Resistant Aphids on the Rise

Resistant Populations Complicate Soybean Aphid Control

Emily Unglesbee , DTN Staff Reporter
Heavy aphid populations can look alarming, but the threshold of 250 aphids per plant is well under yield loss levels. (Photo courtesy Christina DiFonzo, Michigan State University)

ROCKVILLE, Md. (DTN) -- Reports of pyrethroid failures against soybean aphids in Minnesota are surfacing once again. Last year, entomologists there confirmed a number of pyrethroid-resistant populations, not a surprising development given the pest's creepy cloning abilities.

Aphids can duplicate themselves many times over, each time producing female offspring already pregnant with the next generation, all summer long. Every time a farmer treats this army of tiny clones, the same genomes can be exposed to the same insecticide repeatedly -- a recipe for the evolution of resistance.

With this rapid, asexual life cycle, soybean aphids have dramatically increased insecticide use in soybeans in Midwestern states like Iowa and Minnesota since their arrival in 2000. Now the aphid's threat to agriculture has heightened.

"Last year we documented failures of pyrethroids in southern Minnesota," said University of Minnesota entomologist Bob Koch. Specifically, those aphid populations showed resistance to either bifenthrin, the active ingredient in products like Brigade, Tundra, and Hero, and lambda-cyhalothrin, the active ingredient in products such as Warrior. "This year we are starting to see failures again," Koch added.

This development, and the likely insecticide overuse that spurred it, prompted a group of 16 entomologists from 10 universities to write and publish a review of the research on the thresholds for treating soybean aphids, titled "Just the Facts." (You can see it here: http://bit.ly/…)

Spraying prematurely for aphids is driven by an insurance mentality among farmers and some inaccurate marketing claims that have surfaced in the Midwest recently, Koch said.

"There are claims circulating out there that people should be treating at five to 10 aphids per plant and questioning the validity of the 250-aphids-per-plant threshold," Koch said. "We want to make people understand that we've done research and it continues to validate this threshold."

By spraying too early and often for the aphid, farmers risk wasting their money and destroying beneficial predator populations early on, Koch said. Longer-term risks include water quality problems from high insecticide use, which could prompt unwanted regulation, and the rise of resistant populations, which is now underway.

THE ARGUMENT FOR 250

Looking at a soybean plant with 250 aphids on it is quite alarming -- the tiny green insects can cover entire leaves quickly. But don't forget how tough beans are, Koch said.

"Even at 250 aphids, you're not seeing yield loss," he said. "We've deliberately set the threshold below yield loss levels, so you have time to line up an insecticide application."

Keep in mind that even when aphids cause some yield loss, they are not necessarily doing economic injury -- that is, their damage still might not justify the cost of an application. Economic injury depends on other changing factors such as soybean prices and yield potential. Growers are best off sticking to the fixed-rate threshold of 250 aphids per plant, with 80% of plants infested, which is conservative enough to allow for treatment before any yield loss can be measured, Koch said.

A GROWING ALLY

As an invasive pest, soybean aphids have enjoyed many years without some of the predators that keep them in check in Asia. That may be coming to an end, as a growing number of parasitic wasps that prey on the aphid enter the scene.

In Minnesota, entomologists have noticed an encouraging rise in the number of one such species, Koch said. "They lay their eggs in the aphid, and the larvae eat the inside of the aphid, leaving behind mummified black, crusted aphids that you can sometimes see on the plant," Koch explained. "They came over from Asia and have spread through the northeast and Canada." Entomologists are hopeful that the pest will suppress aphid populations in the Midwest as well as they have in Canada, he added.

USDA and university researchers have evaluated other species of parasitic wasps, and Minnesota has received authorization to release some of them over the last couple years, Koch added.

Other natural insect predators include lady bugs, the insidious flower bug and green lacewing larva. Applying insecticides before threshold levels of aphids can knock down these populations early in the season and leave you vulnerable to more aphid infestations later on, Koch noted.

WATCHING FOR RESISTANCE

As upper-Midwest growers spray for aphids in the coming weeks, more resistant populations may become apparent, Koch said.

Keep in mind that insecticide mixtures do not necessarily combat resistant populations in the same fashion that herbicide mixtures do with herbicide-resistant weeds. The Insecticide Resistance Action Committee (IRAC) recently released a statement about this issue, which you can see here: http://bit.ly/….

For more information on monitoring and reporting resistant aphid populations, see Koch's article from the University of Minnesota here: http://bit.ly/….

For more information on how to scout, manage and treat the soybean aphid, see this Iowa State University field guide: http://bit.ly/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/AG)

Emily Unglesbee