During the past several weeks, increasing numbers of producers have discovered that their soybean fields are infested with aphids. These aphids were preliminarily identified as cotton or melon aphids (see the article in issue no. 20 of the Bulletin). Last week, Dr. David Voegtlin, an entomologist with the Illinois Natural History Survey, examined the aphids and identified them as soybean aphids, Aphis glycines. Subsequently, Dr. Voegtlin sent specimens to other taxonomists, and yesterday (August 15) additional confirmation was provided by Dr. Manya Stoetzel, USDA Systematic Entomology Laboratory in Beltsville, Maryland. There remains no doubt that the presence of these aphids in soybean fields in northern Illinois, Wisconsin, and Michigan are aphids that have never been reported in the United States. They are native inhabitants of China, and their route of entry into the United States has yet to be determined. In China, soybean aphids are very common in soybean-growing areas and reportedly have a rather narrow host range. |
Soybean aphids in McHenry County, Illinois. (Courtesy of McHenry County Extension Office.)
In some fields, densities of aphids are so impressive that honeydew from the aphids soaks the clothing of scouts, producers, and others who are monitoring infested fields. Honeydew is a sugary liquid excreted through the anus that consists primarily of excess plant sap taken up by the aphids while feeding. Ants are commonly found in close association with aphids feeding on the sugary excretions left behind. Many species of aphids have complex life cycles, including the soybean aphid. Unfortunately these aphids will remain in producers soybean fields until overall plant vigor and nutritional quality has significantly declined. As soybean plants mature, smaller "versions" of soybean aphids become more common on the lower leaves and stems of plants. Eventually winged forms of these aphids leave soybean fields and fly to their overwintering host buckthorn plants (Rhamnus davuricus). Following mating of the winged form of these aphids, eggs are laid on buckthorn plants and serve as the overwintering stage. The following spring, eggs hatch into very small nymphs and two to three generations of aphids are produced. When soybeans begin to emerge, winged forms of the aphids serve to reinfest fields. In China, injury to soybean plants has been reported throughout the entire growing season with as many as 15 generations developing on soybeans and 18 generations occurring if the overwintering host is included. A Chinese paper (Acta Entomologica Siniga, vol. XI, no. 1, 1962) entitled "Studies on the Soybean Aphid, Aphis Glycines Matsumura," provides considerable information on the life history of this aphid species. In this paper, C.L. Wang and other authors reported that the densities of overwintering eggs on buckthorn and numbers of aphids in seedling soybeans directly relates to the potential for economic infestations later in the growing season. They also reported that the optimal range of temperatures for development was 22 to 25° C (72 to 77° F) with a humidity below 78%.
Because this aphid species was just recently discovered in the United States, no insecticides are registered for control of this pest. However, we have many reports that some products (labeled for use in soybeans) are being applied to soybean fields in an effort to control this pest. Producers are strongly encouraged to evaluate the costs of these treatments against the uncertainty of product performance. Reports from Wisconsin and northern Illinois suggest that late-planted soybean fields have been showing the most signs of stress from injury. Some heavily infested fields are yellowing, and some plants have cupped leaves. In some fields, aphid numbers are beginning to decline, and some producers have noticed impressive densities of natural enemies in soybean fields.
If an insecticide is applied, keep in mind that very thorough coverage of plants is required. Because many of the aphids can be found on leaves beneath the upper canopy, they may escape the blunt of an insecticide treatment. If so, densities of aphids could resurge rapidly. Also, insecticide applications will very likely reduce the numbers of natural enemies within soybean fields. This also may enable aphid populations to "jump back." Obviously we dont have any treatment thresholds for this new pest, and much additional research will be required in the coming years. Entomologists working with plant pathologists have much to learn about the potential for disease transmission by these aphids to soybean plants. Potential diseases include several viruses.
Producers are encouraged to scout their soybean fields, including those south of Interstate 80. If aphids are found, check to see if plants are displaying any signs of injury. Unless plants are beginning to yellow and leaves are cupping, insecticide applications may not be warranted. If an insecticide treatment seems warranted, thorough coverage of the canopy is essential. Also, dont forget that these aphids will be present in soybean fields for the duration of the summer. Retreatments of some fields may be required, in part, due to the removal of natural enemies from the initial application of an insecticide. Decisions regarding the use of insecticides will not be easy in light of the fact that we dont have well-researched economic thresholds.
In the upcoming weeks, well continue to provide additional information regarding this late-breaking story on this insect pest of soybeans.--Mike Gray