Time To Assess Soybeans

Routine scouting will be most critical in the next two to three weeks as soybeans advance to the later reproductive growth stages, according to the Department of Agriculture, Trade, and Consumer Protection.

Mid-season soybean surveys across southern Wisconsin indicate that advanced fields have reached the R2-R3. That’s the full bloom to beginning pod growth stages. DATCP notes that insect pressure is gradually building.

Soybean aphid counts are still overall very low. The typical average is fewer than five aphids per plant or 500 aphids per 100 plants. This is based on observations in 80 soybean fields sampled July 5-18. Aphids haven’t yet dispersed throughout fields. They are generally colonizing less than 20 percent of the plants fieldwide.

As populations increase toward the end of the month, DATCP reminds growers and crop advisors to consider insecticide treatment if the economic threshold exceeds 250 aphids per plant on 80 percent of the plants. Aphid counts have not begun to approach this level in any soybean field surveyed by DATCP as of July 18.

Soybean fields are also showing 1-10 percent of plants with light to moderate leaf injury caused by pests. These include 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. An accurate assessment requires scouting several areas in the field interior, in addition to field edges where Japanese beetles are most numerous.

In addition to aphids and defoliators, scout soybean fields for soybean gall midge and the newly identified soybean tentiform leafminer. Scouts have found these pests in Minnesota, but not yet Wisconsin.