The study, conducted by SDSU and USDA’s Agricultural Research Service insect laboratory in Brookings, suggests that farmers may have to take action against soybean aphids at far lower counts than previously thought. Preliminary findings, by research assistant Eric Beckendorf, indicate that 10 or fewer soybean aphids, if detected when pods are forming, may justify spraying.
"Those 10 aphids have the potential to reproduce into thousands," Beckendorf says.
The critical time for South Dakota farmers to take action is in about late July or early August, adds SDSU extension entomologist Mike Catangui.
Beckendorf is still analyzing data and SDSU is not yet recommending an economic threshold at which to spray.
Beckendorf and Catangui and Walter Riedell, a research plant physiologist at the Agricultural Research Service's Northern Grain Insects Research Laboratory in Brookings.
Catangui says the work is critical because the soybean aphid is new to South Dakota and the U.S. The research hadn't been done to set thresholds and early estimates about thresholds were way too high.
Experts in Wisconsin, where the pest was first found in the U.S., were advising farmers to spray when they counted 250 aphids per plant. Minnesota specialists advised taking action at 200 aphids per plant – the threshold that South Dakota also initially adopted.
But Beckendorf, who used cages to keep out insect predators and then introduced a known number of soybean aphids to plants last growing season – says even small populations of aphids grew to astronomical numbers when left unchecked.
"We know how many aphids we put out there. We wanted to monitor the aphids' reproductive capacity through the growing season," he says. "On one plant we counted over 47,000 aphids. It started out with 10."
Beckendorf infested the soybeans with aphids when the plants were at the V5 growth stage of development on July 7, 2003. The aphid population continued to increase rapidly, reaching more than 47,000 aphids on one plant sampled six weeks after initial infestation, or at the R5 growth stage (beginning seed) on Aug. 21, 2003.
The aphid population drastically decreased after that date as the plant began to drop its leaves and die as it reached maturity.
"The aphids may have also stopped reproducing on the soybean and instead began migrating to buckthorn, the aphids' overwintering host," Beckendorf adds.
He took sample plants from his experiment last year and froze them so that he could count the aphids this winter. The aphids cling to the plant when frozen, making it easy – though time-consuming – to count them.
Catangui first found the soybean aphid in South Dakota in very small numbers in August 2001 after it had already been detected in neighboring states.
Minnesota, where the aphid has been established for one year longer than South Dakota, saw its worst damage to date from the insect in 2003, he says. That suggests South Dakota could see its worst damage so far in 2004.
Soybean aphids cause direct damage to plants by sucking the juice from plant leaves and stems, causing the leaves to curl and yellow, and stunting plant growth.
Beckendorf's research also is testing protein and oil content of the seeds to see what effect the aphids have on soybean quality at different infestation levels.
The South Dakota Soybean Research and Promotion Council has helped support SDSU's soybean aphid work from the outset and is continuing to fund it.