Soybean Aphids and the Search for Natural Enemies
The soybean aphid, Aphis glycines, was discovered in northern Illinois and southern Wisconsin in late July 2000. By September its presence had been documented in 11 states. This year, 2001, was the first opportunity to observe this species throughout an entire growing season and there have been many surprises. One of the first surprises was the discovery of large populations on the perimeter of where the aphid had been relatively abundant in 2000. In New York and Pennsylvania and the province of Ontario, where the aphid had not been recorded, early spring populations appeared, as they did in many areas of Minnesota, Wisconsin, and Michigan where it had been found but had not been abundant in 2000. In most of these areas, high populations developed and were sprayed with insecticides. By September it was known from southeastern North Dakota to one county in Virginia. It is clear that this aphid is here to stay and that it has the potential to become a serious pest of soybeans.
When exotic organisms, like the soybean aphid, arrive in North America their populations may explode and they can become pests. Often this is because the natural enemies that kept populations under control in their native habitats are absent in their new home. One way of controlling exotic organisms is to search for these natural enemies in the native habitats and, after proper testing to insure these natural enemies will not also become pests, introduce them in our infested habitats. This is called classical biological control.
In July, one group of scientists from the University of Minnesota went to China and Bob O'Neil of Purdue and David Voegtlin of INHS went to Japan in search of natural enemies of the soybean aphid. The focus of these trips was to find small wasps that parasitize aphids. These tiny wasps, called parasitoids, kill aphids by laying an egg into the body of an aphid, the egg hatching into a larva that feeds on the inside of the aphid, eventually killing it. The mature larva spins a cocoon inside the aphid and turns into a pupa that emerges later as another adult wasp, one wasp from one aphid. When the parasitoid larva spins a cocoon inside the aphid body, the aphid takes on a puffed-up appearance and is called a mummy.
In Japan, collections were made by myself and Bob O'Neil in both cultivated (Glycine max) and wild (G. soja) soybeans at approximately 60 sites in the northern part of Honshu. Field size varied from plots with dozens of plants (e.g., backyard gardens containing soybean plants) to 50-ha fields (most fields this size were converted from rice paddies). Small blue/black mummies (aphelinids) were the most common parasitoid found in both wild and cultivated soybeans. Parasitoids were found in all sizes of fields and at both low and high aphid densities.
In China, scientists from the University of Minnesota made collections in Harbin, Changchun, and Beijing (eastern China). Soybean field size in China varied but fields were generally larger than those in Japan. Only cultivated soybeans were sampled for aphids, and only aphidiine parasitoids were observed and collected. These mummies have the color and texture of a brown paper bag. As in Japan these mummies were found in aphid colonies of all sizes.
Mummies from both Japan and China were brought back to a U.S. Department of Agriculture quarantine facility at Newark, Delaware. At present, the species from Japan, whose scientific name is Aphelinus albopodus, is being successfully cultured in quarantine. Before this parasite will be approved for release against the soybean aphid, it will have to undergo tests to demonstrate its host range for us to be certain the parasite will not itself become a problem.
Further trips are being planned to continue the search for natural enemies of the soybean aphid in the Asian region.