The latest update on the soybean aphid from researchers at the University of Illinois indicates that it will clearly remain a pest in the state. The aphids, first discovered in large numbers near the end of the 2000 soybean growing season, have been identified as Aphis glycines. Aphis glycines had previously been reported only in Asia, Australia, and some Pacific islands.
Because so little was known about this pest, an interdisciplinary team of experts was assembled at the U of I to monitor the situation. The thirteen-member group includes researchers from the U of I, the Illinois Natural History Survey, USDA, and the National Soybean Research Laboratory. The Illinois Soybean Checkoff Board, USDA, and a Sentinel Grant from the Illinois Council for Food and Agricultural Research (C-FAR) provided funding for the project.
"The aphids definitely spent the winter on buckthorn shrubs in Illinois," says David Onstad, U of I entomologist and team project leader. "Our team of researchers found the aphid on buckthorn in October 2000 and May 2001. The team has also tracked its abundance and distribution in Illinois since it was first found infesting soybean fields on June 20 of this year."
He notes that the aphid was first observed in the northeastern section of the state and along major rivers in the north. By the end of July, it was found all the way from the Wisconsin border to the Kentucky border. Researchers monitored the situation through a network of suction traps at seven locations around the state. Additional teams of entomologists sampled soybean fields across the state throughout the growing season.
"The suction trap at Dixon Springs proved its worth by alerting the team to the presence of the aphid in the far south before we had discovered it in soybean fields," Onstad says. "Within a week of capturing the aphid in the suction trap, the team had collected it at several soybean fields at the southern end of Illinois."
The team found aphids on buckthorn in northern and central Illinois this fall at densities much higher than those observed one year ago. According to Onstad, this probably indicates that the distribution and abundance of the pest will be increasing over the next few years as it becomes established in much of Illinois.
Field studies in Wisconsin, Michigan, and Minnesota have shown that yield losses are often higher than 50% when thousands of aphids infest each soybean plant. Although it was unusual to observe these densities in Illinois, even the smaller infestations common in Illinois probably caused some significant yield loss.
"The growth of populations in soybean fields depends on the variety of soybean planted, the abundance of natural predators, and major rainstorms," Onstad says. "The aphid populations often increased more than ten-fold in a single week. Some populations had peak densities in late July, while others continued to increase after that. A few soybean fields were observed to have over 1,000 aphids per plant on average."
He notes that a major phenomenon in the aphid population dynamics is the formation of winged nymphs as the population grows. Early in the season only 5-10% of the aphids form wings and fly away as adults. In late July, the proportion can rise to 90%, causing populations to crash within a week.
Studies were conducted under controlled conditions to determine if reproduction rates of the aphids would be reduced on certain soybean varieties. Results indicate that some varieties did reduce the rates of aphid reproduction.
In the field, insect-proof cages were used to evaluate aphid colonization on various soybean varieties and on some lines from the wild relatives of the soybean. Some of the wild relatives were rated as highly resistant to the aphid.
"We also confirmed that the aphid can transmit soybean mosaic virus strains in the lab," Onstad says. "Our team also collected isolates of virus from the field. Aphids collected from soybean plants in Winnebago County that displayed severe virus-like symptoms transmitted the symptoms to healthy seedlings in the greenhouse."
He points out that additional tests will be required to determine the identity and the pathogenicity of the causal agent.