Issue No. 16, Article 4/July 13, 2007
Numbers of Soybean Aphids Are Increasing
After some early-season excitement in Michigan and Ontario this year, soybean aphids have been relatively yawn-inducing in most areas in 2007 (good for growers). However, I have just learned of greater-than-threshold numbers of soybean aphids in northeastern Iowa and several locations in Minnesota, and our own data suggest that we need be more alert in Illinois over the next couple of weeks.
Our weekly surveys in 26 fields from Woodford County to Stephenson County have generated mostly zeros thus far, but the numbers increased noticeably (not alarmingly) in Stephenson County on July 9 and 10. The average number of soybean aphids per plant (and growth stage) in the 10 fields sampled in Stephenson County on July 9 and 10 were: 3.85 (R3), 1.05 (R3), 19.8 (R2), 1.85 (R3), 3.35 (R2), 8.55 (R2), 1.9 (R3), 3.1 (R3), 7.6 (R3), and 6.85 (R3). In the field with an average of 19.8 aphids per plant, 95% of the plants were infested, with as many as 50 aphids on one plant. Dave Feltes, Extension IPM Educator in the Quad Cities, also found as many as 50 aphids per plant in a soybean field in JoDaviess County on July 11. He indicated that in a previous visit to the field, the largest number of aphids he found on any given plant was one. As individual winged aphids land on soybean plants and begin producing live young, their numbers can increase rapidly.
These average densities won't cause anyone to lose sleep . . . if they stay where they are. However, with projected temperatures in the upper 70s and lower 80s over the next week, all heck could break loose in some fields. Remember, the doubling time for soybean aphid populations is 2 to 3 days when temperatures are in this range. I learned from Marlin Rice, Extension entomologist at Iowa State University, that the average density of soybean aphids in at least one field in northeastern Iowa increased from about 40 to more than 400 aphids
per plant within about 1 week. If the number of soybean aphids in the Stephenson County field currently with a density of about 20 aphids per plant doubles every 2 to 3 days, the average density would exceed the threshold of 250 aphids per plant in 8 to 12 days. This simple projection conveys potential only because it does not take into account the population regulatory potential of natural enemies. But, the point is clear . . . watch soybean fields closely for soybean aphids during the next couple of weeks.--Kevin Steffey