Recent cross-breeding at Ohio State University has yielded two advanced germplasm lines, HC95-15MB and HC95-24MB, that may offer breeders additional management tactics where leaf feeding is a concern.
The two recently released soybean germplasm lines appear to resist defoliation against bean leaf beetle and western corn rootworm, insects that have been known to cause severe crop damage throughout the Midwest. The results of the study were recently published in the Journal of Economic Entomology.
"These lines have not been released commercially to growers because the yields they produce are not high enough. But they have potential for breeders who are looking for lines that show good insect resistance that in the future could potentially be crossed with high-yielding lines," said Ron Hammond, an entomologist for the Ohio Agricultural Research and Development Center (OARDC). "We wanted to look at them in the field against insect problems to see what value they might have."
Researchers found that over a two-year period, amount of defoliation from the adult bean leaf beetle and the western corn rootworm was significantly reduced in the germplasm lines compared to a commercial soybean cultivar. No commercially grown lines are considered to have any level of insect resistance.
Percent defoliation from 1999 data averaged 22.5% for the commercial cultivar, 14.8% for HC95-15MB and 10% for HC95-24MB. In 2000, defoliation levels were 8.5% for the commercial cultivar, 4.1% for HC95-24MB and 3.9% for HC95-15MB.
"We saw less leaf feeding on the resistant lines," said Hammond, adding that the reduction in feeding may be due to antixenosis, a plant defense mechanism that modifies the behavior of the insect without affecting plant or insect metabolisms.
The resistant lines, however, did not reduce bean leaf beetle or western corn rootworm populations. Hammond said the germplasm lines were originally developed specifically to target insects, such as caterpillars and the Mexican bean beetle, whose larvae develop and feed on plant leaves. Bean leaf beetle and western corn rootworm larvae develop in the soil and feed on the plant roots. It is the adult insects that are defoliators.
In addition, the resistant lines were not effective in reducing the pod injury levels associated with bean leaf beetle feeding. Hammond speculates that the plants' defenses rest solely in the leaves.
"These lines may not have the ability to reduce pod feeding and population density, but in terms of defoliation there is definitely some promise. Most insect problems that have historically been on soybeans have been those insects that defoliate the plant, and we know how defoliation can affect the economics of the crop," said Hammond. "With insects like the bean leaf beetle and soybean aphid present that can transmit viruses, continuing research into new soybean lines is very important."
Bean leaf beetles can cause headaches for soybean growers. Not only do they feed on plant leaves, they cause pod injury that can reduce yields. They also transmit bean pod mottle virus, a disease associated with green-stem syndrome where the stems remain tough and green and beans are too dry to harvest.
Western corn rootworm beetles are normally a concern on continuously grown corn. Rotating corn with soybeans has been the first line of defense. However, a new biotype of the western corn rootworm is laying its eggs in soybean fields that hatch into larvae and feed on first year corn that has been planted into those fields the following year. This prevents rotation from being used as a management practice. Though more problematic in such states as Indiana and Illinois, this new biotype of western corn rootworm has been recorded in Ohio.
The soybean germplasm lines were developed by Hammond and Dick Cooper, a USDA soybean breeder at OARDC, as part of on-going soybean breeding research.