For release: July 22, 2005

Contact: Craig Rosenberg, (605) 367-7877; Mike Catangui, (605) 688-4603

BROOKINGS, S.D. – Producers should scout for soybean aphid now and spray if their fields have economic numbers of the insects, SDSU Extension educators and specialists say.

Minnehaha County Extension Educator Craig Rosenberg said most soybeans that were established at the normal planting date are in full bloom (the R2 stage) or beginning pod set (early R3 stage) now in much of South Dakota.

South Dakota State University research has shown that soybean aphids have a big impact on yield when infestations reach economic thresholds from these growth stages up to beginning seed (R5 stage).

“It takes about 20 to 25 days for the crop to progress from R2 to R5. This means that these next three weeks are a critical time for the soybean crop,” Rosenberg said.

Because soybean aphids are present in most fields in this area, producers should be stepping up scouting activities to detect them and effect treatment where necessary to avoid significant yield loss. SDSU researchers also found that low levels of aphids can cause economic yield loss when infestations occur in these early growth stages.

SDSU Extension Entomologist Mike Catangui and SDSU Graduate Research Assistant Eric Beckendorf have constructed economic threshold charts for several different growth stages and crop yield levels. Find it at Catangui’s Web site at http://plantsci.sdstate.edu/ent/. Click on “Soybean Aphid Economic Injury Levels.”

For example, if the soybean aphid infestation began when the plants had five trifoliate leaves (V5) and they are now at full bloom (R2), the threshold for a crop that will cost $10 per acre to spray and will yield 65 bushels per acre and be worth $7.00 per bushel at harvest time is only 6 aphids per plant. This threshold, however, will change along with the soybean growth stages to 71 aphids per plant at full pod (R4), then 184 aphids per plant at beginning seed (R5). “Our economic thresholds are unique in that they are yield potential-, market value-, control cost-, and stage-specific,” Catangui said.

Aphid numbers in fields can change dramatically so producers need to stay on top of the situation and be ready to react if necessary. The sooner the economic number of aphids are determined and treated, the better will be the yield advantages at harvest, according to 2004 SDSU research. It is not recommended to be treating for soybean aphids after R5 as the damage will have been done at that time.

Catangui recommends examining a minimum of 100 soybean plants on at least 10 locations per 40 acre field to accurately gauge aphid numbers and distribution. Sampling locations must include the center of the field and not just the edges. Soybean aphids are currently being found mainly at the growing points as newly born nymphs barely visible to the naked eye. These newborns were recently deposited by adult winged aphids coming from overcrowded soybean nursery plants. Ants farming these aphids as well as lady beetles feeding on them should reveal the aphids’ location on the field.

Insecticides labeled for the soybean aphid on soybean and their recommended rates and pre-harvest intervals (PHI) in South Dakota are as follows: Asana XL (5.8-9.6 fluid ounces per acre, 21 day PHI); Baythroid 2 (2.8 fluid ounces per acre, 45 day PHI); Decis 1.5EC (1.5 to 1.9 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 MAX (2.8-4.0 fluid ounces per acre, 21 day PHI), Orthene 90S (0.83 to 1.10 pounds per acre, 14 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); Proaxis (1.92-3.20 fluid ounces per acre, 45 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). Different insecticides will offer different yield advantages ranging from 4.7 to 13.1 bushels per acre in 2004. Consult Catangui’s Web site for more details.