FEATURE ARTICLE

Ecological Consequences of Importing Natural Enemies

"Classical" biological control is an approach that involves the collection (usually in foreign localities), importation, release, and establishment of natural enemies in new areas where exotic pests have invaded. The goal of this approach is to add a permanent source of pest suppression to the new environment by using natural enemies that are well-adapted to the pest and, hopefully, capable of reducing and maintaining its populations at low levels. During the past century, hundreds of species of predators, parasites, pathogens and herbivores have been imported and released in the United States as prospective biological control agents of scores of exotic pests. Most of these pest invaders have been insects, mites, and weeds. Many notable successes have resulted in which the target pest has been brought under substantial or complete control. However, as with any form of pest management, some ecological consequences must be borne when using classical biological control. These consequences fall into two categories: (1) those that involve the target pest; and (2) those that affect nontarget organisms (that is, all species other than the intended pest).

With respect to the target pest, the most immediate ecological consequence is the failure to establish the natural enemy in the new environment. A review of the historical records indicates that only about a third of the released biological control agents have become established. Various reasons have been given for the low rate of success in colonizing and establishing natural enemies. These include both genetic and environmental factors. Unfortunately, the particular factors responsible for the failed introduction of any given species of natural enemy is difficult to discern. However, even without this specific knowledge, decisions can be made to increase the likelihood for successful establishment. For example, the probability of establishing a new biological control agent tends to be greater when larger numbers of natural enemies are released per unit time. This trend may simply have to do with having sufficient population numbers to carry over to the next generation or season. It may also guarantee that the full spectrum of a natural enemy's genetic traits are represented which, in turn, ensures maximum flexibility for adapting to the new environment. To some degree, it may also be possible to identify environmental conditions that are the most favorable and least favorable for a natural enemy. Adverse conditions can sometimes be avoided, either by altering the environment or by choosing a natural enemy that is well-suited to the new environment. An example of the latter is climatic matching. Here, a population or species of natural enemy is selected from a location that has climatic conditions that match closely with those where the release is to be made. In general, if greater emphasis were placed on the discovery of factors that promote and inhibit natural enemies, and if measures were taken to implement programs to protect natural enemies before importation and during the time of their release, it may be possible to increase the establishment rates.

Even after new natural enemies become established, the success of a biological control program is not guaranteed. In fact, the impact of introduced biological control agents ranges from negligible to highly significant. When the level of biological control is not high enough, it may be possible to alter the environment to enhance a natural enemy's population performance and, thus, its ability to suppress pests. On the other hand, some problems cannot be overcome by manipulating the environment. For example, if an imported natural enemy interferes with one that already is established, or if two or more imported species establish but do not interact well, there are few changes that can be made to improve the situation. Therefore, which species of natural enemy we choose to import may have major ecological consequences for the long-term effectiveness of biological control programs. Other common problems that can be best avoided by natural enemy selection are seasonal asynchrony with the target pest, and known or suspected vulnerability to heavy predation or parasitism by natural enemies that are already present in the new environment. Careful evaluation of the species to be imported can eliminate candidates that are likely to be ineffective, thus preventing a potentially adverse ecological consequence. However, there is considerable disagreement among biological control specialists over the wisdom of attempting to select "best candidate" natural enemies. Although it may be possible to direct the importation of natural enemies to avoid obvious pitfalls, the complexity of most ecosystems makes it difficult to anticipate the level of pest suppression that will result after a new assemblage of natural enemies has been created.

An ever-present ecological consequence is unintended feeding by the imported natural enemy on economically important plant and animal species. For the most part, this undesirable occurrence has been avoided by evaluating candidate biological control agents for their potential to feed and develop on economically-targeted species based on historical records of host or prey affinities, and by direct tests of species that are somewhat to closely related to the target pest. The most rigorous application of this screening process has been directed towards the protection of food, fiber, and ornamental crops commonly used by humans. Hence, biological control programs for exotic weeds have received the most attention. However, the ecological consequences of imported natural enemies for nontarget organisms go beyond well-known, high-value species. Conservation biologists, whose mission is to preserve native plants and animals, view the importation of natural enemies as a threat to native species. Some reasons for their concern are (1) screening protocols in weed biological control programs historically have not included native plant species; (2) the potential impact of releasing imported natural enemies on native arthropods has not been considered; (3) decisions to approve the release of some natural enemies involve an unacceptably high level of risk to nontarget species; and (4) conservationists have little or no formal input in the decision-making process.

Those who specialize in biological control counter that it is infeasible to test all possible native species that are closely to somewhat closely related to the target pest. There also is disagreement about what level of risk is acceptable, and how to weigh the need to solve a pest problem against potential risks to native species when making decisions regarding importation and releases. Finally, biological control workers cite that failing to use classical biological control as a tool to manage a widespread exotic insect pest or weed poses a greater risk to native species than that of releasing an imported natural enemy-in effect, an ecological consequence of not importing a natural enemy.

In turn, conservationists have called for a broader evaluation of pest management options before decisions are made to import a new natural enemy. This challenge has considerable merit, but it is only one step towards resolving existing conflicts. There is accumulating evidence that previously imported biological control agents are having an adverse impact on indigenous plants and animals. The extent to which this impact threatens the survival of the affected species is debatable. However, some species are clearly at high risk because of biological or environmental constraints. To better protect all interests, changes should be made in the criteria used to set priorities for classical biological control, to screen prospective natural enemies, and to evaluate the long-term ecological consequences of importing natural enemies.

- James Nechols, Kansas State University


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