Nature of Wild - Using Biological Controls a Tricky Business 

Nationwide, more than $120 billion is spent annually trying to control about 800 exotic species or in repairing the damage they cause. Invasive, exotic plants and animals are spreading throughout the United States, displacing native species and devaluing our lands and waters. At least 183 non-native species have already been discovered in the Great Lakes and, on average, about two more show up in the lakes every year.

Facing a problem of such magnitude, resource managers and researchers are looking intensely at a high-risk solution: biological control. Biological control refers to the use of animals, fungi, or microbial diseases to knock down numbers or slow the spread of unwanted species. Over time, it can be less expensive than direct control and can be less harmful to the environment. But it requires a lot of front-end research to make it work. Scientists usually have to turn to the native land of the invasive species to find and study such potential control agents. The risk inherent in this approach is that the controlling species becomes an exotic species when it is introduced. Often, years of research are necessary to ensure that the control species will not, in turn, create additional problems.

The use of biological controls in the U.S. has an interesting history rife with failures. Efforts to control Canada thistle and spotted knapweed by introducing certain insects have not really panned out, and there have been cases in which the control agents ate native plant species. But in Michigan, proponents of biological controls are enthused after recent success in the fight against purple loosestrife and early optimism in the struggle with the emerald ash borer.  

The emerald ash borer is perhaps the best-known of the insects plaguing Michigan woods. It has spread over the past decade from an initial infestation in the Detroit area to more than 40 counties in both the Lower and Upper Peninsulas. Officials say it has wiped out about 4 percent of Michigan’s ash trees including all of the ashes in certain large areas like Southeast Michigan.

One solution scientists in the U.S. Department of Agriculture are banking on is a parasitic wasp from China that affects only the emerald ash borer. About the size of a small seed, the stingerless wasps lay eggs in young ash borer larvae or in ash borer eggs, killing the borers before they morph into beetles. It takes about three weeks for the wasps to emerge from the hosts and be ready to repeat the cycle. A few thousand were released this summer and the results were encouraging.

These wasps appear to be an ideal control agent. They are highly specific to emerald ash borers, don’t pose a threat to humans, and don’t over-winter in Michigan’s cold climate, so the potential for unforeseen ecological impacts is slight. Officials are expected to step up releases over the next few years. They will also continue to evaluate the use of a naturally-occurring fungus—Beauveria basslana—that kills the emerald ash borer.          

The task of controlling purple loosestrife—an exotic plant that invades marshes and streambanks—seemed hopeless until a tiny brown beetle was brought into the fight. Galerucella beetles, which were first introduced into Michigan in 1994, have suppressed purple loosestrife throughout Southern Michigan. 

The beetles chomp on the plant’s tall flowering stalks, sometimes nearly eliminating seed production. Control is usually achieved within three to five years after the beetles are introduced into a dense purple loosestrife stand. Then native vegetation better competes with the defoliated loosestrife, preventing it from coming back.

Dr. Doug Landis, professor of entomology at Michigan State University, points to the Crow Island State Game Area as one of the beetle’s triumphs. “There were hundreds of acres of purple loosestrife in the marsh there before we introduced the beetles. Now, there are very few flowering loosestrife plants within several miles of those release sites,” said Dr. Landis. He emphasizes that biological control agents such as the Galerucella beetle don’t completely eradicate their target plants. Otherwise, they would doom themselves. But they are very effective in reducing the negative impacts of the exotic loosestrife, which is the essence of control. Similarly, there are proven, or at least promising, controls available for other exotic plants such as buckthorn, multiflora rose, garlic mustard, leafy spurge and Eurasian water milfoil. A variety of insects can help keep these plants in check.

Biological control sometimes requires a multi-pronged attack. That’s been the case with the gypsy moth, one of the biggest headaches of forest managers and homeowners with wooded lots.

The gypsy moth is a European and Asian insect brought to Massachusetts from Southern France. By the early 1900s it had defoliated nearly two million acres of hardwood forests and was spreading westward. Scientists responded by bringing in nine different parasitic wasps and flies from Japan and Russia as well as Western Europe. All preyed on the gypsy moth, but the insect nevertheless continued to spread. In the 1960s researchers with the U.S. Forest Service began isolating a virus disease. They later identified other promising viral strains, and eventually developed Gypches, a virus formulation for controlling gypsy moth. Today, new microbial materials including genetically-engineered bacteria are being employed or considered for use against gypsy moths and other major forest insect pests. Bacillus thuringienis or Bt is now commercially available for gypsy moth control.

Biological controls have limitations. Pacific salmon were introduced to the Great Lakes to eat the exotic alewife, but most of the invertebrates and fishes that have invaded the Great Lakes don’t have enemies that we dare import. Research on biological controls is expensive and highly regulated. It takes years of laboratory and small-scale field trials in carefully controlled settings before approval is given for actual applications. But the high threat of exotic species make this high-risk control method worth continued research.

Dr. Patrick J. Rusz
Director of Wildlife Programs

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