West Lafayette, Indiana
August 7, 2003

An agricultural pest that first moved into Indiana soybean fields three years ago could be establishing permanent residency.
 

The soybean aphid, a leaf-attacking insect no larger than twice the size of a period ending a sentence, is reproducing at an alarming rate in fields across northern and east-central Indiana. Purdue University Extension entomologists have been tracking the pest's burgeoning numbers the past few weeks, said Bob O'Neil, a research entomologist. He advised farmers in high infestation areas to check their fields for aphid activity. "In the last couple of weeks we've been seeing some significant increases in aphid numbers," O'Neil said. "We traditionally sample about a half dozen fields along U.S. 30, and we've noticed numbers increasing to 10, 50 or 100 aphids per plant. The increase in numbers got us concerned.

More than 100 soybean aphids collect on the underside of a soybean leaf. Pest feeding can inhibit the plant's ability to make grain, or kill it outright. (Photo/John Obermeyer, Purdue Extension entomologist)

"Then we initiated some sampling along U.S. 20, a little farther north, and began finding numbers closer to 50 to 100 aphids per plant, and some plants with over 500 aphids. The significant aphid numbers are most common in the northern third of the state."

Entomologists are unsure why aphid populations are exploding this year. Aphid numbers were down in 2002, after increasing in 2001 and 2000, when the pest was first discovered in Indiana.

One possible reason for the higher aphid numbers is the low number of the pest's natural predators. Among them are minute pirate bugs, Asian lady beetles and fungal pathogens, O'Neil said.

Aphids are native to Asia, where the pests have caused severe crop damage. The insects infiltrated Midwestern states in 2000, with the greatest infestations in Minnesota, Wisconsin and Illinois.

The insect's rapid reproduction, along with its feeding and disease-carrying characteristics, makes the aphid an especially dangerous pest, O'Neil said. As many as 15 aphid generations can be birthed in a single crop season, he said.

"The aphid, although small, builds up large numbers," he said. "It's that buildup of large numbers that is the problem. The aphid has piercing, sucking mouthparts. They have what you might call a little sharpened straw, and they jab it into the plant and start sucking out plant sap."

A soybean plant with low aphid numbers should survive, O'Neil said. As numbers increase, however, the plant's grain-making ability becomes impaired, or it dies.

"If you have a few aphids per plant, the plant can handle it," O'Neil said. "If you have a hundred aphids per plant, the plant probably begins to notice. If you have a thousand aphids per plant, it's probably causing some pretty significant challenges to the plant, in terms of its ability to produce yield. At very high infestation levels, it'll kill the plant outright.

"Another complication we have with these aphids is they can transmit diseases. Soybean mosaic virus is something we're very concerned about. Fortunately, we haven't seen any of that disease appear in association with the aphids."

The sap aphids consume is later excreted on the plant. This "honeydew" also poses a threat.

"With large aphid numbers there's a lot of honeydew being produced, and it will wet the leaves of the plant with a very sugary solution that allows for fungus growth – something we call sooty mold," O'Neil said. "In heavily infested plants, the leaves will begin turning black. If they turn black they can't produce the energy they need from the sun – photosynthesis – that allows them to grow and develop and produce yield."

O'Neil urged farmers with soybean fields north of I-74 to inspect their crops for aphids. They should examine the underside of plant leaves closely, because simple drive-by inspections aren't enough to spot potential infestation.

"Producers should get into their fields and start looking for aphids, particularly those producers in the northern third of the state," he said.

"An infested field is infested throughout. You don't have to walk very far into your field to find aphids. If you walk more than 10 yards you're probably going to find as many aphids as if you walk a hundred yards.

"Next, I'd pull up about 10 plants in different places and turn them over to see how many aphids you have. You don't have to count them – if it looks like there's a lot, there's probably a lot. They'll be in little yellow masses, and there will be quite a few of them. If you have less than a couple hundred per plant, you're probably in pretty good shape. If you have more than a couple hundred, then another decision has to be made in terms of treatment."

Brand-name insecticides proven effective to control aphids include Lorsban, Warrior and Mustang. Farmers should apply the products according to manufacturer's recommendations. Use of insecticides could kill aphid predators, as well, which is why, besides cost, farmers should be certain they have an aphid problem, O'Neil said.

If farmers find aphids in soybean fields on first inspection, a second inspection is in order.

"I would advise producers to take note of where they're finding aphids and in what fields, and then go back to those fields in a few days," O'Neil said. "Aphids can double their populations in about three to five days. Likewise, those populations can decrease during that time."

More information about soybean aphids is available in Purdue Extension publication E-217-W, "Soybean Aphid." It can be read online at
http://www.entm.purdue.edu/entomology/ext/targets/e-series/EseriesPDF/E-217.htm .

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu
Source: Bob O'Neil, (765) 494-7207, bob_oneil@entm.purdue.edu
Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu;
http://www.agriculture.purdue.edu/AgComm/public/agnews/

Related Web sites:
Purdue Field Crops Integrated Pest Management
Purdue Department of Agronomy Extension Soybean Management