Issue No. 19, Article 1/August 1, 2008
Soybean Aphids Deserve Some Attention Now
Reports from other entomologists in the Midwest and the results of our weekly surveys indicate populations of soybean aphids increased dramatically from the week of July 14 through the week of July 21, when temperatures were ideal for soybean aphid development and population growth. In addition, winged soybean aphids are being captured in the network of suction traps in the Midwest, indicating that populations are building and winged aphids are moving among soybean fields. Although no winged soybean aphids have been captured yet in any of the traps in Illinois, captures in traps in Iowa, Minnesota, and Wisconsin are worth noting, especially if the weather pattern flows through Illinois from the west-northwest. You can review the information for yourself at the North Central IPM Center "Regional Soybean Aphid Suction Trap Network."
During a July 28 teleconference, several entomologists in the north-central states noted marked increases in soybean aphid activity. David Ragsdale, University of Minnesota, indicated that at their Rosemont location, infestations of soybean aphids increased from 30% of the plants infested with about 20 aphids per plant on July 18 to 100% of the plants infested with as many as 500 aphids per plant on July 25. Ian MacRae, University of Minnesota and located in Crookston (northwestern Minnesota), stated that some fields were infested with economic numbers of soybean aphids, but infestations had not yet reached regional outbreak proportions in his area. Kelley Tilmon, South Dakota State University, said densities of soybean aphids were at or greater than the threshold of 250 aphids per plant in many fields and that many alates (winged aphids) have been observed. Jon Tollefson, Iowa State University, shared a report of heavy infestation in northeastern Iowa, with 98% of the plants infested with an average of 60 aphids per plant--one plant, however, sported more than 700 aphids. Numbers of soybean aphids remained low in most fields east of the aforementioned states, including Illinois, Indiana, Michigan, Ohio, and Ontario, Canada.
Numbers of soybean aphids in the 26 commercial soybean fields that are being surveyed weekly in Illinois have been very low thus far, with a couple of exceptions. As I indicated in last week's issue of the Bulletin (No. 18, July 25, 2008), the density of soybean aphids in one field in Stephenson County increased from 6.15 aphids per plant on July 15 to 75.5 aphids per plant on July 23. There were noticeable, although slight, increases in soybean aphid densities in a few of the other fields in Stephenson County, as well as in Lee and Putnam counties. Soybean aphids were found for the first time in five fields in Woodford County on July 23. At the time today's article was written, the data from the July 29-31 surveys was not yet in the database, but you can view the counts for all 26 fields for all sampling dates at www.ipm.uiuc.edu/bulletin/soyaphid. An "ns" in any table cell signifies that the field or plants in a field were not sampled on that date due to a recent pesticide application, either a fungicide or an insecticide (for control of Japanese beetles).
The recent higher temperatures have slowed soybean aphid development and population increases, and predicted higher temperatures suggest that the slowdown likely will continue. However, the relative sudden increase in soybean aphid activity indicates that we need to be more alert. People should sample a minimum of 20 soybean plants, relatively widely dispersed, in a field and determine an average density by counting the aphids on each plant. When the numbers of aphids are large enough, estimates will speed up the process. Sampling more plants will provide a more precise estimate of the average density of soybean aphids, but time is a consideration. Nonetheless, it's important to obtain an estimate of the average number of soybean aphids field-wide. Large colonies usually begin on only a few plants, and the population may or may not build, depending on environmental conditions and the presence or absence of natural enemies, such as multicolored Asian lady beetles, insidious flower bugs, and Syrphid fly larvae. Wet and humid weather also will increase the likelihood of an epizootic of fungal pathogens that can suppress soybean aphid populations.
Remember that most entomologists in the Midwest firmly support the economic threshold of 250 aphids per plant, even with the current higher prices for soybeans. If an insecticide is warranted, there are several products that are effective aphicides. Check out recent issues of on Target to compare efficacies of many of the products registered for control of soybean aphids. Refer to Table 2, page 13, in the 2008 Illinois Agricultural Pest Management Handbook for a list of insecticides and rates suggested for control of soybean aphids in Illinois. Please follow all label directions and precautions.--Kevin Steffey