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Hey! What about those aphids?

By Staff | May 8, 2009

Soybean growers seeking information about what aphids will be doing this year are going to find no firm answers from researchers.

Last winter, soybean growers were told at a variety of crop outlook meetings that the model for predicting the level of infestations by aphids, apparently, no longer works. 2008 was supposed to be a year for lower aphid numbers, said David Wright, director of production technology for the Iowa Soybean Association, but instead it was a record infestation year. The ISA assists a number of organizations in soybean research projects.

Now that planting season has arrived, Wright said there is still no way to help producers anticipate the level of infestations they’ll get.

When asked if 2009 could be the year that aphid counts are down, Wright said, “There really is no way to tell.”

From September through October 2006, the four aphid suction traps in Iowa – Ames, Sutherland, Nashua and Chariton – caught a total of 532 aphids and there was a record infestation in summer 2007. In fall 2007, there was a total of nine aphids trapped in Iowa during the same two-month period and researchers thought aphid populations in summer 2008 would be low. But that proved untrue as another record infestation was recorded.

In fall 2008, the four Iowa traps caught a whopping 1,437 aphids in the same two-month period and researchers have no idea what lies ahead for soybean growers in summer 2009.

Wright said the previous method for predicting aphids was to watch the traps that caught insects in fall flights. The number of trapped insects indicated what the insect population would be the next growing season. But in 2009, when aphids were supposed to be way down in number, they showed up in unparalleled populations.

Wright said there is no truth in the report that aphids are developing immunity to pesticides and to aphid-resistant soybean varieties.

Genetic researchers have inserted genetic resistant traits, called Rag1 and Rag 2, into soybeans to make the beans resistant to the ravages of aphids. However, Wright said, “we discovered that there are some types of aphids who are naturally not affected, or controlled, by these traits.”

Aphids pierce the veins of soybean plants and suck the sap. They also can spread viral diseases like soybean mosaic, which is prevalent in China, where the pests originated.

The yield damage can be extensive without treatment.

Wright said that researchers continue to study the economic threshold for treating soybeans, which is still thought to start at 250 aphids per plant. However, he urged that constant scouting of fields is needed for producers to stay on top of aphid populations, which can grow quickly.

What researchers are concentrating on now, Wright said, is to find that a balance between three different management techniques – economic threshold for treatment, resistant traits in plant genes and natural insect control.

Asian lady beetles, for instance, are a natural predator on aphids. Last year, a parasitoid wasp was made available and tested with some success, Wright said, on Iowa State University research farms. There are other natural enemies of aphids that come from China, which are being studied as well. One is for northern environments and one is for southern environments.

“We are trying to find controls that are site specific,” Wright said. “We’d like to combine the genetics with the (predatory) insects as ways to reduce pesticide use and protect yields.”

But, in the meantime, he added, researchers would continue to refine the economic threshold for spraying.

Contact Larry Kershner by e-mail at kersh@farm-news.com.

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