Soybean aphid, then and now
It was the summer of 2000 when entomologists in Iowa and other Midwest states discovered their first soybean aphids. A new insect in the U.S., it likely arrived by hitchhiking on some ornamental shrubs shipped into the Great Lakes region in the late 1990s. Soybean aphids have been in China and other Asian countries for a long, long time — probably for centuries.
When it first showed up in the Midwest in the summer of 2000, the aphid was more of a curiosity than a threat, not developing into any significant problems that year. The summer of 2001 was another matter. In parts of Wisconsin, Minnesota and five counties in northeast Iowa, we quickly learned to respect just how quickly soybean aphids can populate a soybean field.
In late June 2001, John Rodecap, a farmer just south of Decorah, asked me to look at one of his soybean fields. Within another week the aphid population doubled, so aside from getting out the word about this threat to the soybean crop, we decided to establish some insecticide trials to measure the possible damage this pest might cause. At harvest, the untreated aphid plots yielded half of the treated plots.
This was also the start of conducting annual soybean aphid trials at this site. We’ve been doing this ever since, and sharing weekly population trend data every season with farmers, crop consultants, dealers and others.
Aphids command our respect
So, 10 years ago we had a new and still rather unknown pest in soybeans. We assumed it would take at least five years of research to recognize general population patterns and basic control measures, and probably another five years to more fully understand how to successfully manage the pest.
Many research trials were established in 2002, but the aphid populations were almost non-existent. Was 2001 a fluke? We soon learned it wasn’t.
The 2003 season greatly renewed our respect for the pest, which caused 20% to 30% yield reductions in many fields in Iowa, Minnesota and Wisconsin. The year 2004 was a dud year, but in 2005, aphids again caused problems, expanding the pest’s territory across the upper two-thirds of Iowa.
What emerged was a pattern of aphids causing problems only in the odd years. But this really wasn’t true. In 2006, we had spotty problems in northeast Iowa, and parts of Michigan had huge problems. In 2008 we also had highly significant problems with the pest.
With the establishment of a network of insect suction traps across the upper Midwest, we discovered that even when we have locally low aphid populations, summer migrations of winged aphids can travel long distances and still cause significant infestations. This is why we scout weekly starting in July. These longer summertime migrations are not predictable other than they tend to be most active in northeast Iowa from mid-July through mid-August.
What have we learned?
In 10 years, we’ve learned some lessons about the soybean aphid and how to manage it.
1. Aphids overwinter on common buckthorn, a woody plant that grows in forests and woodlands. Soybean fields near where aphids overwinter will see initial aphid migrations from the buckthorn to soybeans in May through early June. These populations grow and eventually provide ammunition (the winged aphids) for the more widespread migration during July and August.
2. Aphid development is significantly slowed during consecutive days of wet weather and hot weather.
3. Once populations build to 250 aphids per plant on at least 80% of the soybean stand, it should be treated with a foliar insecticide. Here is the difficulty with this recommendation: What if your field reaches 250 aphids, you call the applicator to treat the field, and he says he can’t get there for five days!
What I have learned with my research trials is, once I get to about 100 aphids per plant, I call the applicator and schedule him to come to my field in four or five days. I can check the field one more time (three days later) just to make sure the population is still increasing (now closer to 250 per plant). Or, if the population crashed because of rain or a heat wave, I could still call off the treatment for a while. That way, I improve timeliness with a treatment applied when the population is around 250 aphids per plant.
4. Those smaller, whitish-colored aphids you see later in the summer are counted the same as the larger, yellow-green aphids. They are the same aphid, just not as well-nourished. But they still feed and reproduce, although not as quickly as the larger, somewhat healthier yellow-green aphids.
5. Insecticide seed treatments work well to minimize aphid populations for about 60 days after planting. Then the control seems to wear off. Unfortunately, the summer aphid migration doesn’t get started until mid-July and continues through much of August. So in an “aphid year,” these fields will still need a foliar-applied insecticide.
6. Most foliar insecticides have worked equally well for control of the pest. Continued research might eventually favor a product or chemical family over another, but not yet. Current control efforts should place emphasis on proper application method of the insecticide, nozzle choice, spray pressure, coverage throughout the canopy and timing of the spray.
7. Aphid-resistant soybean varieties are now available for planting, are initially working fairly well and should be improved with broader resistance in the near future. Keep in mind that the currently available soybean varieties that have aphid resistance slow the growth of aphids, but aphids will still survive to some degree, and the fields should still be scouted.
Lang is the Iowa State University Extension field agronomist at Decorah in northeast Iowa.
For more information about managing soybeans, go to www.soybeanmanagement.info.
This article published in the August, 2010 edition of WALLACES FARMER.