Biological Control of Invasive Species
Richard A. Casagrande, Professor
Department of Plant Sciences
Richard Casagrande earned a BS in entomology from Rutgers University and an
MS and a PhD in entomology from Michigan State University. He has been at URI
since 1976. In 1996 he was honored as the Research Scientist of the Year in
the College of Resource Development.
Non-native plants and animals that are acci- dentally introduced into North America typically arrive without the natural enemies that keep them in check in their native habitats. Freed from natural controls, these aliens often reproduce and spread with abandon. In many cases, the problems that they cause can be solved by reacquainting these "pests" with their natural enemies in a deliberate process called Classical Biological Control.
The University of Rhode Island's Biological Control Program, which is entering its third decade, has been a leader in Classical Biological Control. Our program received a major boost six years ago with the construction of our USDA-approved primary quarantine laboratory, the only such university-based lab in the Northeast. Programs underway include research on the birch leafminer, hemlock woolly adelgid, lily leaf beetle, purple loosestrife, Cypress spurge, and common reed (Phragmites australis). Although these target plants and insects are quite diverse, they have several characteristics in common. All, with the possible exception of common reed, are non-native. We generally do not consider introducing exotic natural enemies against native plants and insects, no matter how much we may dislike them. (Poison ivy and greenbriar come to mind.) Native species have evolved in our ecosystems and may play important, if unappreciated and little understood roles, now or in the future. Besides, there is no shortage of exotic pests to work on. There are about 1,700 introduced insects in this country and more than half of them are identified as pests. The 300 species of invasive plants in North America have already infested 100 million acres and take over another three million acres per year (about 4.5 times the land area of Rhode Island). Combined with the exotic invasive animals (such as zebra mussels), these invasive species cause roughly $123 billion in damages annually in the United States.
Work on biological control generally begins in the homeland of the invasive pest. When we don't know where something came from, we look to that part of the world with the greatest concentration of closely related species and/or many natural enemies. Purple loosestrife is a good example. This attractive purple-flowered plant was accidentally introduced into the United States in ship's ballast two centuries ago. Subsequently, it has been purposely introduced and distributed as a garden plant. With no insect natural enemies, loosestrife has spread throughout much of North America's wetlands, displacing native plants and animals in the process. This plant is widespread but relatively uncommon throughout its European homeland where 120 species of insects feed on it.
Once a program is initiated, it takes several years of careful research before a biocontrol agent is introduced against an invasive plant. Again, looking at loosestrife, literature and field surveys begun in Europe in 1987 showed that 14 of the native insect herbivores appeared to be specific to purple loosestrife. This list was further narrowed to six that were considered particularly useful for biological control. These candidates were then subjected to a battery of host specificity tests on 50 European plant species that were either closely related to purple loosestrife or associated with it in wetlands. After proving to be highly (if not totally) specific to purple loosestrife, these insects were taken into quarantine in the United States where they were tested on some North American plants that don't exist in Europe, and then some of our plants were taken to Europe for additional open field tests. When it was ascertained that the insects would reproduce only on purple loosestrife, the USDA gave permits for their release in 1992. URI became involved in this program in 1996 when we obtained some of the leaf-feeding beetles from collaborators in Canada and subsequently released them at the Roger Williams Park Zoo. In the past two summers, the effect of this biological control effort has become apparent. Loosestrife has been totally defoliated for the past two seasons and native vegetation is once again gaining a foothold in the wetland area (see fig. 3). With releases in four other locations throughout Rhode Island, we are expecting a substantial decline in loosestrife populations in the state within a few years.
We have similar hope for our efforts against the lily leaf beetle. Most Rhode Islanders are not yet familiar with this scarlet red beetle which was accidentally introduced into Boston about 1991. But this beetle has spread constantly since its introduction, reaching Cumberland, Rhode Island this year, and it is now found in all of the New England states. Its larvae carry their fecal material on their backs, an effective defense against predators, which also makes them repulsive to gardeners. Adults and larvae feed on all species of cultivated lilies and defoliate natural populations of native lilies in New England. When we realized that this insect came from Europe where it is not a pest, we began exploration there in 1997. To date, we have found four species of parasitic wasps that apparently control the beetle in its native range. One of these wasps (Tetrastichus setifer) is found in large numbers throughout Europe, and we feel it is an excellent candidate to control the beetle. We subjected it to host specificity testing in our quarantine lab against the North American beetles that are most closely related to the lily leaf beetle and a few more distant relatives such as lady beetles. When these tests showed the wasp to be specific to this beetle in North America, the USDA gave us permission to release the wasp in Boston last summer. We are conducting similar host specificity tests in our lab on the other three species and planning some competition tests to determine which, if any, additional species should be released against this pest. If we are successful, populations of the lily leaf beetle should decline a few years after parasitoid releases are made. Common reed is another plant that we are evaluating as a candidate for biological control. However, in this case, it is unclear whether the plant is native or exotic. Based upon its recent explosive spread through wetlands in the Northeast, many researchers believe it is a recent introduction. On the other hand, it has been in North America for a long time; at least several thousand years in Connecticut and perhaps 40,000 years in Arizona (apparently a native). But there are only three native North American insects that feed on this plant, compared to the 128 species that we know to feed on it in Europe, about half of which feed only on this plant. This leads us to believe that the plant originated in Europe. Further evidence is provided by the observation that common reed is not a problem in Europe. To the contrary, populations are declining and Europeans are actually managing for this plant. While colleagues at Yale work to determine the origin of our plants and colleagues in Europe identify potential biocontrol agents, we have surveyed the insects feeding on reed. We were surprised to find that we now have 21 insect species feeding on the reed in the Northeast United States, most of which are recent introductions. Programs in Classical Biological Control have been conducted in the United States for more than a century and with proper safeguards, have provided a safe, economical, and permanent means of reducing pest problems. However, it is important to note that they are not without serious constraints. The permanence that makes biological control so attractive also makes possible a permanent mistake. Once introduced, an exotic natural enemy is probably here to stay. It must be expected to spread throughout its favorable range, which might include much of North America. Pesticides pose many serious environmental problems, but even the worst of them have a half-life of only several years, and they don't reproduce in nature. Thus we must thoroughly examine and debate the risks and benefits of a biological control agent before its release. USDA regulations require this analysis by professionals in biological control to provide adequate safeguards against mistakes. Our best option is to keep exotic invasive species out of the United States, but we have a large backlog of existing problems and accidental introductions continue, apparently at an accelerating rate. With continued vigilance, Classical Biological Control will provide an effective approach to managing these invasive species.