Biological Control for the Public

There are natural control factors that limit the increase in numbers of an organism in an ecosystem. These naturally occurring limiting factors prevent most organisms from becoming pests. This process is known as Natural Control. Natural control consists of two major components, environmental factors (abiotic) and natural enemies (biotic). When natural enemies are killed by man's actions in any habitat or when the pests are introduced to new habitats without their natural enemies, natural control often fails and results in pest outbreaks. To remedy the situation caused by his own actions, man can play an active role in reintroducing the natural enemies to control pests. The use of natural enemies by man to control pests is Biological Control. Thus the difference between Biological Control and Natural control is that the former is brought about through man's action while the latter is not.

Despite the successful use of biological control in the control of insects and weeds for over a hundred years, this concept is still not very well known among the general public. Even in major universities, the concepts and methodology of biological control are not taught in the more general introductory courses, but only in specialized courses that usually have small enrollment. Consequently, less than 10 percent of the students attend classes that include the concept of biological control. This lack of knowledge of biological control among students, growers, and the general public throughout the State of Virginia is readily perceived in my dealings with them during the past 27 years. Very few are aware of the valuable potentials of biological control as a pest problem solving technique, or of the availability of research information on the topic. This unfortunate situation can be readily rectified through a more effective method of dissemination of information.

Pest outbreaks are the result of natural or man-induced factors. Natural factors include migration and environmental changes that allow the pest to increase in numbers. Man-induced factors include the introduction of a new species, a change in crop management, an alteration of the habitat, side effects of pesticide use, monoculture or continuous cropping that provides a concentrated food source for species that can survive and reproduce on them. One of the most common factors is the introduction of insect species by man, directly or indirectly, to new areas without their natural enemies. In the absence of their natural enemies, the pests survive and reproduce rapidly in their new environment. This is often referred to as "expression of their Biotic potentials".

Biotic Potential is the inherent characteristics of an organism to survive and reproduce. Without restraining factors, they will soon colonize the whole world. However, there are various factors that limit the growth of any organism. These natural control factors act as restraining forces that keep a species from unlimited growth. In any habitat, natural control factors suppress pest outbreaks. When the natural control factors are removed, then pest outbreaks occur. These natural control factors are often referred to as environmental resistance.

Environmental Resistance is the sum total of all mortality factors (abiotic physical factors such as temperature, and biotic factors such as natural enemies) in the organism's environment which prevents it from expressing its full capacity to reproduce. Mortality factors can be broadly divided into two groups, biotic and abiotic.

Biotic factors are living factors such as the effects of other organisms

Abiotic factors are environmental factors that include climate, soil, air, space, and light.

Biological control is the study and use of natural enemies by man as control agents for reducing pest populations and number. In 1919, Smith defined it as the use of natural enemies to control insect pests. Harley in 1985 broadened the concept to the study and utilization of parasites, predators, and pathogens to regulate the populations of pests. Biological control agents must be living organisms so that they can seek out the target pest. They can be natural or genetically improved organisms.

The basic principles of biological control are that:

The term natural enemy refers to living organisms that are normally found in the same habitat as the target pest. They keep the pest numbers down. Natural enemies, also called beneficial organisms, can be grouped into four types:

1. Parasite -- an organism that lives and feeds in or on a host.
2. Parasitoid -- an insect parasite that destroys its host.
3. Predator -- a living organism that feeds upon other organisms that are smaller and weaker than itself.
4. Pathogen -- a microorganism that causes disease in its host.

There are some major differences between how parasites and predators act. It is useful to know some of their characteristics. In general, parasites are more host specific, that is, they have a narrow host range. They are also better adapted with the host in the same environment, and in many cases the parasite and host have evolved together. Thus parasites are synchronized with the host as well as its environment. A parasite requires fewer hosts for its development and survival. It feeds less than a predator and the young do not need to search for food. Parasite eggs are usually laid into the target host and as the egg hatches, the immature stage will feed inside the host and can complete its development in a single host.

Predators need to have a wide range of prey. They destroy large numbers of prey quickly and thus their spectacular successes are often noticed. They are more likely to show an immediate impact. However, because they are mobile and move away after they have fed, the effects of many predators are often overlooked.

Natural enemies work quietly, often unnoticed and so their importance is often overlooked. Frequently their effectiveness as biological control agents becomes noticeable only after they have been eliminated through the wide use of pesticides that kill not only the target pests, but these natural enemies as well. The lack of appreciation of natural enemies is not a recent occurrence. As early as 1880, Forbes indicted that "…the annihilation of all the established 'enemies' of a species would, as a rule, have no effect to increase its final numbers". This obviously has been proven incorrect. The importance of natural enemies is now widely recognized among pest management specialists, and current pest control practices emphasize the preservation of natural enemies.

Some of the more desirable characteristics of natural enemies are:

Some examples of natural enemies that keep major pests of Cole crops under control in Virginia are shown below:

There are three major types of applied biological control where man plays a role: Augmentative, Classical, and Neo classical.

1. Augmentation is the increase of native agents for control of native or exotic pests. Usually there is a lack of synchrony where the pest occurs early in the season and the natural enemies are absent. Thus, releasing the natural enemy early in the season will help to ensure that there is no scarcity of natural enemies when the pests first appear. This process is called augmentation.
2. Classical biological control is the use of exotic biological control agents imported from its native home into the target area against exotic pests that have arrived without their natural enemies. In the absence of their natural enemies, the exotic pests multiply rapidly and spread to become a major pest. Thus, classical biological control attempts to introduce the natural enemies also into the new area so that they will reestablish equilibrium with the pest and keep it under control. This is often referred to as an old association because the natural enemy and pest were in the same place or ecosystem.
3. Neoclassical biological control is the use of exotic biological control agents against a native pest. Since they were not in the same area before, this is referred to as a new association.

Not all pests can be controlled by this technique. The pests most suitable are exotic pests that have been introduced into this country without their natural enemies. The goal in biological control is to find out what the effective natural enemies are, where they can be found, and to try to import them from their native area into the affected areas to control the pests.

1. First we need to determine whether the pest is suitable for biological control since not all pests are suitable for this method of control.

Musk thistle USA

The optimum number of biological control agents for one release for control of a given pest has generated much discussion. There are two schools of thought. One group supports the release of the "best" single biological control agent to decrease competition between species. The other supports the idea of using a number of biological control agents, as the combined effects would be greater than that on a single biological control agent. Decision on whether more than one biological control agent should be used should be based on two primary criteria:

• Host acceptance -- to find out which hosts are accepted for egg laying
• Host suitability -- to ascertain whether feeding on a specific host can support the complete development of the biological control agent.

Host specificity tests can be conducted in the laboratory under quarantine in the target area or in its native habitat in the field.

• Feeding (starvation) tests that determine whether the natural enemy will feed or starve on the test plant.
• Developmental tests to determine whether the natural enemy can successfully complete development on the host and produce fertile progeny by the formation and maturation of eggs.

• Free of side effects
• Safe to handle or use
• Occurs naturally
• High degree of host specificity
• Cost effective
• Self perpetuation
• Searching ability
• Survive at low host density

• Slow to achieve results
• Impact often not dramatic
• Partial success
• Can be complex

• Need for quick fix
• Lack of patience
• Potent pesticides
• Industry advocates
• Easy application of pesticides

Musk thistle Head weevil, Rhinocyllus conicus