable for their growth,” she says. “However, the two-spotted spider mite can start building populations in June during years with early-season temperatures greater than 85 degrees F, humidity is less than 90%, and moisture levels are low. These are ideal conditions for the two-spotted spider mite, and its populations are capable of increasing very rapidly.”
Helpful tips for scouting and managing spider mites
Using a hand-held magnifying lens is recommended if you are scouting for two-spotted spider mites. The little critters are less than 1/16 of an inch (less than 1/60 inches). They can be mistaken for specks of dirt to the naked eye (see photo 1).
Two-spotted spider mite larvae have six legs, and nymphs and adults have eight legs, notes Hodgson. Mites can be removed by shaking the leaves of soybean or corn plants onto a white piece of paper, and then look for moving mites. Two-spotted spider mites are typically a cream or green color when feeding on corn or soybean plants. The mites can also be an orange-to-red color when conditions are unfavorable for their growth.
Two-spotted spider mites can aggregate at the field edges, especially if there are weeds surrounding the borders, says Hodgson. Eventually they can disperse with the wind to develop a field-wide infestation. She encourages you to look at the edge rows first to see if mites can be found. If their presence is confirmed, then you can estimate populations throughout the field by walking a “Z” or “W” pattern.
Two-spotted spider mites begin feeding on the bottom of the plant, and move to the top as the plant’s health deteriorates, says Hodgson. Although they lack wings, two-spotted spider mites disperse with the wind to move from dying plants to areas with healthy plants. That’s why it is important to scout healthy areas of an infested field that are downwind from damaged areas.
Recognize the early symptoms of spider mite injury
Early symptoms of two-spotted spider mite injury will appear as small yellow dots or stipples on the lower leaves of the plants. Hodgson says prolonged feeding will cause the infested leaves to turn completely yellow, then turn brown, and eventually the leaf will die and fall from the plant. The webbing is visible on the edges and underside of leaves, and is an indication of prolonged colony feeding (see photo 2).
Hodgson cautions: “The two-spotted spider mite is capable of reducing soybean yield by 40% to 60% when left untreated; drought-stressed plants could experience even more yield loss.”
When does it pay to spray insecticide for spider mites?
“Exact treatment thresholds for spider mites in corn and soybeans do not exist,” says Hodgson. Instead, the decision to treat should take into consideration how long the field has been infested, and the mite density in the field, including the number of spider mite eggs present. Also consider mite location on the plants, moisture conditions, and plant appearance.
A general guideline for soybeans is to treat between the R1 to R5 growth stage of the soybean plants (i.e., beginning bloom through beginning seed set) when most soybean plants have mites, and heavy stippling and leaf discoloration is apparent on lower leaves (see photo 3). Foliar insecticides are recommended to be applied on corn from R1 to R4 growth stage (i.e. silking through dough stage) when most corn plants have mites at or around the ear leaf and are showing 15% to 20% leaf discoloration.
Hodgson notes that entomologists Bruce Potter and Ken Ostlie at the University of Minnesota have developed a two-spotted spider mite rating scale to help farmers and crop consultants make treatment decisions:
0 – No spider mites or injury observed
1 – Minor stippling on lower leaves and no premature yellowing observed
2 – Stippling common on lower leaves and small areas on scattered plants with yellowing observed
3 – Heavy stippling on lower leaves with some stippling progressing into the middle canopy and leaf yellowing and some leaf loss observed; mites scattered in the middle and top canopy [Economic threshold]
4 – Lower leaf yellowing readily apparent and leaf drop common; stippling, webbing, and mites common in the middle canopy; mites and minor stippling present in upper canopy [Economic injury]
5 – Lower leaf loss common and yellowing moving to the middle canopy; stippling and distortion of upper leaves common; mites in upper canopy observed.
Which insecticides are recommended for spider mite control?
Hodgson says organophosphates are the recommended insecticidal chemistry for two-spotted spider mite control (e.g., dimethoate and chlorpyrifos). Bifenthrin is the only pyrethroid to show efficacy against two-spotted mite.
“Insecticides may not kill the eggs, thus a treated field should be scouted seven to 10 days after application to determine if a second application is necessary,” she says. “As always, refer to the label of the insecticide you are using for the appropriate application rates and field re-entry intervals.”
Make sure you get good coverage when spraying insecticide
To improve application coverage, Hodgson suggests you consider increasing the water volume in the insecticide mixture to make contact with spider mites. Treatments along the borders of fields may also be a cost-effective option if heavy spider mite populations are restricted to edge rows.
Treatment of two-spotted spider mites may not be required when temperatures drop below 85 degrees and humidity levels are greater than 90% for an extended time. That’s because a naturally-occurring fungus can control populations. “Mites that are infected by the fungus will appear brown and will not move on the piece of paper you use for scouting,” says Hodgson.
Additional resources on two-spotted spider mites: