Potentially Damaging Soybean Aphids Again Found in Kansas

Populations appear to be light thus far.

Published on: Jul 31, 2007

Soybean aphids, which have the potential to wreak havoc in soybean fields, have again arrived in Kansas.

This year's soybean aphids were first discovered July 12 in a research plot by Kansas State University plant pathology leader Doug Jardine and Lyon County agricultural Extension agent Brian Rees.

The plot is a sentinel plot, used to monitor the possible occurrence of soybean rust and soybean aphids. Both are potential threats to the state's soybean crop, according to Extension entomologist Jeff Whitworth.

Five days after Jardine and Rees' discovery, soybean aphids were found in Riley Co.

"These are the first 2007 reports of soybean aphids in Kansas. They're somewhat earlier than in past years, although approximately in the same time frame as seen in 2005," Whitworth says. "But, populations that year never really became troublesome."

The soybean aphid, Aphis glycines, was first detected in the United States in 2000 and first detected in Kansas in 2002.

"We have had soybean aphids every year since, but only in 2004 did they become a problem of economic consequence, requiring some fields to be treated with insecticides," Whitworth says. "In fact, only one county had verified soybean aphid populations in 2006."

Soybean aphid populations seem to be sensitive to hot temperatures -- specifically temperatures over 95 degrees, he adds.

High summer temperatures may not actually kill the aphids, but it does seem to drastically reduce their ability to reproduce, Whitworth says. And, if they are not rapidly reproducing, this gives some of their natural enemies, such as predators and parasites, the opportunity to help regulate the aphid populations.

Aphid populations can increase very quickly because they reproduce parthenogenicallym, which means they don't have to mate and lay eggs.

Each aphid is a female and thus produces females that produce females and so on, without any lag time of looking for mates and waiting for eggs to hatch. They are born and start feeding and producing young immediately.

Soybean aphids are the only North American aphids to colonize and develop large numbers on soybeans, Whitworth adds. They're relatively small, light-green aphids with distinctive black cornicles or "tail pipes" at the tip of their abdomen. Infestations usually start at the outer canopy on newer soybean leaves. Aphids can be throughout the canopy, but well-established populations most often will be on the undersides of leaves and on stems and developing pods.

"This aphid can transmit many viral diseases, but it also can stress plants and reduce yields by virtue of their feeding, which removes nutrients from the plant," the entomologist said. Infested plants may have a distorted, yellow appearance and may become covered with a slick, sticky substance called "honeydew" - which itself often becomes dark and sooty, due to molds living on the honeydew. Ants seem to be attracted to this honeydew, so "if you see ants in your soybean canopy, that may be an indicator of an aphid infestation," he explains.

The aphid populations discovered in Kansas so far are small and widely scattered, Whitworth says. However, this year's soybean crop in general is a little late in development, and that might make it more vulnerable to insect damage.

Soybeans in the late vegetative and early reproductive stages are probably the most susceptible to aphid feeding, Whitworth says.

States with annual soybean aphid problems have set the economic thresholds for applying controls at around 50 aphids per plant in pre-reproductive plants and about 250 per plant in the R1 (beginning bloom) to R4 (full pod) stages. The thresholds take into account a seven-day lag between scouting and treatment, for time to make the arrangements to treat, he says.

Whitworth encourages producers who find a soybean aphid infestation to report it to their county or district K-State Research and Extension office.

In addition, Extension offices have information about sampling, a new technique called "speed scouting," treatment thresholds, and insecticides registered for use against soybean aphids.

More information also is available on the Kansas Extension Web site: www.entomology.ksu.edu/DesktopDefault.aspx?tabid=668. Or, look at Iowa State University's Web site: www.ipm.iastate.edu/ipm/icm/node/53