Soybean aphid season under way
While Nebraska aphid populations can reach economically damaging populations in late July, it usually occurs in August, when soybeans are in the mid-reproductive stages (R4-R5). In some years there are many fields where the aphid populations peak in late R5 (beginning seed) to early R6 (full seed).
Of course, there are always exceptions to any rule, so one should always be watchful for soybean aphid colonization and population increase.
Soybean Aphid Management
The current recommended economic threshold for late vegetative through R5 stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing. Depending on economic conditions, this generally gives you about five to seven days to schedule treatment before populations reach economically damaging levels. If populations do not increase during these five to seven days, you may be able to eliminate or delay treatment. Determining if the aphid population is actively increasing requires several visits to the field. Factors favorable for aphid increase are relatively cool temperatures, plant stress (particularly drought), and lack of natural enemies.
Soybean aphids in Nebraska usually reach the economic threshold and require treatment in late July through August, with a few fields requiring treatment earlier in July. 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. Also, insecticide treatment will kill many natural enemies, reducing control from predators or other natural means. 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 you have to treat early, make sure to closely monitor the field until early September for a resurgence of aphids.
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 an 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.