Soybean Aphid Biological Control...An Exotic Approach

Parasitic wasp laying eggs in a soybean aphid Photo credit: Bob Ellingson

Dr. Chris DiFonzo, Michigan State University

In its native region, the soybean aphid has all it needs to exist in the landscape, but it also has a cadre of natural enemies that attack and keep it in check. As a result, the soybean aphid is rarely abundant in Asia.

The first soybean aphids invading North America had it made! They found environmental conditions similar to their Asian homeland. They found their food crop, soybeans, planted on millions of acres. They found their overwintering host, buckthorn, abundant in some areas. Initially they didn’t encounter many natural enemies. The soybean aphid population boomed, spreading rapidly across the Midwest. Within a few seasons, native insect predators and pathogens ‘learned’ to attack soybean aphids. Mortality from these natural enemies likely contributes to the current cycling of soybean aphid outbreaks that we've observed across the region.

Soybean Aphid Natural Enemies

An aphid mummy showing the exit hole where the parasitic wasp emerged once it reached adulthood. Photo credit: Peter Sonnentag, courtesy Eileen Cullen, University of Wisconsin

Pathogens, predators, and parasitoids are the three main types of natural enemies of insect pests (read more about these natural enemies at the Soybean Aphid Biological Control website). Mostly missing in North America were sobyean aphid parasitoids. The larvae of these tiny wasps develop inside the aphid body, eventually killing it. Parasitoids often have a narrow host range, specializing on a particular insect group or species. Under favorable conditions, parasitoid numbers can increase rapidly, reducing pest numbers dramatically. Entomologists identified soybean aphid parasitoids as possible agents for classical biological control.

The idea behind classical biological control is to explore the native range of an introduced pest and identify natural enemies for importation into the United States. The long-term goal for soybean aphid biocontrol is to introduce exotic parasitoids that become established (survive and reproduce) in the Midwest and hold down aphid numbers from year-to-year with no further inputs.

Successful examples of classical biocontrol the U.S. include the introduction of parasitoids to control cereal leaf beetle and alfalfa weevil. Both of these non-native insects are now effectively suppressed by biological control, and rarely have to be managed by growers.

The Steps to a Successful Introduction

1. Climate / Habitat Matching
Entomologists, working with Asian cooperators, identified suitable areas for collecting parasitoids. These areas had both soybean production and buckthorn, and were a climate match for important soybean production areas in the United States. Climate-matching increases the likelihood of finding insects adapted to local conditions, increasing the success of an introduction.

2. Foreign Exploration
From 2001 to 2006, entomologists from University of Minnesota, the Illinois Natural History Survey, Purdue, and USDA-ARS worked with scientists in China, South Korea, and Japan to collect natural enemies of soybean aphid. Parasitoid collections were made from both soybean and buckthorn, at different times of the year. These insects were shipped back to U.S. quarantine facilities.

3. Importation
It was an administrative and logistical challenge, requiring many permits, to move live insects from foreign countries to the United Sates. Parasitoids have a short life cycle; insects must reach the U.S. from Asia in a matter of days, or die in transit.

4. Rearing and Testing in Quarantine
Back in the U.S., parasitoids were kept in quarantine at the USDA Beneficial Insect Research Lab in Delaware or at a new MAES/MDA Facility at the University of Minnesota in St. Paul. It is very labor intensive to rear plants and keep aphid colonies to rear the wasps.

Each parasitoid was tested for how effective it was at using soybean aphids as a host. The wasp Binodoxys communis was chosen for further testing to determine if it specialized on soybean aphids or if it attacked other aphid species as well. It is important not only to release an effective natural enemy, but also one with a narrow host range that will not disrupt non-target aphid species. The host-range testing conducted for Binodoxys was some of the most thorough ever done for a parasitoid targeted against an aphid.

5. Release Permits
Once efficacy and host range data were complete, more permits were needed from the U.S. and Canadian governments to make field releases. Permits for the release of Binodoxys were finalized this spring.

6. Field Introductions
The first field releases of Binodoxys will be done in IL, IN, IA,MN, SD and WI this summer. Researchers will determine the parasitoid's efficacy and spread, and it's survival over the winter into 2008.

Watch for updates throughout season!

Information on these pages is from the cooperative effort of researchers throughout the North Central states. Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: NCSRP Plant Health Initiative at www.planthealth.info. Please do not use copyrighted photos without permission.