SDSU research associate Ana Micijevic said she's found aphids on five soybean plants out of 100 in some SDSU plots, in numbers ranging from one to 11.
SDSU soybean research entomologist Kelley Tilmon said scouting is the most important thing producers can do to help manage this pest and keep it from becoming a problem.
"Even though continued scouting can be tedious, it really is the only way to know whether aphid numbers are on the rise to damaging levels," Tilmon said. "Very often we see low aphid numbers in a field that stay low all season long. Under these conditions, the time and expense of spraying for them is simply not worth it, even when using inexpensive, insurance treatments. On the other hand, sometimes aphid populations that start out low can reach damaging levels very quickly. The only way to know what is happening is to maintain a regular scouting program."
Now that aphids have been found in South Dakota soybean fields, producers should start actively scouting and continue weekly through the end of August.
The fields to watch most closely early in the season are those near buckthorn, Tilmon said. Buckthorn is a shrubby plant often found in woodlots and shelterbelts. Soybean aphids spend the winter on this plant and move from this winter host into nearby fields. Often the earliest infestations are in soybeans near buckthorn, with the aphids spreading to other locations a little later.
When scouting for aphids, examine 20 to 30 plants total, spread out over about 10 different places throughout the field. Aphid infestations can be spotty, so it's important not to make your decision based on just one field location that may not represent the whole field. Pick a plant at random, start at the bottom, and work your way up, paying special attention to the undersides of the leaves and the newest vegetation.
"Though counting individual aphids sounds like a daunting task, you will soon get a feel for quickly gauging how many are present," Tilmon said. "A technique called 'speed scouting' shows promise for more rapidly assessing aphid populations. Upcoming research at SDSU will evaluate the usefulness of speed scouting for our producers."
Tilmon's research at SDSU last summer examined a range of different thresholds, at different locations in the state, to determine which would be most appropriate for South Dakota. This work was funded by checkoff dollars through the South Dakota Soybean Research and Promotion Council, the North Central Soybean Research Program and the SDSU Agricultural Experiment Station.
Tilmon found no yield loss when using the threshold of 250 aphids per plant advocated by other Midwestern states.
"Yield was just as high when we used this threshold as when we used lower thresholds. Some producers may feel they should treat before aphids reach an average of 250 per plant. However, a safety margin is actually built into the 250-aphid threshold," Tilmon said.
She explained that when researchers design thresholds, they first determine the aphid population that actually causes damage. Then a lower treatment threshold is calculated to keep insect populations from getting to the higher damaging level. Thus, the treatment threshold level is lower and more conservative than the damage level. Time is built into the treatment threshold (in this instance, seven days). In soybeans, the actual damage level is closer to 650 aphids per plant, Tilmon said, though this can vary with conditions � healthy plants can tolerate more aphids than stressed ones.
SDSU research on controlling soybean aphids is continuing, Tilmon said.