FEATURE ARTICLE

Classical Biological Control:
Importation of New Natural Enemies

Editor's note: This is the first of a series of three articles introducing the three main approaches to insect biological control.

Many insects are serious pests because they are not native to our area; they were accidentally introduced through commerce or the transport of personal belongings. Many of these pests were introduced from overseas with early settlers who unwittingly brought infested foodstuffs or even plant material destined to start new crops in the New World. Today we have modern quarantine laws intended to eliminate the introduction of new pests, but even now serious new pests, such as the Russian wheat aphid, can find their way into the United States, become established, and cause damage.

When an alien pest is accidentally introduced and established in a new area, it is usually without the complex of natural enemies that controls it in its native location. It is generally considered that some of the most effective natural enemies of an organism are those that have coevolved with it in its native habitat. Therefore, some of the most dramatic successes in biological control have resulted from the importation of natural enemies from other countries, a practice often called classical biological control. The first major successful example of this method occurred over 100 years ago and involved the control of cottony cushion scale, a serious pest of the California citrus industry.

The goal of classical biological control is to find useful natural enemies, introduce them into the area of the target pest, and permanently establish them so that they will provide continuing pest control with little or no additional human intervention. The search for natural enemies in other countries is often referred to as foreign exploration.

Classical biological control differs from the other two general methods (conservation of natural enemies and augmentation of natural enemies, highlighted in future issues of MBCN) because it is not directly conducted by the farmer or gardener. International agencies, federal agencies (especially the United States Department of Agriculture), and state agencies (state departments of agriculture and the Land Grant universities) are responsible for identifying potential target pests, locating their natural distributions, searching these areas for candidate natural enemies, and introducing selected natural enemies into the necessary areas. Indeed, there are specific quarantine laws that prohibit private individuals or agencies from introducing alien organisms (including natural enemies) without proper authorization from the USDA. Natural enemies must be carefully screened by trained personnel under rigid quarantine conditions to be certain that (1) they will provide benefit in controlling the target pest, (2) they will not, themselves, become pests, and (3) they do not harbor their own natural enemies that might interfere with their effectiveness.

Many of the successes in classical biological control have occurred in tropical and subtropical locations. California, Hawaii, Texas, and Florida have all achieved significant successes by the introduction of exotic (foreign) natural enemies. Each of these states utilize state revenues to support biological control efforts. Historically, there has been little state support for classical biological control in the Midwest, and the few significant cases of successful biological control in the region have resulted primarily from USDA programs. There are two major benefits to state involvement in, and regional coordination of classical biological control attempts: (1) optimum environmental range of the natural enemies, and (2) control of minor and localized pests.

Optimum Environmental Range of Natural Enemies. All organisms are capable of surviving within a certain broad range of environmental conditions that includes food availability and quality, climatic conditions, and presence of natural enemies and competitors. However, each organism has a much narrower optimum range of environmental conditions within which the species thrives. This holds true for natural enemies. Because of the time and expense involved, and the limited resources available for foreign exploration, often only a few types of natural enemies from limited geographical areas are introduced and established. In a country of the size, geographic diversity, and agricultural complexity of the United States, an introduced natural enemy species will encounter both optimum and sub-optimum areas for survival. In suboptimum areas, only limited or partial control of the target pest will result. In particular, it is important to have a close climatic match between the target release area and the source of the natural enemies. State and regional agencies are more likely to identify climatically appropriate exploration areas, based on specific local needs, than are federal or international agencies.

Control of Minor and Localized Pests. Most USDA foreign exploration programs are for natural enemies of major pests with widespread distribution. However, significant pest problems may occur on minor crops or in limited geographical areas. Such problems must be addressed by state or regional efforts.

There has been increasing interest in biological control by state departments of agriculture and Land Grant universities in the North Central United States. It is hoped that there will be increasing state (as well as federal) support for introduction of new exotic natural enemies specific for pests in this area.

There are over 75 alien pests of crops, livestock, human health, forests, and landscape here in the upper Midwest; many of these are candidates for biological control. Some of the partial and substantial successes of classical biological control in the Midwest are presented in the following table.

Successful cases of exotic natural enemy establishment
in the North Central United States
Pest Crop Established Natural Enemies Source
alfalfa blotch leafminer alfalfa Chrysocharis punctifacies Europe
    Dacnusa dryas Europe
alfalfa weevil alfalfa Bathyplectes curculionis Italy
    Bathyplectes anurus Europe
    Microctonus aethiopoides France
    Microctonus colesi uncertain
    Anaphes luna uncertain
cereal leaf beetle grain crops Anaphes flavipes W. Europe
    Tetrastichus julis Europe
clover leaf weevil clover and alfalfa Biolysia tristis Italy
elm leaf beetle elm Tetrastichus gallerucae Europe
euonymus scale euonymus Chilocorus kuwanae Korea
European corn borer corn Macrocentrus grandii Europe, Asia
    Eriborus terebrans Europe, Asia
gypsy moth trees Over 10 species Europe, Japan
imported cabbageworm crucifers Cotesia glomerata Europe
Japanese beetle turf grass Tiphia vernalis Korea, China
larch casebearer larch Agathis pumila England
    Chrysocharis laricinellae England
Nantucket pine tip moth pines Campoplex frustranae Virginia
oriental fruit moth stone/pome fruits Macrocentrus ancylivorus New Jersey

Although farmers and gardeners are not directly involved in the classical biological control process, they need to be involved in the manipulation of the exotic natural enemies that become established. Recognition of these natural enemies, understanding their benefits, and knowledge of their use in an overall pest management program are important considerations in both the conservation and augmentation of natural enemies, the subjects of future articles.

Adapted from

Mahr, D. L., and N. M. Ridgway. 1993. Biological control of insects and mites: An introduction to beneficial natural enemies and their use in pest management. N. Central Reg. Ext. Publ. 481.


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