Soybean aphids fight back

Soybean aphids are now resistant to insecticides in Manitoba.

That’s not a problem in years like 2018, when there were little to no soybean aphids in Western Canada.

But if there’s another year like 2017, when aphids were abundant, soybean growers could struggle to control the pests.

“Failures of certain pyrethroid insecticides for management of some soybean aphid populations have been observed in commercial fields and resistance to bifenthrin and lambda-cyhalothrin has been documented through small-plot research and laboratory bioassays,” said a North Dakota State University website on soybean aphids.

University of Minnesota testing, conducted with help from Manitoba Agriculture staff, shows that soybean aphid populations in North Dakota, Minnesota and Manitoba have developed resistance to several insecticides.

In 2017, University of Minnesota researchers looked at aphid populations at more than a dozen sites in the region and measured the resistance to the several products, such as bifenthrin. They collected populations of aphids from a particular site and treated the insects with bifenthrin.

From certain sites the bifenthrin killed nearly 100 percent of the aphids. At other locations almost of all of the aphids survived the treatment.

At Newton, Man., the chemical controlled about 50 percent of aphids from two fields. From a third site, bifenthrin killed less than 15 percent of the aphids.

Canadian soybean growers should be concerned because Health Canada has registered a small number of products for soybean aphids, said a University of Minnesota entomologist.

“A lot of growers in Minnesota have a limited tool set, but you guys have a much more limited tool set, in regards to chemicals,” said Robert Koch, who spoke at the Manitoba Agronomists Conference, held last month in Winnipeg.

The summer of 2017 was a nasty season for soybean aphids. The tiny insects infested nearly every soybean field — meaning they covered more than 2.2 million acres of farmland in Manitoba.

Last summer was different — there were no aphids for most of the growing season.

Soybean aphids don’t likely overwinter in Manitoba; most blow in with southerly winds from the United States.

When they do arrive, the aphids pierce the tissues of soybean plants and remove the sap, stealing nutrients from growing plants.

It’s difficult to gauge yield losses but a Manitoba Pulse and Soybean Growers (MPSG) fact sheet suggests that 500 aphids per plant, feeding for 20 days, cuts yield by seven percent.

Koch has been tracking aphid resistance to pyrethroids and other chemicals since about 2015.

The resistance was especially bad in 2017, when more than 20 counties in Minnesota and North Dakota had reports of pyrethroids failing to control aphids.

Koch didn’t provide detailed data for Manitoba, but resistance was likely a problem that year in parts of the province.

Manitoba soybean growers didn’t cause this problem, said John Gavloski, Manitoba Agriculture entomologist. The aphids likely arrived with resistance to pyrethroids because growers south of the border have been overusing the chemistry.

That’s a problem for Manitoba growers because they have fewer options for controlling soybean aphids. However, Health Canada recently registered an aphicide that can be applied to soybean aphids, so that’s good news for producers, Gavloski said.

There are several reasons why soybean aphids are developing resistance to insecticides:

  • Selection pressure — the same insecticides being sprayed on aphid populations year after year, or multiple times in the same year.
  • Spraying when unnecessary, without doing aphid counts.
  • Not spraying at the recommended rate.

To prevent resistance to insecticides or to slow the process, Koch said one factor is critical: only spray when necessary.

Growers should scout fields and count the number of aphids on soybean plants. Manitoba Agriculture said the “action threshold” for soybean aphids is 250 aphids per plant, on average, and when the population is increasing.

“(This) is because the actual economic injury level, where control costs will equal yield loss, is actually about 670 aphids per plant,” Manitoba Agriculture says.

The figure of 250 is used because it provides a producer time to take action before the aphid population explodes.

When spraying is necessary, growers should rotate chemistries and avoid using the same product in a single season.

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