This article was written as an update between issues no. 21 and 22 (August 15 and 22, 2003) and appeared on the Web site. As a matter of policy, we include update articles in the next printed issue.
As we near the end of August, soybean aphids are still the main cause of con-cern in many soybean fields. Several past issues of the Bulletin have highlighted the captures of winged aphids in suction traps in northern Illinois. A capture of 6,775 aphids in the DeKalb suction trap during the week of August 1 was remarkable, but 4,920 from the Eureka trap and 1,912 from the Freeport trap a week later are impressive as well. The movement of winged aphids has brought these insects to all areas of the state. Suction traps in the southern half of Illinois are now reporting levels of soybean aphids similar to those in northern Illinois. These are the counts from suction traps for the week ending August 22:
The counts illustrate the movement of the aphids and increased numbers in the newly affected areas. Judging by the numbers of phone calls and e-mail messages we have received, several Illinois areas are seeing no reprieve from the intense aphid populations, while others are just beginning to experience economic levels. These discoveries lead to the question, Just how long will populations remain high? Several callers have noted an abundance of natural enemies and parasitized "mummies" found in soybean fields. Others have noted a population "crash" in their fields. However, these reports seem to be few and far between. Dr. David Voegtlin, Illinois Natural History Survey, reports that only about 1% to 2% of aphids observed in fields around central Illinois are alatoid nymphs (aphids that are forming wings), indicating that the populations in central and southern Illinois are likely to stay throughout the remainder of the growing season.
Many of the soybean fields in southern Illinois are in full bloom, with a range in maturity from R1 to R6. Also, the Illinois Agricultural Statistics Service reports that as of August 24, 90% of the soybeans in Illinois have set or are in the process of setting pods. Determining the need for insecticide sprays can be quite challenging. With many soybean plants in the later reproductive stages, producers should think twice about insecticide applications. It is essentially unknown what level of aphid numbers a soybean plant can tol-erate and still fill seed, or what benefit insecticide application has on stages of soybeans R5 and beyond. Another consideration at this time of year is the preharvest interval associated with insecticide applications. Nearly all the insecticides labeled for soybean aphids have at least a 3-week preharvest interval (Table 1).
Ultimately the decision to spray is left up to growers, based on their individual situations. However, if you do decide to spray, we recommend leaving a check strip untreated to compare yields. As always, feel free to share any of this information with us.--Kelly Cook