By Janet J. Knodel
Extension Entomologist
Continue scouting for soybean aphids during the R1 (early flowering) to R5 (early seed) crop stages. The IPM scouts in North Dakota and Minnesota found soybean aphids in 71% of the soybean fields surveyed during the time from June 29 to July 11. Percentage of plants infested with soybean aphids ranged from 0 to 100% with an average of 39% of plants infested. The average number of soybean aphids per plant ranged from 0 to 67, still below the economic threshold (E.T.) of an average of 250 aphids per plant, 80% of the plants infested, and increasing populations. Regular scouting will identify any increasing populations of soybean aphids. Temperature in the low 80s F are favorable for soybean aphid reproduction. If temperatures reach above 90 F, aphid reproduction will slow down.
Avoid early insecticide applications for control of soybean aphids. Our research showed that early insecticide application before the E.T. resulted in no significant yield gain compared to spraying when the E.T. is reached. The residual foliar insecticide activity may not last through the entire aphid season if the insecticide is applied early. Consequently, growers may have to re-spray at an additional cost if soybean aphid populations resurge. In some years soybean aphid populations do not increase to the E.T. level due to climatic conditions and/or beneficial insects keeping aphids in check naturally (growers might not have to spray at all). Regardless of the insecticide used, application timing at the E.T. is most important for optimal soybean aphid control. The E.T. has a built in window of approximately 7 days to get that insecticide application on before soybean aphid population growth will reach economic yield loss levels. The USDA NASS reports that 49% of the soybeans were blooming (R1), well ahead of 23% last year in North Dakota (USDA NASS News Release – July 13, 2015).
