HPIPM:Biological Types of Biological Control
Author: David J. Kazmer & Michael J. Brewer
Types of Biological Controls
It is important to first distinguish between "natural" and "applied" biocontrol. Natural biocontrol (or fortuitous biocontrol) is the reduction of a species' populations by natural enemies with no manipulation of the natural enemies by man. In contrast, applied biocontrol is the reduction of a species' populations by natural enemies when man manipulates the natural enemy populations for the purpose of population control.
Although we are typically concerned with applied biological control in pest management, two important features of natural biocontrol should be understood. First, natural biocontrol is an extremely common phenomenon, both in natural and agricultural ecosystems. There are more than 300,000 plant species and 1,000,000 insect species. Very few of these species dominate their habitats or are pests, so it is clear that most populations of most species are suppressed most of the time. A variety of biological and environmental factors are responsible for this suppression. Natural biocontrol is certainly one of these factors because virtually every organism has one or more natural enemies.
Secondly, disruption of natural biocontrol is one of the most common ways pest problems are created or made worse. Applications of broad-spectrum insecticides suppress pest populations but also suppress natural enemy populations. In some cases, this can lead to even greater pest populations because the pest population grows rapidly in the absence of its natural enemies, a phenomenon referred to as pest resurgence. In other cases, the broad-spectrum insecticide suppresses natural enemies of a species that is not normally a pest, leading to a secondary pest outbreak. Many other factors can disrupt natural biocontrol, including the deliberate or accidental introduction of a species into a new area (where the population can rapidly grow in the absence of its natural enemies) and many aspects of annual and perennial cropping systems (for example, large monocultures often fail to provide sufficient food sources and overwintering sites for natural enemies).
Three forms of applied biocontrol are generally recognized based on how the natural enemies are manipulated. In classical biocontrol, exotic natural enemy species are imported and released in the region where the pest occurs. If the introduced natural enemy survives and adapts to its new habitat, it may increase in numbers, disperse throughout the pest region over the course of several years, and suppress the pest population. Often, no natural enemy releases beyond those used to initially establish and spread the natural enemy are needed. Classical biocontrol is often practiced against exotic pest species because these pests usually invade their new habitats without the natural enemies that suppress their populations in their native range. However, classical biocontrol is also practiced against native pests when it is thought that an exotic natural enemy species may be able to suppress the pest better than native natural enemies.
Classical biocontrol can result in complete, sustained, and broad-scale control of the target pest. For example, introduction of natural enemies of the cottony cushion scale, Icerya purchasi, in the late 1800's is credited with having saved the California citrus industry from the ravages of this pest. The cottony cushion scale still remains under control by natural enemies in California and the process has been repeated with equal success in other citrus-growing regions. Although comparable successes have been attained with other pests, classical biocontrol programs are not always successful. In fact, only about one-third of all programs against weed and arthropod pests have resulted in substantial or complete control of the target pest.
Table 1 (see below) provides a summary of recent or ongoing classical biocontrol programs in the High Plains area (not including Montana in this edition). The alfalfa weevil, Russian wheat aphid, and cereal leaf beetle programs have been relatively major efforts. Several natural enemies of the alfalfa weevil have "established" (i.e., persisted for one or more years after release) in our region, but we do not believe they widely suppress alfalfa weevil. This is in contrast to the eastern U.S. where the alfalfa weevil biocontrol program is considered a major success. Probable causes for this difference between the two regions include different alfalfa cultivation practices, different alfalfa weevil strains, and differences in the natural enemy species established. The other two major projects are more recent and it is too early to determine what degree of success will be attained. However, Russian wheat aphid numbers have declined in recent years following natural enemy releases and one exotic natural enemy in particular (Aphelinus albipodus) is becoming quite common.
