Soybean aphids are now showing up in Brown County, Edmunds County and Beadle County fields. Producers there and elsewhere should start monitoring their fields for the insect and take action if necessary says Mike Catangui, South Dakota State University (SDSU) Extension entomologist.

Soybeans are especially vulnerable to soybean aphids and other insects from the R2 or full-bloom stage, through the R6 or full-seed stage, Catangui says. That means producers may have to take serious action against pests in late July or early August. The soybean aphid, a relatively new pest in South Dakota, can cause serious damage at that stage.

Research conducted by SDSU Graduate Research Assistant Eric Beckendorf at the USDA Northern Grains Insects Research Laboratory in Brookings in 2003 has indicated that soybean aphids can severely reduce pod formation in infested plants. Beckendorf's research is the basis of recently developed economic thresholds and control recommendations for managing the soybean aphid in South Dakota.

SDSU has recently developed a preliminary procedure for calculating the economic injury level of soybean aphids in R2 (full bloom) soybean in South Dakota.

Find it by visiting Catangui's SDSU Extension Entomology Web site at http://plantsci.sdstate.edu/ent/. Click on "See Mail Newsletter" to find Catangui's summary of how the procedure works.

Also included on the Web site is an interactive soybean aphid economic threshold calculator that will ask the user to enter the cost of control, soybean market value, and the yield potential of the field.

Economic thresholds do vary with the predicted market value of soybeans, actual cost of the spray and the yield potential of the field. Economic threshold represents the breakeven point between the cost of the spray and the value of the damage that will be prevented if the field were sprayed.

Most soybean aphids found on soybeans right now are the wingless forms. They are about one-sixteenth of an inch long and yellowish to yellowish-green in color. They have syringe-like mouthparts and feed on the soybean sap. A magnifying lens will also reveal that they have a pair of black "tail pipes" on the rear.

Soybean aphids may be found on the growing points, stems, and on the underside of leaves. Soybean aphids are tended by ants, which "farm" the aphids for their honeydew.

In scouting for aphids, producers can watch for ants along the borders of soybean fields as one telltale sign that aphids may be present.

Catangui encourages growers to look for aphids from at least 10 locations on the field to properly gauge the representative aphid infestation of the entire field. Soybean aphids usually infest the borders first then spread into the whole field.

Insecticides labeled for use against the soybean aphids in South Dakota as well as their field performances can be found on Catangui's soybean aphid Web site at: http://plantsci.sdstate.edu/ent/entpubs/sa_insecticides.htm.