Biological Pest Control

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Scientists use different pest control methods that range from choosing a pesticide that will be least harmful to beneficial insects to raising and releasing one insect to attack another. Biological control of horticultural pests is a subject of increasing interest, especially to people who prefer to use chemicals as little as possible. The traditional and familiar use of chemical pesticides sometimes has detrimental effects on the environment (such as with DDT) and harmful effects on humans (such as the increased risk of cancer in individuals exposed to certain pesticides). There are, however, alternative pest control methods, such as biological pest control, that are less harmful to the environment and humans. Unfortunately, biological pest control methods alone are rarely sufficient. Research suggests that an integrated approach, using pesticides, biological pest control, and other techniques may be the most effective.

In general terms, biological pest control is the use of a specifically chosen living organism to control a particular pest. This chosen organism might be a naturally occurring parasite, predator, or disease that will attack a harmful insect. Biological pest control is a way to manipulate nature to increasea desired effect. There are three main ways to use natural enemies against unwanted insect pest populations: classical biological control, augmentation, and conservation.

Classical biological control involves releasing an imported organism that establishes itself and spreads to permanently control a pest. In 1889, for example, the release of 129 imported Australian vedalia beetles resulted in a dramatic reduction of cottony cushion scale disease, which had threatened California's citrus industry. Successful biological control means no further costs are required to keep the pest under control.

Classic biological control may also mean traveling to the country or area from which a newly introduced pest originated and returning some of the natural enemies that attacked it and kept it from being a pest there. New types of insects are constantly arriving, accidentally or intentionally. Sometimes they survive. When they come, their enemies are left behind. If the insects become a pest, introducing some of their natural enemies can be an important way to reduce the amount of harm they can do. Although simple in concept, the process of locating the place of origin of the nonnative pest and then finding and introducing natural enemies from its place of origin presents many ecological and logistical challenges. For example, any introduced pest predator or parasite must undergo exhaustive testing before being released to be sure it will not harm nontarget organisms. Even when challenges are successfully met, projects can fail because of problems relating to such factors as climate differences, prior or current pesticide use, disturbances of the habitat by other agricultural operations, and the removal of noncrop vegetation that might otherwise offer food and shelter to the natural enemies.

Augmentation is a method of increasing the population of a natural enemy that attacks pests. This can be done by mass producing a pest in a laboratory and releasing it into the field at the proper time or breeding a better natural enemy that can attack its prey more effectively. Mass rearings can be released at special times when the pest is most susceptible and natural enemies are not yet present, or they can be released in such large numbers that few pests go untouched by their enemies. In one study, for example, male insects were sterilized by gamma radiation, and large numbers were released into the environment to mate with wild insects. The pest population was dramatically reduced because they were unable to produce viable offspring.

Scientists also grow microbes, such as bacteria and fungal spores, in the laboratory and spray crops and lawns with large numbers of these natural organisms to bring certain pests under control. About 1,100 species of viruses, bacteria, fungi, protozoa, rickettsiae, and nematodes are known to parasitize insects. Japanese beetle populations, for example, once created major crop infestations here in the United States but have been decimated by treating crops and lawns at intervals with spores of a bacillus bacterium that causes milky disease, a lethal pathogen for these beetles. These sporesinfect the Japanese beetle larvae but do not harm other animals in the environment.

Another bacterium, Bacillus thuringiensis, has been extensively exploited in the bacterial control of pest insect populations. Commercial preparations of Bacillus thuringiensis are registered by at least twelve manufacturers in five countries for use on numerous agricultural crops and forest trees for control of pests including the alfalfa caterpillar, bollworm, cabbage looper, fruit tree leaf roller, California oakworm, and fall webworms. Many other commercial bacterial products exist. For many years, a mixture of Bacillus popilliae and Bacillus lentimorbus has been marketed under the trade name Doom.

Viral diseases have also been studied as a means of controlling certain caterpillars and sawfly larvae. Crops are sprayed with a substance prepared from diseased insects in order to start an epidemic of a fatal disease in the pest population.

Augmentation is effective but it relies upon continual human management and does not provide a permanent solution, unlike importation or conservation methods.

Conservation of enemies is an important part of any biological control effort. This strategy involves identifying any factors that limit the effectiveness of a particular natural enemy and changing them to help the beneficial species. Conservation of natural enemies involves either reducing factors that interfere with the natural enemies or providing needed resources that help natural enemies. Use of reflective aluminum strips mixed in with mulch in vegetable fields, for example, has reduced or prevented aphid attack and thus protected cucumbers, squash, and watermelons from infestation. The planting of cover crops, such as providing nectar-producing plants and sources of alternate hosts in and around fields, and the interplanting of different crops to provide habitat diversity are management techniques that lead to the buildup of natural enemy populations and result in enhanced biological control.

Physical energy is also known to kill insects. In a recent discovery, adult Indian-meal moths exposed to certain wavelengths of sound during their egg-laying period reduced their reproduction by 75 percent. These sound waves also had a similar effect on flour beetles. Light waves, high-frequency electric fields, and high-intensity radio frequencies also may offer helpful options. Sophisticated methods of pest control are continually being developed. Cutting-edge research with highly specific insect hormones was also underway at the beginning of the twenty-first century.

Integrated pest control methods, which involve more than one method, may be the best answer. Combining disease-resistant plant varieties with an insecticide that leaves parasites and predators unharmed is one ideal strategy that seems to offer promise. It has been successful, for example, in combating the spotted alfalfa aphid in California. In other cases, first-line defenses such as chemical sprays combined with bait, followed by the sterile-insect technique were highly effective. The most important value of this control method is that far fewer chemical pesticides are used, and so the environment remains unaffected.

Biological pest control methods, in concert with proven agricultural practices such as destruction of crop residues, deep plowing, crop rotation, use of fertilizers, strip-cropping, irrigation, and scheduled planting operations, can further prevent or reduce crop damage.

The battle against spotted alfalfa aphids in California began in the late 1950s when researchers noted growing damage among the crop. Initial tests showed that the aphids had developed a resistance to chemical sprays, and that beneficial insects were dying off because of the chemicals. Over the years, pesticide use was reduced and the aphid problem diminished as the natural predators regained their populations.

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