Beneficial Insects Could be Released as Early as Spring for Aphid Biological Control

Farmers, as stewards of the land, understand the use of insecticides is not ideal as a long-term management strategy, so they have partnered with scientists and agronomists to continue the search for an effective biological control method of the soybean aphid. They have learned finding such a solution brings many challenges, as well as opportunities.

In the past several years, soybean aphids have been few and far between, but this year, aphids were found in fields all across the state of Iowa, except in the southeastern. In order to prevent yield loss, an economic threshold of 250 aphids per plant has been established for insecticide use. 

Low populations mean less spraying of insecticides and more money saved. However in the future, if populations continue to increase, soybean farmers will have to take proper measures to ensure these pests do not reduce yields. One way researchers are working to manage soybean aphids is through the introduction of beneficial insects that prey on the soybean aphid.

Using beneficial insects in an attempt to manage soybean aphids is not a new strategy. A parasitic wasp, Binodoxys communis, was released in 2008. The tiny wasp lays its eggs inside soybean aphids, which then hatches into a larva and feeds on the aphids from the inside out, leaving nothing but aphid mummies.

However effective Binodoxys communis was at destroying aphids, it hasn’t established populations in the United States, possibly due to the harsh Midwest winters or other biological inhibitors. For some United States Department of Agriculture (USDA) researchers, it was back to the drawing board, or in this case, Asia, to find an insect that could destroy aphids, survive brutal winters and not be detrimental to other beneficial insects. Does such a species exist? According to USDA entomologists, the answer is yes.

Kim Hoelmer, research entomologist with the USDA, has traveled several times to China and Korea since 2002 to search for natural aphid predators. Hoelmer says any given trip in the summer often reveals different aphid predators or, if weather or crop conditions are unfavorable at the moment, none at all.

“We believe there is a lot of natural enemy diversity in Asia, and we have not found all of it yet,” Hoelmer says. “Soybean aphids are not as prevalent in Asia because their natural predators and parasitoids have kept them in check. They can be hard to find during exploration in Asia because their aphid hosts are not as abundant as they are in the north central states. In the past years, we have collected more than 20 species of parasitoid wasps, and additional species of predatory flies and lady beetles that are all natural enemies of the soybean aphid. After repeated trips, we are still finding new species, and once that number starts to diminish, we’ll feel confident we have found all of the soybean aphid’s natural enemies.”

Keith Hopper, research entomologist with the USDA, says he is currently studying three parasitic insects that are in the pipeline for future release against soybean aphids.

“We have three species in culture, which host range testing has shown would be safe to introduce,” Hopper says. “I have submitted a petition and received approval from North American Plant Protection Organization for one of them, Aphelinus near engaeus, which we plan to introduce next year.

“In light of the poor establishment of a previously introduced parasitoid, Binodoxys communis, we have tested winter diapause in Aphelinus near engaeus and found it diapauses well.”

Hopper says he is currently working on a petition for another species as well.

“I’m working on Aphelinus near gossypii, which I plan to submit at the end of the summer,” Hopper says. “The third species will require some more host range testing, but we should be able to submit a petition for its release next spring.”

While numerous aphid predators and parasitoids have been found in Asia, introducing them to soybean fields in the United States poses a few challenges. First, any live insects that are purposefully introduced into the United States must be approved by USDA’s Animal and Plant Health Inspection Service (APHIS) for importation to an approved quarantine facility. Soybean aphid natural enemies obtained in Asia are also sent to the USDA’s Beneficial Insects Introduction Research Unit in Newark, Del., for further research by Agricultural Research Services scientists and by cooperating entomologists at the University of Minnesota until they are determined to be safe and specific enough to be tested in fields.

George Heimpel, an entomologist with the University of Minnesota, studies non-Aphelinus species in quarantine and says another obstacle is making sure the aphid predators do not destroy other beneficial insects.

“There’s no risk to humans or the chance of these parasitoid wasps becoming pests themselves,” Heimpel clarifies. “We are looking to ensure the safety of a healthy ecosystem. If they attack a lot of aphid species, we don’t consider them further because of the fear of unintended consequences.”

Hopper says all Aphelinus species only parasitize aphids and not other species.

“Aphelinus near engaeus only parasitizes aphids in the genus Aphis and not all of them,” Hopper says. There are native species of Aphis that are not pests. However beneficial these native species are is unknown, but we’d prefer to introduce parasitoids that do not attack them,”

Hoelmer says beneficial insects from northern China and South Korea are scouted because their climates are similar to those of the north central states. Even if surviving harsh winters may not be an issue, there could be other unknown factors influencing their establishment in North America.

“While parasitoid wasps may use soybean aphids as a host in the summer, they could have alternate aphid and plants hosts that help them get through the winter in Asia that are not found here,” Hoelmer says. “None of these natural enemies have been studied extensively in Asia, and there may be aspects of their biology that are missing or unknown, which might be why Binodoxys communis did not take off. We won’t know which ones will succeed until we have tried to establish them here.”

Heimpel says, for the time being, spraying insecticides will manage soybean aphids in fields, although it’s not a long-term solution.

“Soybean aphids can build resistance against insecticides and aphid-resistant soybean varieties,” Heimpel says. “Biological control is long-term, with the potential to permanently solve the issue. Aphids can’t build up a resistance to parasitoids because they change with them.”

In the meantime, Matt O’Neal, an entomologist with Iowa State University (ISU), says there are numerous ways to keep up-to-date on soybean aphids and how to manage them.

The second edition of the ISA-ISU Soybean Aphid Field Guide is available, which includes information on host plant resistance and several ongoing experiments on how aphids are responding to insecticides out in the field,” O’Neal says.

The field guide and other aphid-management resources can be found on the Iowa Soybean Association’s Production Research Library at www.iasoybeans.com/productionresearch and at www.planthealth.info.