As we begin to approach the tail end of the growing season, it has been a popular time for growers to be walking fields and evaluate the crops. This time of year tends to be quiet in regards to issues seen in the field. However, one pest that has been noticed recently in soybean fields in parts of the Midwest has been the soybean aphid. Due to the cool dry weather and crop stress, some areas are seeing soybean aphids in fields that are exceeding thresholds. This is not a reason to be in panic mode, but to be aware and scout fields in case action needs to be taken.
Aphids are very small insects that are less than 1/16” long. They are yellow/green in color and have distinct black cornicles at the end of the abdomen. Both winged and wingless forms can be observed in the field. They have piercing-sucking mouthparts that remove sap from the soybean plant. Aphids start out in the upper canopy of the soybean and slowly move down the plant to lower leaves, stems, and pods. They move from their overwintering host buckthorn in May to colonize on soybeans through August until moving back to the buckthorn to lay eggs and overwinter.
As stated above, aphids have piercing-sucking mouthparts that remove sap from soybeans. If we see aphids in large numbers, it can cause a reduction in plant growth, vigor, and pod/seed counts. These causes can ultimately reduce yield in soybeans and may warrant treatment if populations are high enough. Soybean aphids when feeding also excrete unneeded plant sugars known as “honeydew” that leave plants with a shiny appearance. “Honeydew” also promotes the growth of sooty mold fungus which looks like a black powdery coat on the leaves. This can reduce photosynthetic capabilities of the plant.
Scouting and Thresholds
Scouting for aphids on soybeans can begin during mid to late June until the R6.5 stage. The most damaging time for soybean aphids is around the R1-R4 stage. Economic threshold for soybean aphids occurs when populations reach 250 aphids/plant and are increasing. It is recommended to take many random samples across the entire field (20-30 plants), excluding field borders, to get a good representation. Focus your search on the undersides of the leaf, even though at higher populations they can move to the stem of the plant.
Scouting regularly is advised since populations can increase fast if conditions are favorable (cooler temperatures, dry conditions, and plant stresses). Predators or beneficial insects include Asian Lady Beetle and Minute Pirate Bug that can regulate soybean aphid populations. The chart below (from Purdue University-Entomology) is easy guide for monitoring soybean aphids and when treatment is effective.
This picture shows various populations of Soybean Aphids on the leaves. This can be used to visualize how these levels of aphids appear when scouting fields.
If populations reach thresholds described above, there are several foliar insecticides that can be used to control soybean aphids. Complete coverage is key when treating for soybean aphids. High volumes of 20gal/acre and fine droplets utilizing a ground rig help with this. It is important to only treat when we approach thresholds due to economics and environmental stewardship. After an application, it is advised to still monitor the field for aphid populations and secondary pests. Some pyrethroid herbicides can flare up mite populations such as spider mites if environmental conditions are favorable (hot and dry). If this is observed, consider switching to an alternative insecticide.
If you have questions regarding scouting, treatment or thresholds, feel free to contact your local LG Seeds Agronomist for more guidance.
References and additional information:
Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
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Download a copy of this Technical Bulletin: Tech_345 - Scouting for Soybean Aphids