Many of our most severe agricultural insect and weed pests are native to Europe, Asia or Latin America, where their populations often are held in check by a variety of natural enemies. The "classical" approach to biological control involves introduction of exotic, nonindigenous natural enemies to control pests that are not native to the area in which they have become established. Classical biological control seeks to reconstruct agricultural (or other) ecosystems by assembling a complex of natural enemies in a pest's new home in order to reduce the pest population below damaging levels. There has been outstanding although infrequent success with classical biological control, usually involving establishment of parasitoids and predators that are highly specific in their preference for feeding upon the target pest. Control of the alfalfa weevil and the cereal leaf beetle by respective specific parasitoid complexes are recent examples from the Midwest.
Recently, some proponents of biological control have suggested that this impact might be even more effective if we establish natural enemies that feed on species related to the pest, not necessarily just on the pest in question. The suggestion is that in their native homeland, ecosystems are "co-evolved"; that is, pests will have developed some natural resistance to their specific natural enemies, whereas in a "new association," pests might be less resistant, and therefore mortality due to the natural enemies may be higher. This is sometimes called the "neoclassical" or "new associations" approach to biological control. It implies that effective biocontrol agents will have impacts beyond that on the immediate target species on which they are expected to feed, because they already feed on nontarget species.
Introduction of nonindigenous species amounts to artificially assembling components of ecosystems in the expectation that specific populations (of pests, we hope) will come under more efficient control. No management practice is entirely without impact, and therefore none is entirely without risk. Recently, practitioners of biological control have come under criticism because of potential impacts of nonindigenous natural enemies on "nontarget" species in both target (agricultural) and nontarget ecosystems. While some of this concern may be overstated, it is worthwhile that biological control practitioners at least consider this concern and evaluate it accordingly.
Indeed, there are some horror stories regarding impact of nonindigenous natural enemies. Most pertain to generalist vertebrate predators such as mongooses eating birds in Hawaii and other Pacific and Caribbean Islands, or predatory snails driving other snails to extinction, again in places like Hawaii or Tahiti. Among insect natural enemies, impact is often a small part of greater ecosystem disruption and is therefore harder to document. The greatest negative impact of introduced arthropods may have occurred on islands, where, not coincidentally, classical biological control has resulted in some resounding successes. Francis Howarth (University of Hawaii) is probably the most vocal critic of classical biological control, and he reports that fifteen species of native Hawaiian moths are threatened with extinction as a consequence of introduced tachinid flies, whose larvae parasitize the caterpillars. Five of the 20 butterfly species native to Guam have become extinct since 1945, after introduction of several species of Hymenoptera for control of sugarcane pests. The actual direct impacts of these biological control programs are difficult to evaluate owing to the wholesale general environmental changes due to expanding human populations, agriculture, and overall economic (including military) activity on these and other tropical islands.
It is instructive to look at a few examples closer to our region. The seven-spotted lady beetle (Coccinella septempunctata), nicknamed "C7" is native to Europe (where it is the original "Ladybug, Ladybug" of nursery rhyme fame). For many years the USDA tried without success to establish C7 in North America for biological control of aphids in agricultural crops. The beetles apparently became established on their own in Canada about 1966 and have spread, aided by federal and state personnel, throughout most of the U.S. including all of the Midwest. In many places they have become the most common lady beetle not only in agricultural crops but in abandoned fields, wetlands, and prairie remnants. In Ohio, I have seen a decline in the numbers of native lady beetles in these "nontarget" habitats following the establishment of C7, and this trend has been noted in other states as well. Of particular interest is the apparent disappearance of the closely related native 9-spotted lady beetle (C. novemnotata) which was once common in the eastern U.S. and has not been seen in many states since the mid-1980's. It is not clear that C7 is contributing directly to the decline of native species, although laboratory studies indicate that it is a superior competitor, and from a biocontrol standpoint, more aphids are probably being eaten than ever before. However, C7, like many lady beetles, eats other things too, including eggs and young larvae of Lepidoptera, some of which may be threatened or endangered. The federally-endangered Karner blue butterfly, for instance, disappeared from Ohio in 1988, about when C7 became the most abundant lady beetle in the butterfly's habitat. The butterfly was declining due to habitat alteration long before C7 arrived on the scene, but the chance that C7 was a factor in the demise of the tiny remnant population should be acknowledged.
