PUTNAM COUNTY — Farmers have to be prepared every year for insect damage in their growing crops. Weather often determines which insect pest will be the most problematic during the growing season. One may believe that dry weather would cause egg deposits to dry up and decrease the survival rate of larvae, or make it more difficult for adults to find host plants. This may be true for some insects, but others are quite adaptive. Dry weather generally favors insects or insect-like pests that we do not see in normal years, such as spider mites. They will often show up in field borders first as they move in from other habitats, such as nearby ditches that have recently been mowed or the vegetation has become dry from moisture stress.

Spider mites are difficult to see. Look for injury signs – small yellow spotting on the upper side of leaves. Stippling is another word that entomologist may use for the yellow spotting. In soybeans, this damage usually begins in the lower canopy and progresses upward as the mite population increases. Heavily infested leaves may also have light webbing similar to spider webs.

There are no number-based thresholds available for mites, in part because it is impractical to count them in a scouting context (they are almost impossible to see with the naked eye and tough to see with a hand lens). During drought, populations can increase rapidly so farmers should scout fields every 4 to 5 days during dry conditions.

Farmers or crop consultants will walk a broad pattern in the field and examine at least two plants in each of 20 locations. Plants will be given a 0 to 5 point value following an integrated pest management scale developed by the University of Minnesota. A miticide is recommended when fields reach level 3. Since the two-spotted spider mite is an arachnid and not an insect (eight legs versus six), standard insecticides are generally not effective. Common choices for spider mite control in soybeans are products containing chlorpyrifos, dimethoate, or bifenthrin.

Here is the point system: Zero points for no spider mites of injury observed. One point for minor stippling on lower leaves, no premature yellowing observed. Two points for stippling common on lower leaves, small areas on scattered plants with yellowing. Three points when heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common and some lower leaf loss. Begin Spraying at this time. Four points start when lower leaf yellowing readily apparent. Leaf drop common. Stippling, webbing and visible mites are common in middle canopy. Mites and minor stippling present in upper canopy. Economic loss has occurred and will continue without an insecticide application. Level five starts when lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy

Dry weather also tends to increase grasshopper populations and damage, often along field borders. Grasshoppers are much easier to kill when they are small, so timely treatment is helpful. A general defoliation threshold can be used for grasshoppers and other leaf-feeding insects. The other leaf feeding insects include Japanese beetles, green cloverworms, and other caterpillars. If soybeans are in pre-bloom they can tolerate up to 40 percent defoliation before treatment is advised, and 15 percent defoliation from bloom to pod-fill. These percentages refer to whole-plant defoliation, not just a few leaves. Most local fields are in full bloom to early pod development.

A word to the wise about treating for defoliators: A good reason to avoid spraying too soon, apart from simple economics, is that many pyrethroid products will actually make spider mite problems worse by killing off the natural predators. We often see the most intense spider mite flare-ups in fields that have been treated with a broad-spectrum pyrethroid.

Farmers will be checking their fields over the next several weeks for mites and insects. Dry weather does not prevent their feeding. Farmers have to be vigilant and control these pests even in drought years. Just another challenge and cost they face besides the weather to bring food to our tables.

For additional mite and insect information, the Ohio State University Extension Field Guide for Corn, Soybean, Wheat and Alfalfa may be purchased at the Putnam County Extension Office.

Acknowledgments: Dr. Kelley Tilmon, and Dr. Andy Michel, OSU Extension Field Crop Entomologists and Ed Lentz recently shared the above information.