The two-spotted spider mite can be a problem in soybean fields, and sometimes in corn fields, in drought years. "We recommend scouting corn and soybean fields for mite infestations this year because this insect can thrive in hot, dry conditions," says Clarke McGrath, Iowa State University Extension field agronomist at Harlan in western Iowa.
The tiny pest gathers at field edges, then can disperse with wind to develop a field-wide infestation. ISU agronomists recommend checking 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.
Depending on what you find, you may need to spray an insecticide. McGrath says you should work with your area ISU Extension field agronomist, local crop consultant or ag chemical dealer for help with treatment decisions. McGrath writes a column, called "Corn-Soybean Insight" each month in Wallaces Farmer magazine.
Keep an eye on spider mites on soybeans, as mite populations can explode
McGrath has spent a lot of time the past two weeks checking fields and watching spider mite colonies increase. "We've picked out some sentinel plots in the area to do our spot checks, and growers and dealers are watching as well," he says. "Levels of infestation are generally still below what we would consider treating in many areas so far. If the weather continues to stay hot and dry, levels can blow up quickly. We plan on checking the sentinel plots every couple days."
What is the economic damage threshold you should use for deciding when it pays to treat for this pest? Treatment thresholds for spider mites do not exist, notes McGrath.
He points out that ISU Extension entomologists say "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 soybeans is to treat the crop between R1 to R5 stage of growth (bloom through beginning seed set) when most plants have mites, and if there is heavy stipling and leaf discoloration apparent on lower leaves."