The U.S. Drought Monitor shows southeast and south-central Iowa experiencing prolonged heat and moisture stress this summer. In early August, infestations of two-spotted spider mites were detected in corn and soybean fields.
Iowa State University Extension entomologist Erin Hodgson recommends scouting fields for mite infestations this month, especially in these drought-affected areas.
Spider mites generally reach economically damaging levels in late July or early August when conditions are favorable for growth of this pest.
“However, two-spotted spider mite can start building populations in June during years with early-season temperatures greater than 85 degrees F, humidity less than 90% and low moisture levels,” Hodgson says. “These are ideal conditions for the two-spotted spider mite, and populations are capable of increasing very rapidly.”
Hodgson offers the following information and recommendations for scouting and managing this pest in corn and soybean fields.
Scouting fields for spider mites
A handheld magnifying lens is recommended for use when scouting for two-spotted spider mites. These insects are tiny creatures, less than 1/60 of an inch. They can be mistaken for specks of dirt to the naked eye.
Two-spotted spider mite larvae have six legs, and nymphs and adults have eight legs. Mites can be removed by shaking leaves 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.
Photo by Frank Peairs, ipmimages.org
HOT AND DRY: Spider mites thrives in hot, dry weather.
Two-spotted spider mites can aggregate at the field edges, especially if there are weeds surrounding the borders. Eventually this insect can disperse with the wind to develop a field-wide infestation. Hodgson encourages people to look at the edge rows first to see if mites can be found. If their presence is confirmed, then you need to estimate the populations throughout the field by walking a Z or W pattern.
Mites ride the wind
Two-spotted spider mites begin feeding on the bottom of the plant, and then move to the top as the plant’s health deteriorates. Although they lack wings, two-spotted spider mites disperse with the wind to move from dying plants to areas with healthy plants. Thus, it’s important to scout healthy areas of an infested field that are downwind from damaged areas, Hodgson says.
Early symptoms of two-spotted spider mite injury will appear as small yellow dots or stipples on the lower leaves of the plants, he says. Prolonged feeding will cause the infested leaves to turn completely yellow, then brown, and eventually the leaf will die and fall from the plant. Webbing is visible on the edges and underside of leaves and is an indication of prolonged colony feeding.
Two-spotted spider mite can reduce soybean yield by 40% to 60% when left untreated; drought-stressed plants could experience even more yield loss.
Photo by Adam Sisson, ISU
MANY MITES: Here’s what a heavy spider mite infestation on a corn leaf looks like.
Exact treatment thresholds for spider mites in corn and soybeans do not exist, Hodgson says. Instead, the decision to treat should take into consideration 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 a soybean field is to treat between R1 to R5 growth stage (i.e., beginning bloom through beginning seed set) when most plants have mites, and heavy stippling and leaf discoloration is apparent on lower leaves.
Foliar insecticides are recommended for spraying spider mites in corn from R1 to R4 (corn silking through dough stage). That’s when most plants have mites at or around the ear leaf and show 15% to 20% leaf discoloration.
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 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 used for scouting.
Photo by Whitney Cranshaw, ipmimages.org)
SYMPTOMS: Injury from spider mites on soybean plants includes yellowing of leaves.
Making treatment decisions
University of Minnesota entomologists Bruce Potter and Ken Ostlie developed a two-spotted spider mite rating scale to help 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
Organophosphates are the recommended insecticide chemistry for two-spotted spider mite control (dimethoate and chlorpyrifos). Bifenthrin is the only pyrethroid to show efficacy against the 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. As always, refer to the insecticide product label for appropriate application rates and re-entry intervals.
To improve application coverage, consider increasing the water volume to make contact with spider mites, Hodgson says. Border treatments may also be a cost-effective option if heavy spider mite populations are restricted to edge rows.
For more information on pesticide groups labeled for spider mite control in corn and soybeans, visit ISU Extension.
Source: Iowa State University