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Corn+Soybean Digest

Scout Now for Soybean Aphids

Producers should begin scouting for Soybean aphids now, a South Dakota State University specialist said.

SDSU Extension Entomologist Mike Catangui said 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.

He added that soybean aphids often appear first in areas close to buckthorn, the host plant on which the aphids overwinter in South Dakota.

Catangui's recent publication, SDSU Extension Fact Sheet 914, "Soybean Aphid in South Dakota," gives more detailed information about the pest. Ask for the fact sheet at county Extension offices. Or find it online at


Soybean aphids are very small insects but can still be seen with the naked eye. A magnifying lens can help in identification. The soybean aphid is the only species that multiplies on soybeans in South Dakota.

"If a producer sees aphids on South Dakota soybeans, they are probably soybean aphids," Catangui said.

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.

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.

The mere presence of soybean aphids does not mean that the field needs to be sprayed. An average of 200 aphids per plant is currently considered economically damaging.

Research conducted by Walt Riedell at the Northern Grain Insects Laboratory has indicated that soybeans are relatively tolerant of the soybean aphids during the vegetative stages, but are quite sensitive to aphid injuries during the pod-fill stages.

Riedell, Catangui, and graduate student Erick Beckendorf are currently studying the basic biology, ecology, and economic thresholds of the soybean aphid in South Dakota soybeans. Funding is being provided the SD Soybean Research and Promotion Council with matching funds from the USDA-ARS Northern Grains Insect Laboratory, and SDSU.

SDSU research in 2002 conducted by Catangui at the Southeast Research Farm near Beresford has shown that spraying for the soybean aphids during R5 (beginning seed) stage in early August can improve yield by 2-11 bushels (8-27%) per acre depending on the insecticide and rate used. Aphid population in the non-treated soybeans was about 870 aphids per plant. The insecticides were able to provide from 87-98% reductions in aphid numbers. Graphs summarizing the research results can be found at:


Insecticides labeled for the soybean aphid on soybean and their recommended rates and pre-harvest intervals (PHI) are as follows: Asana XL (5.8-9.6 fluid ounces per acre, 21 day PHI); Dimate (0.50-0.75 fluid ounces per acre, 21 day PHI); Furadan 4F (0.5 pint per acre, 21 day PHI); Lorsban 4E (1 to 2 pints per acre, 28 day PHI); Mustang (3.0-4.3 fluid ounces per acre, 21 day PHI); Mustang MAX (2.8-4.0 fluid ounces per acre, 21 day PHI), Penncap-M (1 to 3 pints per acre, 20 day PHI); Pounce 3.2EC (4-8 fluid ounces per acre, 60 day PHI); and Warrior (1.92-3.20 fluid ounces per acre, 45 day PHI). Always read and follow label directions. Consult the label for restricted entry intervals (REI).

Soybean aphids were first detected in South Dakota in late September 2001 in Brookings, Moody, and Minnehaha counties. The first record of soybean aphids in the United States was made in Wisconsin during the summer of 2000.

For more information, visit Catangui's soybean aphid Web site at:


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