URBANA--With support from the Illinois Soybean Checkoff Board, scientists at the University of Illinois have continued to track the distribution and abundance of a wide variety of soybean pests across the state during the past year.
Lead researchers on the project are agricultural climatologist Scott Isard from the Department of Geography and entomologists Joseph Spencer and Eli Levine from the Illinois Natural History Survey.
Results from extensive surveys across the state indicate that bean leaf beetles, in particular, have become relatively common in Illinois soybean fields over the last several years. "This beetle serves as an effective vector for Bean Pod Mottle Virus," Isard said. "Infected adult beetles that overwinter in woods and crop residues can transmit the virus to soybean seedlings during the following growing season. Other bean leaf beetles can acquire the virus from wilds native legumes in the early spring and transmit it to the young soybean plants."
Isard notes that the beetles were especially abundant last year in the north half of the state. During the previous year, however, the populations were relatively low in Illinois soybean fields.
"Because the beetles overwinter as an adult in the woods, grassy areas, and crop residues, the populations are very sensitive to weather conditions during winter and early spring," he said. "If winter and spring are mild, we would expect large numbers of beetles in soybean fields at the beginning of the growing season, especially in the northern part of the state."
Another pest that has been increasing in numbers is the Japanese beetle. While feeding by this pest is generally not severe enough to require treatment in soybean fields, beetle injury can be severe in combination with other defoliating insects.
"Japanese beetles have become abundant in soybean fields in eastern Illinois," Isard said. "Because they prefer vegetable gardens and ornamental plants, they are often found in large numbers in soybean fields close to residential areas. Evidence from our surveys indicates that area of high infestation in eastern Illinois is likely expanding to the west as a result of our recent mild winters."
He points out that the problems from the soybean aphid, which was first reported in Illinois soybean during the 2000 growing season, remained relatively infrequent during the past growing season.
"The numbers of soybean aphids were lower in 2002 than during the previous year in Illinois," he said. "This pest caused little economic damage last year. The aphids were present in numbers of less than one to three per plant in most fields that we sampled in northern Illinois and were seldom found last year in soybean fields in the southern part of the state."
The scientists are also tracking the distribution of grape colaspis, which has recently emerged as new pest problem in the state. Isard reports that larval injury from this pest occurs more frequently than adult injury. Damage is usually confined to areas where soybeans are planted after corn and on soils that are poorly drained.
"Early planted crops are generally more susceptible to this insect, and there is speculation that grape colaspis may become commonplace if we continue to plant early in the season," Isard said. "As has been the case for the last five years, this pest was widespread in the state but not abundant in our samples. There is still no evidence of a general increase in numbers of this minor insect pest across the state."
Of much more concern has been the spread of the western corn rootworm variant that has adapted to lay eggs in soybean fields, making this pest resistant to crop rotation as an effective control measure.
"The area affected by the rotation-resistant variant has continued to expand," Isard said. "The greatest expansion has been from its place of origin in Ford County toward the northeast and is believed to be driven by the passage of summer storm systems."
He adds that this new variant has caused many growers who previously used crop rotation to resort to applications of soil insecticides in the major affected areas in Illinois.
"According to our sampling, the majority of fields with large number of these western corn rootworms are located in the east-central part of the state," Isard said. "We also have found that the problem is slowly expanding into northern Illinois. At the same time, the abundance of the variant western corn rootworms in fields did not reach the levels reported in some earlier years."
He notes, however, that rotation remains an effective means of control for western corn rootworms in most western and southern counties of Illinois.
"Nevertheless, growers in all parts of the state are encouraged to monitor western corn rootworm abundance in soybean fields to assess the risk of economic injury to first-year corn planted in those field during the following spring," Isard said.