Classical Biological Control

    Is the practice of importing, and releasing for establishment, natural enemies to control an introduced (exotic) species.

     Biological control is especially effective when a weed (pest) has been accidentally introduced into a new geographic area without its natural enemies.  These introduced species are referred to as exotics or weed species.  To obtain the needed natural enemies (biocontrol agents) of the weed, we turn to the weeds native range.  The first step in the process is to determine the origin of the introduced weed and then collect their naturally occurring enemies from their native range.  The natural enemy is then passed through a quarantine process to ensure that it will not harm native or crop species. It is then reared and released onto the pest species as a biocontrol agent.  After release, follow-up studies are conducted to determine if the biocontrol agent was successfully established at the site of release and to observe its long-term benefits/effects.  The goal is to lower the density of the weed species below the economic damage threshold (in other words, to make the pest species a controlled, natural part [small part] of the ecosystem).
    Classical biological control is long lasting and in the long term is inexpensive. Other than the original costs of the collection, importation, quarantine, testing, rearing, and releasing, little expense is included. When a biocontrol agent is successfully established it rarely requires additional care and it continues to kill the weed (pest) with no direct help from humans and at no cost. It often spreads to new areas all by itself.  Classical biological control at its best will never completely wipe out the pest species rather, it will make the exotic a small controlled part of the ecosystem.
    Unfortunately, attempts at classical biological control do not always work. It is usually most effective against exotic pests. The reasons for failure are many, but may include the release of too few individuals, poor biocontrol agent adaptation to the local environmental conditions at the release site, disturbance or predation by other local species, and disease.

(1) Cornell University., Classical Biological Control, [Website]

(2) Todd Breitenfeldt, (personal interview) Whitehall Schools science teacher, E-mail:, S-mail:, Whitehall Schools, P.O. Box 1109, Whitehall, MT 59759.

By: Shawna Smith

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