Robbie Longest, Essex County Extension Agent
Carrie Ortel, Extension Soybean Agronomist
Tim Bryant, Extension Entomologist
Recently, soybean aphids have been confirmed at higher populations than typically expected in August. While many folks are scouting for worms and stinkbugs, we also recommend looking for aphids in your soybean fields as well. The key variables to consider regarding managing soybean aphids are the number of aphids per plant and the soybean growth stage. Information from our Pest Management Guide on Soybean Aphids in summary including info on thresholds and sampling are as follows (VCE Field Crops PMG, Publication 456-016):
- Number of aphids per soybean plant
The current economic threshold for aphids is an average of 250 aphids per plant, on two consecutive field visits spaced about 5-7 days apart. This is because aphid populations can “crash” quickly due to heavy pressure by natural enemies like lady beetles, parasitic wasps, and fungal diseases. When scouting, choose a “Z” or “W” shaped pattern to cover the entire field and sample at least 20 to 30 plants per field by examining the entire plant, including stems and upper and lower leaf surfaces. Use the aphid/plant average for determining the need for treatment.
- The soybean growth stage
The threshold of 250 aphids per plant applies to soybeans through the R5 growth stage (3 mm long seed in the pod at one of the four uppermost nodes on the main stem), after which time plants can tolerate 1,000+ aphids with no threat to yield. If an insecticide is applied for aphids, pyrethroids (e.g. bifenthrin, Warrior II, Mustang Max, etc.) can be effective for management, but choosing a more selective insecticide can preserve natural enemy populations and limit future flare ups of aphids or other pests.
More information on soybean aphids can be found from Purdue University here: https://ag.purdue.edu/department/entm/extension/field-crops-ipm/soybean/soybean-aphids.html as well as a recent article published by University of Nebraska-Lincoln here: https://cropwatch.unl.edu/2023/soybean-aphid-scouting-and-management/.
See the Virginia Tech pest management guide for a full list of labeled insecticides for soybean aphids. Please feel free to reach out to Robbie Longest (robbiel7@vt.edu) or Tim Bryant (btim2@vt.edu) with any additional questions.