Biological control is an ecological approach to pest management that uses living organisms, known as natural enemies, to regulate populations of pests, diseases, and weeds. This method relies on predators, parasitoids, or pathogens to reduce pest numbers, offering a sustainable alternative to traditional chemical treatments. Unlike broad-spectrum chemical pesticides, biological control agents are highly targeted and minimize harm to the wider ecosystem. The strategy aims for long-term population suppression below economically damaging levels, rather than eradication.
Classical Control Establishing Permanent Solutions
Classical biological control focuses on achieving a permanent, self-sustaining reduction in an invasive pest population. This method involves the deliberate importation and release of a natural enemy from the pest’s native geographic range into the new area where it has become a problem. The goal is the long-term establishment of the beneficial organism, which will then reproduce and provide continuous control without repeated human intervention.
A historical example is the introduction of the vedalia beetle (Rodolia cardinalis) from Australia to California in the late 1800s to control the cottony cushion scale, a pest devastating the citrus industry. Before release, any new species must undergo rigorous testing in quarantine facilities to ensure it is highly specific and will not attack non-target native species. This testing is crucial to prevent unintended negative impacts on local biodiversity. Once established, the introduced natural enemy maintains the pest population at a low, stable level indefinitely.
Augmentative Control Utilizing Mass Releases
Augmentative control involves the mass production and timely release of natural enemies to provide immediate or seasonal pest suppression. This approach does not rely on the permanent establishment of the agent, but rather on overwhelming the pest population with sheer numbers. The agents used are often commercially reared insects, mites, or microbial organisms like bacteria and fungi.
This method is further divided into two primary strategies based on the number and timing of the releases. Inundative releases involve releasing extremely large numbers of agents to achieve a rapid, temporary reduction in the pest population, essentially acting as a “living pesticide.” For instance, millions of Trichogramma parasitic wasps may be released to attack moth eggs, or large quantities of the bacterium Bacillus thuringiensis (Bt) may be applied to control caterpillars. The control is delivered by the released individuals themselves, and the effect is often short-lived.
In contrast, Inoculative releases involve releasing smaller numbers of natural enemies early in the season or before pest pressure is high. The agents are expected to reproduce and build up their population over time, with their offspring providing control for the remainder of the growing season. This strategy is common in controlled environments like greenhouses, where predatory mites such as Phytoseiulus persimilis are periodically released to manage spider mites, or the mealybug destroyer lady beetle (Cryptolaemus montrouzieri) is released to manage mealybugs. This offers a more extended period of protection than inundative releases.
Conservation Control Supporting Existing Ecosystems
Conservation biological control focuses on modifying the environment to protect and enhance the effectiveness of natural enemies already present in the area. This approach recognizes that existing predators, parasitoids, and pathogens are often the most adapted control agents, but their activity can be limited by human practices or a lack of necessary resources. Tactics center on improving the habitat and reducing factors that interfere with the beneficial organisms’ survival.
One practice involves providing refuge and overwintering sites, such as establishing hedgerows, perennial cover crops, or “beetle banks” within or adjacent to a field. These structures offer shelter from harsh weather and undisturbed habitat for natural enemies during periods when pests are scarce. Modifying the farm landscape also includes providing alternative food sources for beneficial insects. Many adult parasitoid wasps and predators require pollen or nectar to survive and reproduce when their prey is not abundant.
This can be achieved by planting insectary strips—flowering plants that supply these resources throughout the season. The selection and timing of pesticide use are also modified to be less harmful to beneficial species, often by using selective compounds or avoiding broad-spectrum sprays. By changing cultural practices, this method sustains the native natural enemy complex, allowing them to exert more consistent and effective control over pest populations.