A more recent arrival is the Multicolored Asian Lady Beetle (Harmonia axyridis), whose introduction was recommended for control of aphids on pecans. Establishment occurred (either planned or accidental) in Louisiana in the mid-1980's. Its populations have expanded explosively; we first noted it in Ohio in 1992 and by 1995 it had become by far the most abundant lady beetle in orchards and on forest trees. No doubt it is consuming billions of aphids, but we have no idea of its overall impact on forest ecosystem function. Moreover, it has an undesirable habit of forming huge overwintering aggregations on or in manmade structures, not only homes and cabins, but urban high-rise office buildings, where the beetles get into file drawers, computers, and vending machines. All this increases the level of irritation among people who pay taxes to support biological control programs and ask just who is responsible for introducing these beetles?
The gypsy moth is a cyclic defoliator of forest and ornamental trees, originally from Europe, that has slowly spread from an initial infestation in Massachusetts to Wisconsin, Michigan, and Ohio. More than 50 species of natural enemies of the gypsy moth have been introduced into the USA without notable long-term impact on the moth's numbers. One of these natural enemies is a tachinid fly, Compsilura concinnata, which parasitizes more than 200 species of butterfly and moth caterpillars. The fly was introduced to New England early in this century and quickly spread from sea to shining sea; in 1970 I found that it was the most frequent tachinid parasitoid of tent caterpillars in oak woods of coastal California. Researchers in Michigan have found that C. conccinata's populations increase greatly immediately after gypsy moth outbreaks, just when gypsy moth numbers decline (usually due to a virus). This leaves thousands of C. concinnata present in the forest, looking for caterpillars to parasitize. The nontarget hosts of the parasitoid include larvae of the large and showy giant silk moths (family Saturniidae). These moths (luna, io, cecropia, polyphemus, etc.) are favorites with collectors and the general public. Several species of Saturniidae have declined in areas where they were formerly abundant, and although much of this decline may result from a host of largely unknown ecological factors (like an increase in outdoor lighting), parasitism by C. concinnata may be partly responsible.
Although outside our region, a historic case of classical biological control, mentioned in all the lectures and textbooks, is the introduction of the moth Cactoblastis cactorum from South America to Australia to control prickly pear cactus. (Cacti are not native to Australia but do very well on livestock grazing lands in the semiarid climate.) Cactoblastis is a very efficient eater of prickly pear, which is great news for Australian stockmen. However, Cactoblastis has recently become established in southern Florida, probably resulting from an accidental introduction, and is eating its way through the last remaining stands of an endangered native prickly pear species. Once again, the native cactus initially became rare due to agricultural and real estate development, and Cactoblastis is a late arrival on the scene, but the tale is of interest to those of us in the Midwest where a vigorous biological control program is underway using European leaf beetles and weevils to control the wetland weed purple loosestrife. Native relatives of purple loosestrife live in restricted wetlands and could be impacted by the nonindigenous beetles.
Whatever manipulation we make to natural or managed ecosystems has an effect. Critics of classical biological control argue that once a biocontrol agent is released into the environment and becomes established, it is virtually impossible to remove the natural enemy if a negative impact is noted. Supporters counter that biological control impacts have to be compared to impacts of continued use of other techniques such as broad-spectrum chemical insecticides, rather than to a policy of doing nothing. The impact of addition of another insect to the fauna in no way compares to the impact of habitat alteration that accompanies clearing land and preparing soil for agriculture, nor to that of diverting agricultural and natural lands to housing, freeways and shopping malls. Although major detrimental impacts from classical biological control using natural enemies have not occurred, biological control practitioners should cautiously acknowledge that we need to be careful and proceed slowly when introducing species into managed, artificial ecosystems whose interactions we do not yet understand very well.
- David J. Horn, The Ohio State University
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