Scout Now for Soybean Aphids

July 29, 2016

Scout Now for Soybean Aphids

            Soybean aphids have been found in recent field surveys in northeast Nebraska. I checked a few local fields and found an occasional plant with a few, usually less than 10 aphids/plant. These numbers are extremely low, which is typical this time of year, but it does signal it’s time to start scouting.

            Aphids will be found on the underside of the newest leaves near the top of the plant first. The good news is, I also found the soybean aphid’s natural enemies when checking fields, so they may hold the populations in check, or at least slow their population growth.

            Relatively mild weather, between 70 and 85ºF, favors soybean aphid development, so the hot weather we’ve had and is predicted may also slow their development. However, soybean aphid population growth can be quite rapid, so regularly monitoring fields for soybean aphids is key to effective management.

            The soybean aphid is soft bodied, light green to pale yellow, about 1/16 inch long, and has two black-tipped cornicles, or tailpipes, on the rear of the abdomen. It has piercing-sucking mouthparts, like a microscopic hypodermic needle it uses to suck out sap from the undersides of the newest leaves near the top of soybean plants.

            Symptoms of soybeans infested by aphids may include yellowed, distorted leaves and stunted plants. A charcoal-colored residue also may be present on the plants. This is a sooty mold that grows on the honeydew that aphids excrete. Honeydew by itself makes leaves appear shiny.

            Aphids are most likely to concentrate at the very top of the plant initially, although they will move onto stems and within the canopy as populations grow or the plant reaches mid- to late reproductive stages. If trees are adjacent to a soybean field, be sure to sample near these areas. Soybean aphids are often found first in the parts of soybean fields closest to wooded areas.

            Counting aphids is not as difficult as it may at first seem. First, walk to a random spot in the field. Pull a plant and turn it upside down and give it a quick scan to see where the aphids are located. Count the individual aphids on a few leaves to get a feel for what 10 or 20 aphids look like and count by 10s or 20s.

            The economic threshold for late vegetative through R5, or soybeans in the pod fill stage, is 250 aphids per plant with 80% of the plants infested and populations increasing. Begin scouting soybean fields weekly. Check 20 to 30 randomly selected plants in various areas of each field. Thresholds for early R6 have yet to be determined, but are likely in the 400-500 aphids/plant range. Insecticide treatments done during or after mid-late R6 have not been documented to increase yield.

            Watch for the presence of the soybean aphid’s natural enemies such as lady beetles, green lacewings, insidious flower bugs, aphid mummies, fuzzy aphids, and other insect predators. Predators and parasitoids may keep low or moderate aphid populations in check. One can often find soybean aphids by examining plants where lady beetles are observed.

            Good insecticide coverage and penetration is required for optimal control of soybean aphid because aphids feed on the undersides of the leaves and within the canopy. For ground application use high water volume (15 gallons/acre) and pressure (30 psi). Aerial applications also work best when higher water volume is used (3 gallons/acre).

            Many insecticides are labeled for the soybean aphid. Pyrethroids have a relatively long residual, and work best at temperatures below 90ºF. Organophosphates have a fuming action, and may work better in heavy canopies or at high temperatures.

            Soybean aphids, if they reach economic thresholds, usually require one treatment in late July through August to keep their population from resurging because there is not enough time for them to build up before they would naturally leave the fields in late August and early September. Insecticide treatments also kills many natural enemies, so any aphids that do re-infest a field are not constrained by the predators and other natural controls, so avoid treating fields if they are not at the threshold.

            For more information on soybean aphid control, contact your local Nebraska Extension office.