The basic approach in biological pest control is to either introduce or foster an organism that exerts some control over the pest population. This organism can be a predator, parasite, competitor, or disease of the pest.
Biological control, of course, occurs widely in nature, much without us even recognizing it. It also occurs in agricultural systems, again often without being noticed - an example of one of those "ecosystem services" that we discussed earlier in the term. In fact, most pests never become serious because of controls exerted by other organisms.
While people often think of "Biological control" as synonymous with the introduction of pest antagonists from other regions of the world (that is, of nonnative species), in fact research in this area often include studies of how to enhance native populations of antagonists as well. Researchers study how adjacent crops, planting densities, or other features of the agroecosystem and its surroundings be manipulated to enhance them.
For example, some reports indicate that bird and bat densities are greater in organic than in conventional farming systems (WorldWatch Jan/Feb '05). Given that birds eat insects, some of whom are crop pests, and also eat weed seeds, and that bats eat insects as well, it appears that some aspects of the organic systems accomplish this enhancement of naturally occurring control agents.
Another example of fostering natural control agents that I can attest to from my own experience involves the apple scab fungus. This fungus causes scabs on apples (you've surely seen these?) as its name implies. The fungus overwinters on fallen leaves and apples, and then sporulates in spring, infecting new fruits. SO, sanitation of the area around the trees, via removing fallen leaves and apples reduces disease. Guess who does this sanitation for us free of charge? Many naturally occurring invertebrates and other critters, including earth worms, which pull the leaves underground and consume them. If you spray the area with a pesticide to which the worms are sensitive, you diminish this control; if you don't spray then the area will be hospitible to worms and your scab will be reduced!
Still another example involves deer mice, which consume huge numbers of weed seeds from the surface of the ground -- and don't hibernate so continue to munch weed seeds over the winter. Studies are on-going to develop tillage and other practices that are beneficial -- or at least are less injurious -- to deer mice, so that they will be encouraged to hang around fields controlling weeds.
The more widely known aspect of biological control involves importing antagonists, as discussed below.
Considerable attention focuses on deliberate introduction of biological control agents (biocontrols) into agricultural systems, some of which are native to the area and some of which are introduced from other regions of the US or abroad. Such introduction must be done with caution, of course, lest the organism become a pest in its own right or interfere with the operation of some naturally-occurring pest control agents, usually in some unanticipated fashion. An incomplete understaning of their ecology could wreak havoc (see article by Pimentel et al. on environmental risks of biological controls on the supplemetary reading list for this unit for some examples). In general, the more specialized the introduced "critter" is, the safer it is likely to be (that is, those with narrow host ranges are preferred over those with broader host ranges). While examples of introductions "back firing" are relatively few, some do exist, and are sobering. A few examples follow:
Yes, the USDA regulates importation of invertebrate weed control agents and evaluates their safety before allowing import. The USDA and EPA jointly regulate importation and use of microbial pest agents (USDA for those imported for weed control and EPA for those imported to control other kinds of pests). Prospective biocontrols must first be tried out under quarantine conditions in greenhouses, then small-scale field trials are allowed, and then larger scale trials must be conducted before commercial release is permitted.
The search for safe and effective agents is expensive and time-consuming (see article on the supplementary reading list on "The Quixotic Quest..."). Only about 1/3 of attempts to introduce a biocontrol agent do result in establishing it as a new "natural" enemy, and in about 1/2 of those cases, the control is essentially complete. (Frontiers in Ecology and Env. 4 (2006). Despite the challenges, there are an increasing number of success stories!
As with so many things, decisions about whether to attempt use of a biocontrol agent must weigh the risks of it creating problems against the risks associated with NOT using it (e.g., diminished crop yields or use of chemical pesticides instead).
There are an increasing number of success stories regarding imported biocontrol agents. Over 50 species of weeds worldwide and over 500 insect pests worldwide are being controlled to some degree by introduced control agents (1999).
As already mentioned above, lady bugs (and preying mantis) are notoriously successful for aphid control, and are widely used by home gardeners. Some examples are more exotic: one aggressive root decay fungus can spread to neighboring trees after partial harvest of a forest by infecting the cut stumps and then spreading through roots to uncut trees. Spores of a competing fungus can be mixed in chainsaw oil to inoculates stumps as trees are cut with a competing fungus that doesn't cause problems in its own right and keeps the harmful fungus in check so that it doesn't spread to neighboring trees!
Cassava is a staple crop in tropical Asia, Africa and Latin America. Its production in Africa has been threatened by several introduced insects (accidentally introduced on nonquarantined cassava roots), but the Africans have had great success in controlling some of them by introducing their natural predators as well.
Dr. Peter McEvoy here at OSU in the Entomology Department has had considerable success with using introduced insects to control the weed ragwort (Senecio jacobaea). Some successes have also been reported with insect control of Klamath weed (Hypericum perforatum) here in the PNW. In the US, about 41% of projects focused on using insect biocontrols for weeds have evidence of some control and about 20% have significant control. Testing and approval for release of a new insect biocontrol agent is time consuming and costly.
Scotch broom (Cytisus scoparius) is a shrub that you may know -- green stems, small leaves, covered with bright yellow blossoms in spring. It is a troublesome invasive exotic species -- in OR alone, it is estimated to cause ~ $47 million per year in damage to timber plantations, pastures, and powerline rights-of-way. OR Department of Agriculture (ODA) introduced a weevil from Europe into some Scotch broom populations in OR with good results -- the larvae eat ~ 80% of its seeds. ODA is experimenting now with another introduced beetle in the hopes that it will consume the rest.
Leafy spurge is another troublesome weed, introduced from Eurasia. It is toxic to livestock and crowds out native species. Good control of this weed is being obained using flea beetles, also from Eurasia -- the adults attack foliage and the larvae eat the roots -- what a trick! The flea beetles are, importantly, very host-specific. In some cases, two species of beetles are used. Once a population of beetles becomes well-established in an area, however, it is possible that they will eat themselves out of a food supply! Efforts to circumvent this self-limitation involve capturing beetles from areas in which they've become well-established and moving them to other infestations.
There have also been successes with biocontrol of some insect pests, such as cottony cushion scale and red scale on citrus in S CA.
One example that, again, many of you may have used in your gardens (and that you may have heard about in the context of discussions about genetically modified organisms ("GMO's") is "bt" (Bacillus thuringiensis) -- a naturally occurring soil inhabiting bacterium. This bacterium produces a toxin that is effective against many lepidopterans (larval forms such as cabbage worms). When bt is ingested, toxins (which are in crystals in the bacterium) are released and kill the lepidopteran. More and more bt strains are being found that are specialized against certain groups of lepidopterans. While use of bt may seem harmless, even it can cause unanticipated problems. For example, recent reports indicate that when bt is used to kill gypsy moths and spruce budworm in forests, bt (the variety btk, specialized for only certain caterpillars) greatly reduces the numbers and diversity of other non-pest moth and butterfly species for at least two yrs. Researchers (including Dr. Jeff Miller from OSU's Entomolgy Department) are also concerned about its effects on Townsend's long-eared bat, which feeds on moths and is a candidate for Federal listing as Threatened or Endangered under the Endangered Species Act. (Organic growers can use sprayable Bt and still be considered organic, incidentally.)
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This page is maintained by Patricia Muir at Oregon State University. Page last updated Dec. 28, 2007.