Soybean aphids already at threatening levels in some counties Soybean aphid infestations have been found in Brookings, Clay, and Turner counties and soybean growers statewide should scout their fields now, a South Dakota State University specialist said.
�The numbers are higher this year and the aphid is more widespread,� SDSU Extension Entomologist Mike Catangui said. �We�re seeing more than 200 aphids per V4 plant in some locations, which is very worrisome.�
SDSU and USDA Agricultural Research Service Northern Grains Insects Research Laboratory (USDA-ARS-NGIRL) research in 2004 have shown that a population of 100 aphids per plant at V5 (five-trifoliate) stage (July 14 in 2004), left untreated, multiplied to 18,572 aphids per plant just after R4 (Aug. 18) then cut the soybean yields by 82 percent at harvest, Catangui said. One aphid per plant at V5 became 3,057 aphids per plant at R5 and caused a 15 percent yield loss at harvest.
Catangui and SDSU graduate research assistant Eric Beckendorf, in collaboration with the USDA Northern Grains Insects Research Laboratory in Brookings, have developed economic thresholds and control recommendations for managing the soybean aphid in South Dakota.
The updated threshold information is available at the SDSU Extension Entomology Web site, http:// plantsci.sdstate.edu/ent/. Click on �Soybean aphid economic injury levels.�
�Producers can scout and treat as early as the V5, or five trifoliate stage, and as late as R5, or beginning seed state. That�s essentially an eight-week window,� Catangui said. �My preference would be that they scout and treat early. R2, or full bloom, is the preferred stage for treating.�
Catangui said the work by SDSU and the USDA-ARS-NGIRL shows that the longer soybean aphid infestations go untreated, the more soybean yields decline. It�s not recommended that producers spray for soybean aphid after the R5 stage because it will not do much good at that point, Catangui added.
The economic thresholds take into account a number of variables, including yield potential of the field, market value of soybeans, and cost of pesticide application. It is also stage-specific, considering the impact of soybean aphid infestation at various growth stages.