By Isaac Popp 

Is something eating soybean aphids?

Let’s hope so!

One of the best ways to achieve long-term control of soybean aphids is to foster their natural enemies.

Lady beetles, lacewings, parasitic wasps and other insects that feed on soybean aphids can be very effective at regulating aphid populations. Now that some of our chemical weapons are breaking down due to increasing aphid resistance, natural enemies are more important than ever.

To protect these valuable insects, it’s important for growers to practice good integrated pest management. That means scouting for aphids and waiting to spray until aphid numbers reach economic thresholds.

RECOGNIZE MUMMIES: Look for black- or tan-colored aphid mummies attached to soybean leaves when scouting. Here, aphids parasitized by wasps are mixed in a soybean aphid colony. The stingless wasps themselves are smaller than a gnat and hard to see. (Photo by Jonathan Dregni)

A beneficial invader
One of the most important groups of natural aphid enemies is the parasitic wasps. These tiny, stingless insects inject their eggs into soybean aphids. When the eggs hatch, the larvae devour the aphids from inside, then form cocoons within their dead hosts’ bodies, which are called “aphid mummies.”

Sounds like a horror movie, right?

In their native Asia, parasitoids keep aphids in check most of the time without insecticides. The USDA has attempted to introduce two species of Asian parasitic wasp into Midwest soybean fields — a tactic known as classical biological control. So far, though, these introduced wasps have failed to take hold, says Jonathan Dregni, a University of Minnesota assistant scientist.

However, another beneficial foreign wasp called Aphelinus certus has become established recently in North America. The aphid-eating species was first detected in Pennsylvania in 2005 and is spreading fast. The wasps arrived in Minnesota about 2011, and since then have been seen in soybean fields throughout the state, Dregni says.

BENEFICIAL NATURAL ENEMY: A parasitic wasp from the genus Aphelinus parasitizes a soybean aphid. (Photo by Matt Kaiser)

Nobody knows how Aphelinus certus got to this country.

“It wasn’t intentionally introduced,” says Robert Koch, U-M Extension entomologist. In fact, the USDA had studied Aphelinus certus, but decided not to release it because it’s not an especially picky feeder. “It likes to feed on lots of aphid species,” Koch says.

Protecting friendly insects
Aphelinus certus can lower spring soybean aphid colonization, reducing later outbreaks, Dregni says. In a recent field experiment, he compared the effectiveness of Aphelinus certus to neonicotinoid seed treatment for early-season aphid control. He found that four weeks after soybean planting, just two female wasps reduced aphid densities as well as the seed treatment.

Unfortunately, broad-spectrum insecticides, such as seed treatments and foliar sprays, not only kill soybean aphids, but also friendly insects.

How can you protect Aphelinus certus and other natural aphid enemies?

The most important thing you can do to minimize collateral damage — and avoid unnecessary expense — is to adopt economic thresholds for spraying. Scout for aphids beginning in mid- to late July through R6.5. Don’t spray unless aphid numbers reach 250 aphids per plant on more than 80% of plants in the field.

I know this sounds like a lot of aphids, and growers may be tempted to spray sooner. Remember, though, that economic injury to soybeans does not occur until aphid numbers reach 650 per plant. The economic threshold of 250 aphids gives you a good five days to spray before yield losses begin.

Be sure to scout three to five days after you spray to make sure the treatment was effective. Failures of pyrethroid insecticides on soybean aphids have been reported for the past three years in Minnesota. To manage aphid resistance, you must also rotate insecticide modes of action from year to year, and within the season.

Survey results show the populations of soybean aphids in Minnesota in 2017. (Counties shaded in tan were not sampled.) The highest aphid densities were found in central and northwestern Minnesota last year. The survey will be repeated in 2018.

Surveying wasps
Natural enemies like Aphelinus certus can also help manage insecticide-resistant aphids.

The Minnesota Department of Agriculture and U-M have been surveying Aphelinus certus for several years.

“The wasp has spread throughout Minnesota,” Koch says, “and the numbers are increasing over time. That’s a good sign for aphid management.”

One of the goals of the survey is to understand how much parasitism is required to suppress aphids without added inputs, Dregni says. A 2014 Minnesota survey found Aphelinus parasitism rates as high as 88% of the aphids in a field. Eventually, researchers hope to incorporate natural enemy densities into soybean aphid treatment thresholds.

Survey results show the populations of the parasitic wasp Aphelinus certus in Minnesota in 2017. (Counties shaded in tan were not sampled.) Aphelinus certus, a beneficial natural aphid enemy, first arrived in Minnesota about 2011 and has been spreading and increasing in number. Researchers are seeking Minnesota cooperators for a 2018 survey.

In 2017, scouts visited 43 Minnesota counties, collecting thousands of aphid mummies. The survey will be repeated this summer, and researchers are looking for additional cooperators. Researchers hope to collect samples from most Minnesota soybean-growing counties in 2018.

If you are willing to participate in this important survey, email coordinator Jonathan Dregni at [email protected], and include soybean field locations. The 2018 survey will take place between July 15 and Aug. 15.

Consider aphid-resistant soybean varieties
Another sustainable way to manage soybean aphid resistance and protect natural aphid enemies is to plant aphid-resistant soybean varieties.

Soybeans that possess resistance genes called Rag genes have proved effective at suppressing aphids without a yield penalty, says Koch. Commercial soybean varieties that contain Rag genes are still limited for northern relative maturity groups. However, they offer promise for future aphid management.

Popp is a certified crop adviser with Centra Sota Cooperative in Little Falls, Minn. Find information and links to Minnesota CCAs at mcpr-cca.org.