HORTICULTURE

BIOLOGICAL CONTROL

The chemical control of plant pests became necessary as a result of human intervention and manipulation of the natural world. By selec­tively producing masses of desired plants and reducing or eliminating the natural diversity of a region’s flora, horticulturists, foresters, and
farmers destroyed the check-and-balance system that kept plant pest populations in proper proportion with their host plants. When a field of natural growth is cleared and replaced with thousands of plants of the same species, the pests that thrive in the new, simplified environment also increase. Simultaneously, the factors that once held those pests in check may also have been eliminated. The solution for many years has been to reach for the sprayer.

Chemical control of pests also owes its acceptance and develop­ment to the public’s desire for plant products that are blemish-free and visually attractive. While leaves with a few spots, or shrubs with a few insects, or a golf course fairway with a few weeds should be regarded as natural and expected, the economic reality is that consumers have been conditioned to view such conditions as unacceptable. It has followed that growers, landscapers, and property managers do whatever is neces­sary to meet the expectations of their customers, including chemically controlling plant pests.

Biological control is an attempt to return some of the natural inhibi­tors of insect, pathogen, and weed injury to the production of desired crops, including the ornamentals. The techniques currently being used include:

• Plant extracts—Natural chemicals produced by certain plants that can repel and/or impede the development of some insects. These extracts include pyrethrum, extracted from chrysanthemums, and rotenone.

• Physical features—Structural characteristics such as thick, waxy cuticles or an especially pubescent leaf surface can protect plants from insect damage or pathogen entry.

• Plant emissions—Some plants secrete chemicals from their roots that are toxic to certain pathogens such as nematodes, and to certain nearby plants that might otherwise compete for available water and nutrients. Still other plants repel insects by exuding a strong scent that the insects cannot tolerate.

• Predators and parasites—Insects and other animal pests that feed on plants are, in turn, the food of choice for a variety of other insect and animal predators. Additionally, there are bacteria and fungi that will infect and kill a number of insects and nematodes, and even other bacteria and fungi.

• Alternate hosts—Some plant pests have a preference of one host species over another. When an economically valuable species is interplanted with a species preferred by the pest, the alternate host becomes a decoy, permitting pest-free production of the desired species.

By selecting host species that possess features which discourage pests, or by incorporating into production sites those plants that will discourage or repel pests, or by releasing into the production area pred­atory or parasitic agents that will infect the undesirable pests, horticul­turists use the constraints of the natural world against biotic irritants of all types. The major drawback to a more widespread use of biological controls is the smaller number of ornamentals that possess the physi­cal or physiological features needed to discourage the pests that affect

them. Genetic engineering by plant breeders may increase the number of ornamental species possessing characteristics that protect against pests. That progress will be predictably slow in coming. Still, propo­nents of chemical and biological control of plant pests have begun to work together for a common purpose. Nothing exemplifies that more clearly than the increasing acceptance of integrated pest management throughout the green industry. The use of chemical pesticides is being reassessed, not to eliminate them from use, but to incorporate them into a more broad-based pack­age of control measures. That package returns some of the balance of the natural world to horticultural production by allowing beneficial insect and microbial predators of destructive insects and pathogens to exist within the production range. By so doing, the predators keep the population of harmful insects and pathogens to a level that does not necessitate excessive use of chemical pesticides. When they are used, chemical pesticides are applied only when and where needed, rather than when their use is excessive, wasteful, and expensive. This multi­faceted approach to pest control is termed integrated pest management (IPM). It applies the principles of control by carefully balancing the rela­tionship existing among the crop hosts, the production environment, and the plant pests to which the crops are susceptible (Figure 6-20). The techniques of IPM vary with the circumstances of production. In greenhouses, for example, the production facility is closed and con­tained. This structure permits the screening of vents and, therefore, the introduction of predatory insects into the crop with the knowledge that they will remain and justify their cost. Table 6-4 lists some of the preda­tors currently available for insect and disease control in greenhouses. Due to the openness of the production site, field and container nurseries are less able to use predatory insects in their IPM programs. Instead, they rely heavily on cultural practices such as proper spacing and pruning, balanced watering, fertilizer monitoring, elimination of weeds and other alternate hosts, and the use of resistant varieties to

Production environment

Plant pest

• Regulate humidity, temperature, air movement, nutrient/ph levels

• Pesticides

figure 6-20. Integrated pest management balances the relationship between host, pest, and environment (Delmar/Cengage Learning)

reduce pest populations. Other aids to control include horticultural oils that suffocate the insects, insecticidal soaps that kill insects on contact, insect growth regulators, biological pesticides, and when necessary, chemical pesticides. Table 6-5 lists some of the biological products cur­rently used to control insects on ornamental and turf crops.

Briefly summarizing how the various biological controls work:

Plant extracts are chemicals found naturally in certain plants that are known to have an insecticidal or herbicidal action. When extracted, concentrated, and applied to vulnerable pests, these natural chemicals can interfere with the growth and development of certain pests.

Microorganisms that can be used as antagonists against other organisms include various species of bacteria, fungi, protozoa, viruses, and nematodes. They are applied (often as sprays) to host crops where they infect and destroy the target pest.

Microbe-derived pesticides use the natural antibiotics and other compounds extracted from microorganisms to control plant pests. Products vary in their targets and in their toxicity to non-target species.

Pyrethroids are synthetic versions of the natural extracts, pyrethrins. They tend to control a wider range of pests and remain effective longer than the natural pyrethrins.

Soaps are light oils made from plants. The insecticidal soaps work against soft bodied insects by penetrating their outer layer and dehy­drating their interiors. The herbicidal soaps have a similar effect against the outer cuticle of the leaves of target weeds.

Insect growth regulators disrupt the growth and development of insects. The effect is usually hormonal and interferes with the metamor­phosis of the insects.

Choronicotinyls are similar to insect growth regulators that act by impeding the normal functioning of the insect nervous system. They are systemic and can be applied as either granulars or as sprays.

Pheromones are chemical cues used by insects to communicate with other insects. The sex pheromones are used to attract mates. When synthesized, sex pheromones can be used as baits to attract and trap certain insects. It is more useful as a management and monitoring tool than as a control measure.