Augmentation biocontrol is the periodic releases of natural enemies into the habitat where the pest occurs. It differs from classical biocontrol in that releases are continually made and the natural enemies may be native or exotic species. Periodic releases of natural enemies may be necessary when existingnatural enemies fail to colonize the pest habitat each season, colonize the habitat too late, or otherwise fail to attain numbers sufficient to control the pest. Types of augmentative releases range from from seasonal inoculative releases, in which the released natural enemies serve to inoculate a new crop and control is provided later by offspring of the released natural enemies, to inundative releases, in which all control is provided by the released natural enemies themselves. Natural enemies used in augmentative release programs are usually purchased from commercial companies. The natural enemies are laboratory-reared or field-collected and then sold to end-users or other distributors.
Augmentation biocontrol has been successfully used against many pests, especially greenhouse pests. Features of greenhouse growing systems that favor successful use of augmentation biocontrol include the comparatively small treatment area, a closed and stable environment, and the (typically) high cash-value of the cultivated plant. These features favor retention and population increase of released natural enemies and justify the costs of purchasing and releasing natural enemies. Because most of the major High Plains cropping systems do not have these features, few opportunities currently exist to employ augmentation biocontrol in these systems. However, augmentative release programs are being developed for several pests in our area including the European corn borer and spider mites.
Classical and augmentation biocontrol directly manipulate natural enemies through natural enemy releases. Conservation biocontrol, in contrast, fosters existing natural enemy populations indirectly by making the environment more favorable for natural enemies. This can involve the removal of factors that negatively influence natural enemies, or the addition of factors that positively influence natural enemies. Often, conservation biocontrol practices seek to minimize disruptions to natural biocontrol. However, conservation biocontrol is also an important part of any classical or augmentation biocontrol effort.
Pesticides are one of the most common factors that negatively influence natural enemies. Several strategies can be used to minimize the effects of pesticides on natural enemies while retaining adequate effects on the target pest. One strategy is to use physiologically-selective (or narrow spectrum) pesticides or formulations. Such pesticides or formulations kill or weaken the target pest on physical contact but have no or minimal effects on contact with natural enemies and other nontarget organisms. A second strategy is to make pesticide applications "ecologically selective" by minimizing physical contact between the pesticide and natural enemies yet maintaining adequate contact between the pesticide and pest. Specific techniques for making pesticide applications more ecologically-selective include reducing the amount of pesticide applied per area, reducing the area treated (e.g., treating alternate rows or alternate swaths in aerial applications), using pesticides with shorter residual times, altering the timing of pesticide applications, and using pesticides only when necessary. Often, these more selective methods can provide comparable target pest control, reduce treatment costs, and minimize problems with secondary pests.
Modern cropping systems often lack adequate resources for natural enemies. These resources include food such as nectar, pollen and honeydew, alternate prey or hosts, overwintering sites, and other types of refugia. Management practices that can conserve natural enemies by providing these resources include cover cropping, intercropping, strip harvesting, proper crop residue management, maintenance of diverse plant communities at crop borders, and the direct provisioning of missing or deficient resources (e.g., food supplements).
Most pests in the High Plains area have several existing natural enemies. Thus, there is an abundance of opportunities to practice conservation biocontrol, especially through selective pesticide use. Indeed, one of the most common recommendations in this guide is to apply pesticides only when necessary in order to conserve natural enemies.
Table 1 Status of classical biological control projects against arthropod pests in Colorado (CO), Nebraska (NE) and Wyoming (WY). "Establishment" in this context refers to whether or not the natural enemy has been found one or more years after it was released. Note that some natural enemies are indicated to be established in the absence of releases. This could occur because the natural enemy has moved into the area from surrounding states or records indicating release of the natural enemy are missing.
