Biology behind biocontrol
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The behaviors and life cycles of natural enemies can be relatively simple or extraordinarily complex, and not all natural enemies of insects are beneficial to crop production. For example, hyperparasitoids are parasitoids of other parasitoids. In potatoes grown in Maine, 22 parasitoids of aphids were identified, yet these were attacked by 18 additional species of hyperparasitoids. This guide concentrates on those species for which the benefits of their presence outweigh any disadvantages.
What makes a good biocontrol agent?
A successful insect, mite, or nematode natural enemy should have a high reproductive rate, good searching ability, host specificity, be adaptable to different environmental conditions, and be synchronized with its host (pest). A high reproductive rate is important so that populations of the natural enemy can rapidly increase when hosts are available. The natural enemy must be effective at searching for its host and it should be searching for only one or a few host species. Spiders, for example, feed on many different hosts including other natural enemies. It is also very important that the natural enemy occurs at the same time as its host. For example, if the natural enemy is an egg parasitoid, it must be present when host eggs are available. No natural enemy has all these attributes, but those with several characteristics will be more important in helping maintain pest populations.
The conservation of natural enemies is probably the most important and readily available biological control practice for growers and gardeners. Natural enemies occur in all production systems, from the backyard garden to the commercial field. They are adapted to the local environment and to the target pest, and their conservation is generally simple and cost-effective. With relatively little effort the activity of these natural enemies can be observed. Lacewings, lady beetles, hover fly larvae, and parasitized aphid mummies are almost always present in aphid colonies. Fungus-infected adult flies are often common following periods of high humidity. These natural controls are important and need to be conserved and considered when making pest management decisions (for example, applying insecticides). In many instances the importance of natural enemies has not been adequately studied or does not become apparent until insecticide use is stopped or reduced. Often the best we can do is to recognize that these factors are present and minimize negative impacts on them. If an insecticide is needed, every effort should be made to use a selective material in a selective manner.
The habitat or environment can also be manipulated to support natural enemies that are already present in the environment (or those that are released; see augmentative biocontrol). This tactic involves altering the cropping system to augment or enhance the effectiveness of a natural enemy. Many adult parasitoids and predators benefit from sources of nectar and the protection provided by refuges such as hedgerows, cover crops, and weedy borders.
Natural enemies can benefit from a source of nectar or pollen and can be supported by a variety of flowering plants. Mixed plantings and the provision of flowering borders can increase the diversity of habitats and provide shelter and alternative food sources. They are easily incorporated into home gardens and even small-scale commercial plantings, but may require more planning to incorporate into large-scale crop production. There may also be some conflict with pest control for the large producer because of the difficulty of targeting the pest species and the use of refuges by the pest insects as well as natural enemies. Examples of habitat manipulation include growing flowering plants (pollen and nectar sources) near crops to attract and maintain populations of natural enemies. For example, hover fly adults can be attracted to umbelliferous plants (members of the carrot or parsley family) and other plants in bloom. Work in California has demonstrated that planting prune trees in grape vineyards provides an improved overwintering habitat or refuge for a key grape pest parasitoid. The prune trees harbor an alternate host for the parasitoid, which could previously overwinter only at great distances from most vineyards.
Caution should be used with this tactic because some plants attractive to natural enemies may also be hosts for certain plant diseases, especially plant viruses that could be vectored by insect pests to the crop. Although the tactic appears to hold much promise, only a few examples have been well-researched and developed in commercial production settings.
In many instances the complex of natural enemies associated with an insect pest may be inadequate. This is especially evident when an insect pest is accidentally introduced into a new geographic area without its associated natural enemies. These introduced pests comprise about 40% of the insect pests in the United States. One example of an introduced vegetable pest is the European corn borer, one of the most destructive insects in North America. To obtain the needed natural enemies, we turn to classical biological control. This is the practice of importing, and releasing for establishment, natural enemies to control an introduced pest, although it is also practiced against native insect pests. The first step in the process is to determine the origin of the introduced pest and then collect appropriate natural enemies (from that location or similar locations) associated with the pest or closely related species. The natural enemy is then passed through a rigorous quarantine process, to ensure that no unwanted organisms (such as hyperparasitoids, or predators of non-pest species) are introduced, then reared, ideally in large numbers, and released. Follow-up studies are conducted to determine if the natural enemy successfully established at the site of release, and to assess the long-term benefit of its presence.
There are many examples of successful classical biological control programs. One of the earliest successes was with the cottony cushion scale, a pest that was devastating the California citrus industry in the late 1800s. A predatory insect, the vedalia beetle, and a parasitoid fly were introduced from Australia. Within a few years the cottony cushion scale was completely controlled by these introduced natural enemies. Damage from the alfalfa weevil, a serious introduced pest of forage, was substantially reduced by the introduction of several natural enemies. About 20 years after their introduction, the alfalfa acreage treated for alfalfa weevil in the northeastern United States was reduced by 75 percent. Many classical biological control programs for insect pests and weeds are under way across the United States and Canada. Classical biological control is long lasting and inexpensive for growers and gardeners. Governments bear the initial costs of collection, importation, and rearing. When a natural enemy is successfully established it rarely requires additional input and it continues to kill the pest with no direct help from humans and at no cost. Unfortunately, classical biological control does not always work. It is usually most effective against introduced pests and less so against native insect pests. The reasons for failure are often not known, but may include the release of too few individuals, poor adaptation of the natural enemy to environmental conditions at the release location, and poor matching between the life cycle of the natural enemy and host pest.
This third type of biological control involves the supplemental release or application of natural enemies. Relatively few natural enemies may be released at a critical time of the season (inoculative release) or literally millions may be released (inundative release). Additionally, the cropping system may be modified to favor or augment the natural enemies that are released. This latter practice is frequently referred to as habitat manipulation, and may also be considered a form of conservation biocontrol, if it supports natural enemies that are already present in the environment.
An example of inoculative release occurs in greenhouse production of several crops. Periodic releases of the parasitoid, Encarsia formosa, are used to control greenhouse whitefly, and the predaceous mite, Phytoseiulus persimilis, is used for control of the two-spotted spider mite.
Lady beetles, lacewings, or parasitoid wasps such as Trichogramma are frequently released in large numbers (inundative release). Recommended release rates for Trichogramma in vegetable or field crops range from 5,000 to 200,000 per acre per week depending on level of pest infestation. Similarly, entomopathogenic nematodes are released at rates of millions and even billions per acre for control of certain soil-dwelling insect pests. An application of a biopesticide is also often an example of augmentative biocontrol.