The aphids were first discovered in large numbers in soybean fields near the end of the 2000 growing season. After careful scientific investigation, they were identified as Aphis glycines, which had previously been reported only in Asia, Australia and some Pacific islands.
Researchers from the National Soybean Research Laboratory at the University of Illinois (U of I), however, have recently discovered three soybean lines with strong resistance to the aphids which could eventually be incorporated into new commercial varieties.
The lead scientists in this effort are Glen Hartman, USDA plant pathologist at the U of I, and Senior Research Associate Curtis Hill. Primary funding has been provided by the Illinois Soybean Checkoff Board.
"Once the aphids infest a field, the most common means of control is to spray the field with an insecticide that can cost $20-25/acre or more," Hartman says. "If resistant commercial varieties were available, the savings to growers could be substantial."
As part of a screening process, they first evaluated the various commercial soybean varieties that had been submitted to the yield trials at U of I for resistance to the aphids. All of the initial screening work was done in an isolated, air-conditioned greenhouse room on the U of I campus.
"After screening more than 700 varieties, we found that all of them were basically susceptible to this pest," Hartman says. "We also determined that there had not been any reported resistance from the germplasm screened in the part of the world where the aphids originated, which is China. That was surprising to us, and we wondered at that point if we would find any resistance at all."
As the next step, they began screening about 100 cultivars that had been identified as the major genetic contributors to modern soybean varieties. Those ancestral lines account for more than 90% of the genetic variation in our current soybeans.
"If resistance exists, it seemed likely that it would come from this group," Hill says. "Luckily we found resistance in three different cultivars. One is called Jackson, which is an old southern cultivar. Another was Dowling, which also is an old variety grown in the south. The third was a plant introduction adapted to more northern conditions."
As part of the experimental design, the three ancestral resistant cultivars were tested in a specially designed field cage with several commercial varieties and were treated with an insecticide or left untreated.
"Even with a large numbers of aphids present, we found virtually no difference in yield and agronomic traits whether these resistant lines were treated with an insecticide or not," Hartman says. "At the same time, the commercial varieties were severely damaged when they were not treated with an insecticide, with many of the plants actually dying."
He emphasizes the resistance from these lines remained effective throughout the extensive field tests carried out over the last growing season.
"These are the first reported sources of resistance in soybean to the aphid found anywhere in the U.S.," Hartman says. "This marks a major first step in dealing with the problems from this newly introduced soybean pest. Work is already well under way to develop resistant germplasm that will be made available to both public and private soybean breeders interested in developing new resistant commercial varieties."