No. 21 Article 3/August 12, 2005

Will the Soybean Aphid Situation in 2003 Repeat Itself in 2005?

The title of a 2003 article about soybean aphids in issue no. 21 (August 15) of the Bulletin was "Problems with Soybean Aphids South of I-80." Both that issue and this one are the last seasonal issues of the respective years. The densities of soybean aphids per plant reported in that 2003 article were not dissimilar to the densities reported in Table 1 for seven counties (10 fields) sampled on August 8, 2005. So do these similarities between the 2 years suggest that soybean aphids will threaten soybean yields as much as they did in 2005? We will have a hindsight answer to this question by the end of the month, but decisions about how to respond to soybean aphid infestations right now will have to be made before then.

There are some significant differences between the current soybean aphid situation in Illinois and the outbreak in 2003. In 2003, economic infestations of soybean aphids were widespread in northern Illinois counties before economic infestations began to show up in central Illinois. In 2005, economic infestations of soybean aphids have been much more localized. Consequently, the numbers of winged aphids currently being captured in our network of suction traps are considerably lower than those captured at the same time in 2003. For example, for the week ending August 1, 2003, 787 (Freeport), 6,755 (DeKalb), and 401 (Eureka) soybean aphids were captured in suction traps. For the week ending July 29, 2005, the numbers captured at these same locations were 16, 30, and 5, respectively. These data clearly indicate that the numbers of winged aphids produced in these areas are a great deal lower in 2005 than they were in 2003. You can make comparisons for yourself at our Web site or at the North Central IPM Center Web site, where you can also compare the 2005 Illinois captures with captures from traps located in Indiana, Iowa, Michigan, Minnesota, and Wisconsin.

Another major difference between 2003 and 2005 has been the much higher temperatures through June and July in 2005. As we have stated more than once in previous articles, temperatures greater than 90EF slow soybean aphid development, whereas temperatures between the low 70s and mid-80s are ideal for development (research from the University of Minnesota, previously cited). The higher temperatures in 2005 undoubtedly impacted development of soybean aphid populations, and the numbers of fields harboring economic levels of soybean aphids have been far fewer in 2005 than in 2003.

A return to temperatures in the low to mid-80s could speed up soybean aphid development again, so fields throughout Illinois still need to be scouted, especially if aphid densities are approaching 100 per plant. A doubling of aphid numbers every 3 to 4 days could result in densities greater than the threshold of 250 aphids per plant. But another reminder is needed here. The threshold of 250 aphids per plant is very conservative and allows lead time for scheduling insecticide applications. Some research conducted by entomologists at the University of Wisconsin indicated densities of 1,000 or more aphids per plant were necessary before yield loss occurred in R3-stage or older soybeans.

Kelly Cook and I received separate reports from individuals indicating that widespread application of insecticides for control of soybean aphids was occurring in some east-central and central Illinois counties. The individuals who provided the reports had been scouting soybeans diligently and had not found many fields where densities of soybean aphids were at or greater than 250 aphids per plant. Our own numbers do not suggest widespread, economic infestations. It is also very important to note that yield benefits are not consistent and not predictable when R6 or more mature soybeans are treated for control of soybean aphids. All of the soybeans in the 10 fields we surveyed were at either the R5 or R6 stage of development. We also remind you that the preharvest interval for several insecticides is 21 days or more for all of the products suggested for control of soybean aphids. So with this information as a backdrop, we strongly encourage producers to consider whether an insecticide application for control of soybean aphids in central Illinois is really necessary. Judge each field independently. Our survey numbers (Table 1) reveal that it is common for two soybean fields in the same county to harbor 0 aphids and detectable densities of aphids, respectively.

One other concern has been raised regarding the economic threshold for soybean aphids. "In soybean fields suffering from environmental stresses or from twospotted spider mite infestations, should the threshold be lower than 250 soybean aphids per plant?" We have addressed this very issue with other insects this summer, and the same general response holds true for soybean aphids. It makes sense that the threshold should be lower in stressed soybeans than in relatively healthy soybeans. However, we have no quantity (how much less?) to offer, so you'll have to use your best judgment. Consider the yield potential for each field, and make the decision accordingly.--Kevin Steffey

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