Soybean farmers should watch for spider mites as this warm, dry stretch of weather continues. In fact, several fields throughout Kansas have been treated for spider mites recently, according to Kansas State University Extension Entomologist Jeff Whitworth.
"Spider mites are usually a problem in soybeans when the weather turns warmer and drier, and plants are experiencing drought stress," says Whitworth.
Whitworth said the spider mites typically feed on the underside of the leaves, creating a webbing that is often in the middle part of the crop canopy. The placement of the webbing makes it difficult to treat. Initially, the leaves turn yellow, then gray-green and finally bronze. Severe infestations can cause the leaves to fall off completely.
Although areas that received rain recently are at lower risk for spider mite activity, growers in dry regions need to decide if they will treat their plants for spider mites, according to Doug Shoup, K-State Research and Extension agronomist in the Southeast Area Extension office.
"If spider mite activity is in the mid-canopy approaching the upper canopy, it might be a good idea to treat the infested areas in the field," Shoup says. "Reducing the spider mite pressure will help alleviate the stress on the soybeans over the next week to 10 days until our next chance of rain."
Variable yield reductions have been recorded, but generally 40-60% reductions are possible when fields are infested during late vegetative or early reproductive stages.
"This seems to be the growth stage (podding) of most of the beans treated thus far," says Whitworth, who advises monitoring soybean fields after treatment as spider mites are difficult to control, noting that spider mite populations can quickly increase within a matter of days.
Whitworth says that if growers do decide to treat for spider mites, drop nozzles can better cover the underside of the leaves and promote more effective treatment.
"It is important to get complete coverage of upper and lower leaf surfaces and even then sometimes mite populations may recover quite rapidly after treatment," he says.
A list of insecticides registered for control of spider mites is available at local K-State Research and Extension offices or at the entomology website.