The best option for long-term control of the soybean aphid is the reestablishment of their natural enemies. The soybean aphid is only a sporadic pest in most of their native areas of China, Japan and Korea because of the presence of parasitoids and predators that keep them in check.

Pathogens, predators, and parasitoids are the three main types of natural enemies of insect pests. These include many species of lady beetles and their larvae, lacewing larvae, larvae of the syrphid or hover fly, adults and nymphs of the minute pirate bug, the incidious flower bug, Orius insidiosus, and parasitoid wasps. Many of these are present in Wisconsin.

The most common naturally-occurring soybean aphid parasitoid observed so far in Wisconsin is a non-stinging wasp called Lysephlebus testaceipes. Soybean aphids parasitized by this wasp become tan mummies. These wasps seem to be increasing in Wisconsin since 2000, perhaps due to a shift in their host pattern towards the soybean aphid. Parasitism numbers in 2003 were estimated at about 1%. This is just a beginning of aphid control, as the amount of combined parasitism required to suppress aphids is considered to be around 20-30%.

Learn to recognize evidence of parasitized aphids when scouting. The parasitoids themselves are not easily seen, but the results of their work is: the aphid mummies. Photo credit above: University of Minnesota; below: Bob Ellingson

UW entomologists have released another common aphid parasite called Aphelinus albipodus. This wasp was released in 2002 and 2003, with the goal of getting a critical mass of wasps into the environment so that they can form a self-sustaining population which can spread and increase on its own. This process will take several years. The wasps successfully attack soybean aphids in Wisconsin under typical field conditions. The question remains as to how well they survive Wisconsin winters, and investigations are in place to answer that question.

Importation biological control project
Since 2005, a group of scientists from five midwestern states, including Wisconsin, and the USDA, with funding from the North Central Soybean Research Program, have been conducting research on importation biological control of soybean aphid. The idea behind importation, or "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 parasitoids that become established 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.

Bindoxys communis
Soybean aphid parasitoids are not as common in North America as they are in Asia. Entomologists have identified soybean aphid parasitoids as possible agents for classical biological control. Over several trips to Asia, a number of aphid parasitoids were identified and brought back to the U.S. where they have been in quarantine. Before field testing, several years of research have been conducted on their biology, safety and impacts on non-target aphid species.

So far, a tiny parasitoid called Binodoxys communis is the most promising. It appears to be very well adapted to soybean aphid and exhibits fairly high levels of host specificity in the laboratory. This means that the chance of this parasitoid disrupting other native species is low and that soybean aphid is it's number one target. It was approved for release by the US Department of Agriculture in 2007, and releases were made in summer 2007 and 2008. Six states (IA, IN, MN, MI, SD, WI) participated in the release program in 2008, and a total of 32 releases have been made across the region. Of these, 24 were in grower fields and 8 were on university research farms.

An aphid mummy that was parasitized by Binodoxys communis. Photo credit: Dan Mahr

Many of these releases were made later in the summer than originally planned, due to the unusually late population increases of the soybean aphid throughout the region. The suction trap network documented the virtual absence of soybean aphid early in the season, and the widespread late season (August) flight of winged aphids.

Preliminary indications are that Binodoxys communis has been recovered from most of the 2008 release sites. However, the parasitoid must survive the winter and be recovered in 2009 before truly successful establishment can be determined. No recoveries were reported in 2008 from limited Binodoxys communis releases made in 2007. Several states are planning to release Binodoxys communis near buckthorn (Rhamnus cathartica), the overwintering host used by soybean aphid, to try to enhance the parasitoid’s survival chances.

It appears that these parasitoids complement existing natural enemies. In Minnesota, the interaction between B. communis and resident aphid predators was assessed by comparing aphid densities either in the presence of resident predators alone (primarily lady beetles), resident predators with Binodoxys or cages with only B. communis. The results showed that the lowest aphid densities were found in the treatment combining resident predators and B. communis.

Read more about the importation project and soybean aphid biological control at the Soybean Aphid Biological Control website»