Soybean Aphid Numbers Rising

July 31, 2008 07:00 PM

Pro Farmer Editors


Reports from entomologists in the Midwest 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.

According to a bulletin from University of Illinois entomologists, during a July 28 teleconference, several entomologists in the north-central states noted marked increases in soybean aphid activity. They report:

  • 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.

University of Illinois entomologist Kevin Steffey says recent higher temps 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," he says.

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Steffey says producers 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," he says.

Most entomologists in the Midwest 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.




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