No. 2 Article 5/April 3, 2009

Will Soybean Aphids Continue Their Threat in 2009?

As most everyone knows by now, soybean aphids definitely broke their every-other-year cycle of outbreaks in the Midwest in 2008. Since the discovery of soybean aphids in North America in 2000, most widespread outbreaks have occurred in odd-numbered years, although regional and localized outbreaks also occurred in even-number years. In 2008, however, the outbreak was widespread and economically significant, especially in the northern Midwest (northern Iowa, Minnesota, Nebraska, North Dakota, and South Dakota). Although the outbreak was less severe in Illinois, populations of soybean aphids reached economically threatening numbers in many northern counties late in the season (late August and early September).

The situation in 2008 leaves us wondering what will happen in 2009. We have often relied on the numbers of winged soybean aphids captured in suction traps in the fall to make broad predictions about the potential for outbreaks during the following year. Roughly speaking, large numbers of aphids captured in the fall of one year have suggested a good potential for an outbreak the following year. Conversely, small numbers of aphids captured in the fall of one year suggested little potential for a widespread outbreak the following year. Captures of winged soybean aphids in a network of 42 suction traps in the Midwest can be viewed at the North Central IPM Center "Regional Soybean Aphid Suction Trap Network."

So what do the data tell us thus far? In a response to several questions from colleagues, Dr. David Voegtlin, entomologist with the Illinois Natural History Survey, provided the following information:

"Over the 9 years of operation of the suction traps, the following has been learned. In years following captures of small numbers of aphids during the fall flight period, the field populations in areas where the winter host (buckthorn) is uncommon have been low. Suction trap captures in September and October represent the migration of aphids to buckthorn. As such, they demonstrate the potential for soybean aphid infestations the following year. A lot of biology takes place between the migration phase to buckthorn in the fall and migration of aphids from buckthorn to soybean the following spring. It would be wonderful if the fall numbers accurately predicted what would happen the following year, but now we know they do not.

"In September and October 2008, record numbers of soybean aphids were captured in the suction trap network. The only other year during which similar numbers of aphids were captured in the fall was 2006. However, during the fall of 2006, the aphids successfully colonized the winter host and produced eggs in astonishing numbers. Random samples of buckthorn twigs in Illinois, Indiana, Michigan, and Ohio revealed close to 100% infestation; in some areas, every bud on every twig had aphid eggs. In October 2008, a similar sampling of buckthorn twigs at the same sites revealed 0 soybean aphid eggs. The reason for the dramatic difference in numbers of soybean aphid eggs in fall 2006 and fall 2008 is not clear; however, in late summer 2008, there was widespread aphid mortality caused by entomopathogenic fungi in soybean fields throughout most of the Midwest. Migrant aphids carried the spores from soybean fields to buckthorn and most likely infected and killed the colonies on the buckthorn before egg deposition occurred.

"I cannot speak for areas outside of those visited last fall, but I have heard from other entomologists who study the soybean aphid that they were not able to find eggs (on buckthorn) last fall. The soybean aphid will most certainly have overwintered successfully somewhere in the Midwest, and there will probably be treatable populations in these areas. I believe that for much of Illinois and Indiana, where summer populations begin by migrations from more northern infested soybean fields, the field populations that do develop will be late and most likely will stay below threshold."

So the best advice for making preparations for soybean aphid management in Illinois in 2009 is, as always, to stay alert from late June through August and to be prepared to react with an insecticide application if densities exceed 250 aphids per plant. This advice does not differ from the advice offered in all of the previous growing seasons. However, because there are no consistently effective preventive management tactics, vigilance is the only recommendation that makes sense all of the time.--Kevin Steffey

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