Most of the fields have decreasing populations of soybean aphids now in North Dakota. Shorter day lengths and the maturity of the soybeans have triggered aphids to develop wings and fly back to its overwintering host, buckthorn, or late-planted soybean fields. Bruce Potter of UMN Extension already found soybean aphids on buckthorn on August 15 in SW Minnesota.
The USDA NASS report for North Dakota indicated that most of the soybeans are in the R5 (full pod) growth stage in ND, and after R5 no insecticide treatments for soybean aphids are recommended because of no increases in yield returns. In north central North Dakota, soybeans were planted later and are only in the R3 crop stages. See D. Waldstein’s article on increasing soybean aphids in late planted soybeans in the Around the State section.
The Extension Entomology office has been getting several calls and emails on whether to be concerned with soybean aphids populations that are below the economic threshold of 250 aphids per plants, and when these populations continue to exist for several weeks while the soybeans are in R4 (beginning pod) to R5 (full pod) growth stages. The answer is “not to worry about these sub-economic threshold populations because they need to reach the 250 aphids per plants level to cause yield loss.”
Below, I have summarized the results of 2009 Insecticide Efficacy Trial for Soybean Aphids at Prosper, ND where we had low numbers of soybean aphids for several weeks during the growing season when soybeans were R4-R5. Pre-spray aphid counts indicated that aphids were present in all treatments, and were fairly evenly distributed. After applying insecticides at the R4 growth stage, post-spray aphid counts indicated differences among treatments. At 7, 14, and 23 days after treatment (DAT), the untreated check had significantly more aphids/plant than all insecticide treatments, and there were no significant differences among insecticide treatments. However, there were no significant differences among all treatments for yield. This is not unexpected, as the economic threshold was never reached and current research in the midwest indicates that aphid populations below the economic threshold do not significantly impact yield.