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Soybean Aphid Scouting and Management

By John Wilson, Extension Educator  

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 (less than 10) aphids/plant. These numbers are extremely low, which is typical for this time of year, but it does signal it’s time to start scouting.

Aphids will be found on the newest leaves at the top of the plant first. Ths indicates they recently colonized the plant, probably within the last week. The good news is the soybean aphid’s natural enemies have also been found in these field surveys, so they may hold the populations in check, or at least slow their population growth.

Relatively mild weather, between 70 and 85F, favors soybean aphid development, so make sure to check fields at least once a week. Soybean aphid population growth can be quite rapid, and regular monitoring of soybean aphid populations is key to effective management.

Soybean Aphid Description

The soybean aphid is soft bodied, light green to pale yellow, less than 1/16 inch long, and has two black-tipped cornicles, or tailpipes, on the rear of the abdomen. It has piercing-sucking mouthparts and typically feeds on new tissue on the undersides of leaves near the top of recently colonized soybean plants. Later in the season the aphids can be found on all parts of the plant, feeding primarily on the undersides of leaves, but also on the stems and pods.

Soybean Aphid Injury to Soybean

Soybean aphids injure soybeans by sucking out plant sap with their needle-like mouthparts. Symptoms of soybeans infested by soybean aphid may include yellowed, distorted leaves and stunted plants. A charcoal-colored residue also may be present on the plants. This is sooty mold that grows on the honeydew that aphids excrete. Honeydew by itself makes leaves appear shiny. Soybean plants appear to be most vulnerable to aphid injury during the early reproductive stages.

Aphid Scouting Methods

The economic threshold for late vegetative through R5, or pod fill, stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing. Begin scouting soybean fields once or twice a week now. Check 20 to 30 randomly selected plants in various areas of each field.

Aphids are most likely to concentrate at the very top of the plant, although they will move onto stems and within the canopy as populations grow and/or the plant reaches mid to late reproductive stages. If a tree line or woodlot is adjacent to the soybean field, make sure and include a few sampling locations near these areas. Soybean aphids are often found first in the parts of soybean fields near 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. Get a feel for what 10 or 20 aphids look like and count by 10s or 20s.

The current threshold for late vegetative through R5 stage soybean is 250 aphids/plant with 80% of the plants infested and populations increasing. Thresholds for early R6 have yet to be determined, but are likely in the 400-500 aphids/plant range. Insecticide treatment done during or after mid-late R6 has not been documented to increase yield.

Soybean Aphid Management

Look for the presence of aphid 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.

Note whether the plants are covered with honeydew or sooty mold, or stunted, and aphids are still present at threshold levels. An insecticide treatment may still be of value but the optimum time for treatment has passed.

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 application works well when high water volume is used (3 gallons/acre).

Several 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 well in heavy canopies or high temperatures. Dimethoate is least effective.

Soybean aphids, if they reach economic thresholds, usually do so and require treatment in late July through August. One treatment during this period usually is enough to keep aphid populations from resurging because there is not enough time for populations to build-up before they would naturally leave the fields in late August and early September. The earlier a field is treated, the greater the chance that any surviving aphids can later reproduce or new aphids can repopulate the field.

Remember, insecticide treatment also kills many natural enemies, so any aphids that do re-infest a field are not constrained by predators and other natural controls. Even insecticides with a relatively long residual cannot last when insecticide treatment is done in early or mid-July, particularly during a year when aphid populations are thriving. If one has to treat early, make sure to closely monitor the field until early September.

Another practice that can result in aphid population resurgence is unwarranted insecticide treatment, either because fields were treated well before the threshold was met or fields were treated along with a herbicide (in some cases a fungicide), regardless of aphid presence. These treatments kill natural enemies and are usually done relatively early so there is plenty of time for aphids to resurge, or re-colonize a field.

Aphid populations below or even at the economic threshold do not cause yield loss, so treating before populations reach 250 aphids/plant only increases the probability of aphid resurgence. In addition, we have observed that many fields support a non-increasing, low population of aphids (e.g., less than 100 aphids/plant) through August. Treating these fields would be a waste of time and money.

Tank-mixing insecticides with glyphosate or other herbicides can be problematic because application methods for herbicides (e.g., lower pressures, large droplet producing nozzles) are not optimal for good insecticide efficacy. Tank-mixing with fungicides can be effective because application methods for fungicides and insecticides require high water pressure for adequate penetration and coverage, however, only conduct this practice IF soybean aphid thresholds are met.