April's Ag Advice By April Borders Soybean producers are encouraged to be out monitoring their fields for the presence of the soybean aphid. Soybean aphids have already been found in Clay, Turner, Yankton and Bon Homme counties. Some fields have already been sprayed due to high aphid populations.
Plant injury is caused by the insects feeding on the plant sap which can cause yellowing, cupping and wilting of soybean leaves. Plant growth may also be stunted. Soybean aphids are also capable of transmitting mosaic viruses that can cause soybean diseases, such as soybean mosaic virus, bean yellow mosaic virus and alfalfa mosaic virus.
Scouting for soybean aphids should start as soon as the plants produce two trifoliate leaves (V2) and continue until the beginning seed stage (R5) of soybean development. The insects can be found mainly on the young leaves and stems of the soybean plant early in the season. Count soybean aphids from at least 10 plants at 10 representative locations in the field. The entire plant needs to be examined as the aphids could be in the stems, growing points, flowers, pods, or underneath the leaves.
Based on findings from research at SDSU, the Extension Service recommends that soybean aphids be controlled at R2 (full bloom stage). This stage also marks the beginning of the most sensitive stage in soybean growth and development. Remember the old saying: �Soybean yield is made in August.� Dr Mike Catangui, SDSU Extension entomologist, would like to add to the saying: �Protect thy bean in early August.�
Field scouting is very important. We really need to be out scouting our soybeans as we enter into the blooming stage of growth. Monitor the new growth for signs of the aphids as the new growth is more attractive to them.
SDSU Extension Service now recommends that the economic threshold be determined using the guidelines that can be found on the plant science Web site. Dr. Mike Catangui�s Web pageis http://plantsci.sdstate.edu/ent. Just click on the soybean aphid and it will take you to a series of scenarios and charts. He has set up charts that take into consideration plant growth stage, cost of application and market value. Control efforts should be considered using these charts.
For more information on soybean aphids or if you have questions about field scouting contact the Clay County Extension Office at 677-7111.