Pictured: Soybean aphids. Photo by K. Hamilton DATCP
Mid-season soybean surveys across southern Wisconsin indicate that advanced fields have reached the R2-R4 or full bloom to full pod, growth stages. With that, insect pressure is increasing.
Soybean aphid counts are still very low overall, but a small percentage of fields have moderate counts of 25-50 aphids per plant. The typical average is less than 10 aphids per plant, based on observations in 125 soybean fields sampled July 3-16. Aphids have not yet dispersed throughout entire fields and are generally colonizing fewer than half of the plants fieldwide.
As populations begin to increase more rapidly toward the end of the month, remember that the treatment threshold for soybean aphid remains at 250 aphids per plant on 80 percent of the plants. It’s not economical to apply insecticides until you meet the threshold. Aphid counts were still far below this level in the soybean fields surveyed by DATCP in the past two weeks.
In addition to aphids, soybean fields are also showing 1-10 percent of plants with light to moderate leaf injury caused by Japanese beetles, bean leaf beetles, green cloverworms, silver-spotted skipper larvae, and other defoliators.
The sampling method for defoliators is to select 10 plants throughout the field, choosing a trifoliate from the upper, middle, and lower canopy on each plant, for a 30-leaf sample. Compare the 30 leaflets with an online defoliation estimating guide to determine the average percent defoliation. Defoliation that meets the 20 percent threshold between the bloom and pod-fill stages and 30 percent in the pre-bloom soybean may warrant control.
Scouting several areas in the field interior, in addition to field edges where Japanese beetles are most numerous, is required for accurate assessment. Routine scouting will be most critical in the next three weeks as soybeans advance to the later reproductive growth stages.
Learn more about soybean aphids: https://cropsandsoils.extension.wisc.edu/articles/managing-soybean-aphids-in-wisconsin-soybean-fields/
Listen to a recent podcast with the DATCP Pest Survey Program:
