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Specialist: Soybean aphid might break with tradition in 2006

By Steve Leer
Department of Agricultural Communications
January 13, 2006

 

WEST LAFAYETTE, Ind. - A tiny insect with a history of causing problems every other year for soybean growers could become a troublemaker two years in a row, said a Purdue University Extension entomologist.

The soybean aphid, a pinhead-size pest that damages soybean plants by feeding on plant sap, entered the winter in a position to come back strong in 2006, said entomologist Christian Krupke. Farmers in northern Indiana counties battled aphids this past year and should monitor their fields closely for aphids in the coming crop season, he said.

Aphids can reduce soybean yields by 10 percent to 15 percent. Since aphids were first detected in Indiana in 2000, the pests have been particularly active in Hoosier soybean fields in odd-numbered years.

"There is the potential for that every-other-year cycle to be broken this crop season," Krupke said. "One of the primary reasons is that we're not seeing the number of predators going into overwintering that we've seen in past big aphid years."

Predators help regulate aphid populations by consuming the pests.

"The Asian lady beetle is the major predator of soybean aphid," Krupke said. "In a big aphid year we get a lot of Asian lady beetles, and a lot of them go into overwintering. The following spring they are there to feed on aphids and lower the aphid numbers. Their cycle sort of follows the aphids' cycle, except that it is delayed by one year."

In its five years as a United States soybean pest, the aphid has defied biological convention by not appearing in fields year after year.

"The aphid is an anomaly among insects," Krupke said. "There really aren't many pest insects that do this boom-and-bust in alternating years.

"One hypothesis on this trend is that the aphid is so new to the Midwest that the pest is still coming to equilibrium, or finding its place. Its eventual equilibrium will most likely be somewhere between the peaks and valleys we're seeing in alternate years."

Most aphids that attack Indiana soybeans are likely migrants that fly in from other Great Lakes states. Aphids take flight when soybean fields in Michigan, Illinois, Wisconsin and Minnesota are occupied by other aphids.

"That may be one reason why the northern half of Indiana has a greater aphid problem," Krupke said. "About the only thing that prevents them from moving farther south in Indiana is their overwintering host, which is a noxious weed called buckthorn. The highest concentration of buckthorn is right around the Great Lakes."

Because aphid populations could be high this spring and summer, soybean farmers are encouraged to keep an eye out for the pest in their fields. Farmers should inspect soybean leaves for the yellowish-green insects and be prepared to act quickly if they find 250 or more aphids on a plant.

"Growers could benefit from doing a bit of scouting in their fields," Krupke said. "Scouting for aphids is very easy. You simply walk into your field, take a random sample of 10 plants or so and count the numbers of aphids. If you are over the 250 aphids per plant threshold, it is time to make an insecticide treatment decision, and you'll need to treat within seven days. Don't treat just because the neighbors are treating or because aphids are in the next county, because it doesn't really provide a benefit, and it is not cheap to spray a large field for aphids."

Insecticide applications cost about $10 per acre.

Krupke and fellow Purdue entomologists will conduct aphid-related research this year. One study will look at the long-term effects on soybean yield from sub-threshold aphid feeding, while another study will examine the effectiveness of soybean seed treatments labeled to control the pest.

"Previous Indiana studies indicate that seed treatments might not remain active for a long enough period of time to protect soybeans from aphids," Krupke said.

In addition to the research projects, Purdue entomologists plan to operate a network of suction traps at six locations across Indiana. The tall, cylindrical traps act as an outdoor vacuum cleaner, sucking up aphids as they fly past. Entomologists use the traps to calculate the severity of aphid infestation and overwintering potential.

For more information on soybean aphids, scouting and treatment, read Purdue Extension publication E-217-W, "Soybean Aphid," by Krupke and Purdue entomologists John Obermeyer and Larry Bledsoe. The publication can be downloaded online.

Sources:
Christian Krupke
494-8761

Writer:
Steve Leer
765 494-8415

Related Web sites:
Purdue Extension publication E-217-W, "Soybean Aphid"

Field Crops IPM
http://www.entm.purdue.edu/fieldcropsipm/

Purdue Department of Entomology
http://www.entm.purdue.edu/