Research confirms SDSU soybean aphid threshold A second year of research at the USDA-ARS Northern Grain Insects Research Laboratory and South Dakota State University confirms that even very small populations of soybean aphid can multiply rapidly enough to cause substantial yield loss in soybeans.
"Even initial populations of under five aphids, if untreated, can grow to the point where they do significant damage," said Eric Beckendorf, a graduate researcher working toward his master's degree in entomology at SDSU. "The cost that it takes to control the aphid is very low compared to the yield loss that you will suffer if you don't control them."
Beckendorf's research is coordinated jointly by SDSU and the USDA Agricultural Research Service's Northern Grain Insects Research Laboratory in Brookings. The South Dakota Soybean Research and Promotion Council funded a portion of Beckendorf's research with soybean checkoff funds.
SDSU Extension Entomologist Mike Catangui, one of the scientists overseeing Beckendorf's research, said even three aphids, placed on a soybean plant during the V5 stage, or two weeks before full bloom, multiplied enough to cut yields by more than 40 percent if untreated. At R2 or full-bloom stage, one aphid per plant, if not controlled then, reduced yield by about 25 percent.
Beckendorf put a known number of aphids on plants at different stages in the plant's development. He used large cages to keep other pests such as grasshoppers and bean leaf beetles out to better determine how the known aphid populations, left to themselves, would affect yields. The cages also kept predators out to some degree, but tiny parasitic wasps, minute pirate bugs, and fungi were able to freely move inside the cages.
Catangui added that separate research at the Southeast Experiment Farm near Beresford in 2004 showed that yields were up to 13 bushels an acre greater if plants were sprayed for soybean aphid during the R2 or full-bloom stage. Delaying spraying by even one week, to the R3 or beginning pod stage, resulted in a loss of more than 1 bushel per acre.
"If you delay, you will lose bushels," Catangui said, adding that South Dakota producers must be especially vigilant in late July when plants enter full bloom.
Plant physiologist Walt Riedell of the ARS Northern Grain Insects Research Laboratory in Brookings, who also helps oversee Beckendorf's research, said producers can also look for a shiny glaze or sheen on the leaves. That's the sugary solution excreted by aphids as they feed on the sap of soybean plants.
Riedell said the soybean plant responds differently to the insect depending on the stage of growth it is at when aphids were allowed to build up. If aphid numbers were allowed to build up starting at the V5 stage, two weeks before full bloom, there's a tremendous loss in the number of seeds per plant and the weight of those seeds is less. Oil content also falls from about 19 percent to about 17.5 percent in the seeds. But protein, oddly, increases in those seeds from about 40 percent to about 43 percent.
If aphid numbers were introduced and allowed to flourish starting at full bloom, the number of seeds doesn't drop as much, Riedell said, but the test weight of the seeds does drop significantly.
Catangui said he will be issuing a slightly updated threshold recommendation based on Beckendorf's research as the 2005 growing season approaches. During the 2004 growing season, Catangui had recommended that growers take action if they saw about three soybean aphids per plant (or 30 aphids per foot of row) at R2 or full bloom stage soybean.
"Detailed yield loss and aphid population growth data from the past two years confirm and support what we have been recommending to South Dakota soybean growers," Catangui said.
SDSU's soybean aphid economic threshold recommendations take into consideration the predicted market value of soybean, the cost of spray plus application, as well as the yield potential of the field. Details can be found on the Internet site entitled "Soybean Aphid
in South Dakota" at http:// plantsci.sdstate.edu/ent/entpubs/soybean_aphid_SD.htm. Producers will hear more about the results of 2004 research during various county Extension meetings starting in December.