Your soybean checkoff.
Delivering Results.
June 2006 e-newsletter
All about aphids
"In this issue, Dr. David Ragsdale, University of Minnesota entomologist and head of NCSRP's soybean aphid research team, provides an update on aphids, what we've learned to date, and what checkoff-funded researchers are tackling now." - DW
From aphids 101 to advanced IPM
"We've already developed good basic tools, like the 250 treatment threshold and knowing which insecticides are effective in the field," says Ragsdale. "The next steps are harder and they take longer, such as developing resistant lines, assessing the benefits of native enemies and establishing exotic natural enemies. Once that's done, our next challenge will be integrating resistant plants, natural enemies and biological controls into an advanced IPM system for growers." He adds that 70 percent of IPM programs stop after establishing a threshold. "Ultimately, we want to help develop the tools soybean growers need to go beyond the 'spray at 250 threshold.' To do this, we know the value of all the natural enemies in the field, and rely more on these beneficial insects to reduce aphids - and less on insecticides whenever possible."
Resistant varieties in test plots
"We're currently looking at more than 20 lines thought to have some aphid resistance," says Ragsdale. "We have some early material from soybean breeders in Illinois, Iowa, Kansas and Minnesota, and next year we'll have crosses from soybean breeders in Michigan and South Dakota. We are also evaluating commercial lines." They've all been planted in replicated plots, and researchers will begin counting aphids in those plots in mid-June. Ragsdale's best guess for when growers can get their hands on resistant varieties: "Possibly in two to three years there will be enough seed for commercial production of varieties adapted to the North Central region. We're getting close."
A two-year cycle for aphids
Other than in Minnesota - where there is an abundance of buckthorn along the Mississippi River Flyway, which provides an ideal overwintering spot - aphids commonly appear every other year. "We think this two-year cycle is due to aphids' natural enemies, which include ladybird beetles (not only the multicolored Asian lady beetle, but several species of native ladybugs), fungi, green and brown lace wings, nabids, syrphids (also known as flower flies or hover flies) and the insidious flower bug," says Ragsdale. "It's the insidious flower bug that's doing such a good job of keeping aphid populations in check throughout Indiana."
The interaction of fungicides, Asian rust and aphids
Checkoff-funded researchers also are looking at how applying fungicides for Asian rust affects aphids. According to Ragsdale, "We suspect that some fungi might be very important in late-season control of aphids, especially when nights get cooler and dew points are reached each evening, fungi really take off. Often it is these beneficial fungi that cause aphid populations to decline in the fall. So when a producer applies fungicides, especially later in the season, these beneficial fungi are suppressed, and he may wind up with more aphids the next spring. That might bump aphids off the biennial cycle and make it an annual pest." Scientists are testing that hypothesis now. In the meantime, Ragsdale offers this suggestion: "If you have a legitimate need to apply fungicides, do it. But if you're just trying to get a yield bump and you don't know what pathogens you're controlling, don't spray."
Waiting for the wasps
One potential biological control that has shown great promise in quarantine is a parasitoid - a tiny, stingless wasp - that hails from Northern China. "It has a narrow host range and is climate-matched to the Upper Midwest," says Ragsdale. "USDA APHIS forwarded our request to release this wasp to the North American Plant Protection Organization for review. We need approval from Canada and Mexico before we can release it," he adds. "We also have a number of other species in the pipeline, and foreign exploration is continuing. Previously we've searched for late-season and mid-season parasites, and now we're looking at early-season."
What about seed treatments?
"Seed treatments are good material for bean leaf beetle control," says Ragsdale. "But with a seed treatment, we only see 35 to 45 days of control after planting for soybean aphid. So you get good aphid control early, but after the V3 or V4 stage, there's no toxicity remaining. So it doesn't protect soybeans during critical points. A single application of a foliar product applied at threshold is a better strategy, but growers do need to scout their fields to use the 250 aphid threshold."
Best advice: scout your fields
"Aphids can't be predicted," Ragsdale notes. "So your best strategy is still scouting your fields at appropriate times."