Agronomic Crop Insects

Bill Cissel, Extension Agent – Integrated Pest Management;

Continue to sample alfalfa for potato leafhoppers. Sample weekly starting seven days after cutting until final harvest. Ten sweep net samples should be taken in 10 random locations throughout the field when the alfalfa is dry. The threshold for alfalfa 3” or less is 20 leafhoppers per 100 sweeps, 4-6” tall is 50 per 100 sweeps, 7-10” tall is 100 per 100 sweeps and greater than 11” is 150 per 100 sweeps. If the field is more than 60 percent bud stage or if it has experienced “hopper burn”, the alfalfa should be cut instead of sprayed.

For more information on the identification, biology, and management of potato leafhoppers, please review our fact sheet:

Here is a link to our Insect Control in Alfalfa Recommendations (pure stands only):

Here is a Youtube video discussing how to sample for potato leafhoppers:

Two-spotted spider mite populations are increasing in some fields.
Despite the recent rain, Two-spotted spider mite (TSM) populations continue to increase. Scout for TSM by examining the underside of 5 leaflets in 10 locations for mites, noting the presence of mite eggs and the amount of leaf damage. The threshold for TSM during bloom to podfill is 20-30 mites per leaflet and 10% of plants with 1/3 or more leaf area damaged.

Concentrate scouting efforts on field edges for initial detection, especially edges bordered by grass and road ditches (it’s not unusual to also find hot spots in the interior portions of the field). TSM typically develop on grasses and other plants on field borders before ballooning into fields. Once TSM are detected, scout the interior portions of the field to determine if they have spread throughout the entire field. If only concentrated on field edges, spot treating may be an option. If spot treating on field edges, extend the treated area about 100 feet further into the field from the damaged area.

A hand lens is necessary when scouting for spider mites to see mite eggs and nymphs. Here is a short Youtube video demonstrating how to use a hand lens:

Here is a link to our Soybean Insecticide Recommendations for chemical control options:

Also note that in 2016, two miticides were registered for use on soybeans; Zeal SC, (Valent U.S.A Corporation) and Agri-Mek SC, (Syngenta Crop Protection, LLC). These are the only labeled formulations of these products. Please consult the label for rates, additional restrictions, and adjuvant requirements.

Zeal SC Supplemental Label for use on soybean:

Agri-Mek SC Label:

Continue to scout for defoliating insects. Continue to scout soybeans for defoliators including grasshoppers, bean leaf beetles, Japanese beetles, and green clover worms.

During reproductive growth stages (Bloom-Podfill), the threshold is 15% defoliation. Prior to blooming, the threshold is 30% defoliation. When estimating defoliation, randomly select leaves from the entire plant, not just the newest growth or leaves with the greatest amount of defoliation.

Here is an image to help in estimating the amount of defoliation:

Right on time, Dectes stem borer adults are beginning to emerge and can be found in soybean fields throughout the state. Dectes stem borers started showing up in soybean fields throughout the state and can be seen “hanging out” on leaves and easily captured with a sweep net.

Adult Dectes stem borer

Check out this week’s Guess the Pest for information on the Dectes stem borer.