Issue No. 20, Article 4/August 10, 2007
Soybean Aphid Update
Despite the high temperatures we have experienced recently, populations of soybean aphids continue to increase in many fields in northern Illinois. Although in most fields densities are still below the economic threshold (250 aphids per plant, 80% or more of plants infested), in some fields densities have exceeded 1,000 aphids per plant. For example, in the field in Mercer County where we have released the parasitoid Binodoxys communis, the average density on August 6 was 1,020 aphids per plant, with a range of 564 to 1,723 aphids per plant on the 20 plants sampled. The soybeans in this field were at stage R2 in development. The average density in this field on July 30 was 287 aphids per plant. We observed a similar explosion of soybean aphids in one of our weekly surveyed fields in Stephenson County. On July 30 the average density in the field was 71.30 aphids per plant; on August 6, the average density in the same field was 916.35 aphids per plant.
Many fields of soybeans have been and are being treated to control soybean aphids in "hot spots" throughout northern Illinois. However, I reiterate that densities of soybean aphids vary widely in affected areas, so scouting of individual fields is essential. And despite the dramatic increases in numbers of soybean aphids in the two examples just provided, densities have reached plateaus in other fields, the growth of the populations having been suppressed by high temperatures. It's also important to note that in some heavily infested fields, predators, especially the multicolored Asian lady beetle, are showing up in force. This lag between buildup of soybean aphid populations and populations of lady beetles has been typical during years when soybean aphids reach outbreak proportions. It is likely that these late-season predators will have an impact on populations of soybean aphids that will overwinter.
We continue to receive reports of people encountering soybean aphids dubbed "white dwarfs" that are smaller and lighter-colored than the typical yellow-green soybean aphids. Again, these are not "babies," necessarily, nor are they a different species. White dwarf soybean aphids are merely smaller versions of the soybean aphid and have developed in response to some type of change, such as high temperatures, shorter day length, or lower nutritional quality. We speculate that these aphids do not cause as much injury as "normal" soybean aphids, but this is speculation. We still include them in overall counts of aphids. It is also important to note that many people are finding alate (winged) soybean aphids. Winged adults are produced when populations of soybean aphids become crowded, typically in heavily infested fields. These alates fly to other fields, where they land in a less crowded environment and begin producing young almost immediately. The end result is new colonies of soybean aphids. By the way, these winged aphids are the ones being captured in the network of suction traps in the Midwest. You can view records of captures of soybean aphids this summer at the North Central IPM Center Web site, "Regional Soybean Aphid Suction Trap Network".
Alate (yellow circle) and white dwarf (orange circle) soybean aphids, amidst white flies on a soybean leaflet, Champaign County, August 7, 2007 (photo courtesy of Joe Spencer, Illinois Natural History Survey).
At the time this article was written, our weekly survey of 26 commercial soybean fields had not been completed. Some of the fields have been sprayed with insecticides, and some of the cooperators have agreed to leave untreated check areas in the fields, in which we will continue sampling. We hope to obtain yield comparisons for some of our study fields when the fields are harvested later this season. We invite others who have untreated areas in otherwise treated fields to provide us with data, including stage of soybean development and average density of soybean aphids at the time of treatment. We are particularly interested in obtaining data for fields of R5 or R6 soybeans that were (will be) treated. Most data to date suggest that an insecticide application to R6 soybeans infested with soybean aphids will not pay for itself in yield benefit. However, we certainly are willing to examine and discuss data that address this relationship.--Kevin Steffey