| Pest Species | Crop(s)/host
plants |
Natural Enemy Species
(Type)1 |
Release Status2 | Establishment2 | |||||
|---|---|---|---|---|---|---|---|---|---|
| CO | NE | WY | CO | NE | WY | ||||
| Thistle Aphid
(Brachycaudus cardui) |
Artichokes | Hippodamia variegata (Pr) | + | - | - | ? | ? | ? | |
| Propylea quatuordecimpunctata (Pr) | + | - | - | ? | ? | ? | |||
| Scymnus frontalis (Pr) | + | - | - | ? | ? | ? | |||
| Russian Wheat
Aphid(Diuraphis noxia) |
Small Grains | Aphelinus albipodus (Pa) | +++ | ++ | +++ | ++ | ++ | ++ | |
| Aphelinus asychis (Pa) | +++ | ++ | +++ | + | ? | + | |||
| Aphidius colemani (Pa) | +++ | + | ++ | ? | ? | - | |||
| Aphidius matricariae (Pa) | ++ | + | ++ | ? | ? | + | |||
| Aphidius picipes (Pa) | - | + | + | ? | ? | - | |||
| Aphidius rhopalosiphi (Pa) | + | = | ++ | ? | ? | - | |||
| Coccinella septempunctata (Pr) | - | - | ++ | ++ | ++ | ++ | |||
| Coccinella ancoralis (Pr) | + | - | + | ? | ? | - | |||
| Diaeretiella rapae (Pa) | ++ | ++ | + | ++ | ++ | ++ | |||
| Ephedrus plagiator (Pa) | +++ | - | + | ? | ? | - | |||
| Eriopis connexa (Pr) | - | - | - | ? | ? | - | |||
| Eupeodes nuda (Pr) | ++ | + | ++ | ? | ? | - | |||
| Hippodamia tredecimpunctata (Pr) | + | - | ++ | ? | ? | - | |||
| Hippodamia variegata (Pr) | +++ | + | +++ | ? | ? | - | |||
| Leucopis atritarsus (Pr) | - | - | + | ? | ? | - | |||
| Leucopis ninae (Pr) | +++ | + | ++ | ? | ? | - | |||
| Oenopia conglobata (Pr) | + | - | + | ? | ? | - | |||
| Praon gallicum (Pa) | - | - | + | ? | ? | - | |||
| Propylea quatuordecimpunctata (Pr) | +++ | + | ++ | ? | ? | - | |||
| Scymnus frontalis (Pr) | +++ | + | ++ | ? | ? | ||||
| Semiadalia undecimnotata (Pr) | ++ | - | + | ? | ? | - | |||
| Sphaerophoria scripta (Pr) | ++ | - | + | ? | ? | - | |||
| Range Caterpillar
mMoth (Hemileuca oliviae) |
Rangeland
Grasses |
Brachymeria intermedia (Pa) | + | - | - | ? | ? | ? | |
| Alfalfa Weevil
(Hypera postica) |
Alfalfa | Bathyplectes anurus (Pa) | - | - | ++ | + | ? | + | |
| Bathyplectes curculionis (Pa) | - | - | - | ++ | ++ | ++ | |||
| Bathyplectes stenostigma (Pa) | - | - | - | + | ? | + | |||
| Dibrachoides dynastes (Pa) | - | - | ++ | ? | ? | ||||
| Microctonus aethiopoides (Pa) | - | - | ++ | +(?) | ? | +(?) | |||
| Microctonus colesi (Pa) | - | - | + | ? | ? | +(?) | |||
| Tetrastichus incertus (Pa) | +++ | - | + | + | ? | ? | |||
| Fall Webworm
(Hyphantria cunea) |
Many De-ciduous Trees | Brachymeria intermedia (Pa) | + | - | - | ? | ? | ? | |
| Western Tent
Caterpillar (Malacosoma californicum) |
Many De-ciduous Trees | Brachymeria intermedia (Pa) | ++ | - | - | ? | ? | ? | |
| European Corn
borer (Ostrinia nub-ilalis) |
Corn | Lydella thompsoni (Pa) | - | + | - | ? | ? | ? | |
| Cereal Leaf Beetle
(Oulema mel- anopus) |
Small Grains | Anaphes flavipes (Pa) | - | - | +++ | ? | ? | - | |
| Tetrastichus julis (Pa) | - | - | ++ | ? | ? | + | |||
| 1General type given parenthetically after species name: "Pr" = predator; "Pa" = parasitoid. | |||||||||
| 2ReleaseStatus: “-” = not known to have been released; “+” = limited releases (< 10 sites); “++” = moderate releases (< 50 sites); “+++” = extensive releases (>50sites). | |||||||||
| 3Establishment: “-” = not established; “?” = unknown status; “+” = established at least at some sites; “++” = widely established. | |||||||||
