L. T. Kok
Victoria T. Kok
B.Sc (Honors, Botany), MLS
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.
To achieve this goal, the National Biological Control Institute (NBCI), United States Department of Agriculture (USDA), Animal and Plant Inspection Service (APHIS) has provided funds for this project to make biological control information more readily accessible. The objective of this project is to develop a program that provides information on the general principles and methodology of biological control in a format that is easily accessible in a compact disc (CD). Included are examples of biological control of weeds and insect pests, with color images of selected examples. This CD will be made available at minimal cost, to all interested individuals or groups, such as schools, county agents, foresters, 4-H, environmental groups, master gardeners, farmers, landowners, and highway departments. To complement this effort, a website on biological control containing information on insects and weeds and their natural enemies will also be established. This website will be continuously updated so that currency of information will be maintained.
Pest Outbreaks and Population Dynamics
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.
Concept of Biological Control
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.
Ecological Principles of Biological Control
The basic principles of biological control are that:
To be effective in biological control, an agent must be host specific so that they do not affect nontarget organisms.
Effects of Biological Control Agents
Biological control agents exert either a direct or indirect effect through their actions. In the formal case, biological control agents attack and kill the pest directly. In the indirect action, the agents weaken the hosts so that they are unable to reproduce at their normal rate.
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:
Parasites and Predators
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 indicated 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.
Attributes of Natural Enemies
Some of the more desirable characteristics of natural enemies are:
Examples of the importance of natural enemies in Virginia
Some examples of natural enemies that keep major pests of Cole crops under control in Virginia are shown below:
|Cotesia orobenae (wasp)||Cross-striped cabbageworm|
|Diadegma insulare (wasp)||Diamondback moth|
Types of Biological Control
There are three major types of applied biological control where man plays a role: Augmentative, Classical, and Neoclassical.
Pests most suitable for Biological Control
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.
Biological Control is most suited for single, dominant, exotic pests that are not closely related to indigenous beneficial species. It is not appropriate for a complex of pests, or pests with many related members, which are of economic importance because it may attack them as well.
How to import Biological Control Agents
Before any biological control agent or natural enemy can be released, permission needs to be obtained from the Federal and State Governments. The USDA, APHIS is the federal agency responsible for granting permission. For individual states, the State Entomologist reviews requests for release of biological control agents. Host specificity studies of potential biological control agents carried out by research scientists are submitted to the federal and state agencies for review. Only when approval is obtained can any exotic biological control agent be released in the USA.
History of Biological Control
Parasites and predators have long been recognized in agriculture as biological control agents. Predation (use of predators) was first noted in ancient China where the citrus growers used predaceous ants (Oecephylla smaragdina) to control pests of citrus crops. Parasitism was first reported in 1602 when a parasite (Apanteles glomeratus) was used to control the common cabbage butterfly (Pieris rapae).
Early use of biological control in the USA began in mid nineteenth century. In 1856, Asa Fitch used parasites against the wheat midge, and in 1867 Benjamin Walsh continued using parasites against the same pest. C. V. Riley introduced parasites to control the plum curculio in 1870 and the cabbage butterfly in 1883. Crawford in 1886 and Koebele in 1887 introduced parasites to control the cottony cushion scale.
Procedures of Biological Control
The major steps involved in biological control are briefly described as follows:
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.
2. Next we select suitable and effective natural enemies (biological control agents) for pests that are considered appropriate for biological control.
3. We conduct safety tests to ensure that the potential biological control agents will only attack the target pests and not turn out to be pests themselves. This means that the host range has to be carefully studied.
4. We develop techniques to rear the biological control agents so that there will be sufficient for distribution if they are approved for release.
5. Upon approval for release, we conduct field establishment studies of the biological control agents.
6. Lastly we conduct evaluation surveys to determine how effective the biological control agents are in controlling the target pest and also to determine whether nontarget speceis are affected.
Major landmarks in Biological Control as a discipline
The first important success in the biological control of insects occurred in 1888 when the ladybeetle Rodolia cardinalis was imported from Australia and New Zealand for the control of the cottony cushion scale in the USA. The first major success in the biological control of a weed was in 1940 where the cactus moth Cactoblastis cactorum introduced from Argentina controlled the prickly pear cactus in Australia.
There are many examples of successes in the use of biological control. Perhaps the best documented example is the use of the vedalia ladybeetle to control the cottony cushion scale in California in 1888. Since that time, many more successes have been reported. Some examples are shown in the following table.
Examples of Successful Biological Control Programs
|Cottony cushion scale||USA|
Successes in Biological Control of Insects
Biological control Agent
Pediobius foveolatus (Parasitic wasp -Eulophidae)
|Mexican bean beetle|
|Microctonus aethiopoides (Parasitic wasp - Braconidae)||Alfalfa weevil|
Rodolia purchasi (Predator,ladybeetle - Coccinellidae)
|Cottony cushion scale|
Successes in Biological Control of Weeds
Biological control Agent
|Cactoblastis cactorum (Cactus moth - Phycitidae)||Prickly pear cactus|
Chrysolina quadrigemina (Leaf beetle - Chrysomelidae)
Success of Pathogens in Biological Control
Nuclear polyhedrosis virus (Virus)
Bacillus thuringiensis (Bacterium)
Colletotrichum gloeosporioides (Fungus)
General Rules for Releasing Biological Control Agents
There are some general rules governing the release of biological control agents. These include number and size of the releases, and where the releases should be made. The releases should be in each climatic zone that is occupied by the host so that the natural enemy will have a chance to develop in all areas where the host occurs. The releases should be large enough to ensure rapid establishment. Often more than one release in an area is needed to have successful establishment. Releases should be synchronized with the occurrence of an abundance of the preferred host growth stage, i.e. when the preferred host stage is present in large numbers. It is important to make sure that the biological control agents are not adversely affected by ongoing activities in the area of release. Thus, temperature during the time of day that encourages the biological control agent to stay and not disperse should be taken into consideration for release. Also, pesticide applications that are routinely carried out in the area before release should be avoided so that they will not kill the biological control agents. To find out whether establishment has occurred, adequate recovery collections should be carried out. These should be made several times in the season and in as many sites as possible.
Time span in Biological Control
One of the commonly asked questions is how long it will take for biological control to be easily observed. In general, for effective control, it will take at least three years or three host generations. Thus, biological control is a long-term project and not a short-term process.
Single versus Multiple Biological Control Agents
The optimum number of species of biological control agents to be released 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 species of biological control agents, as their combined effects would be greater than that of a single biological control agent. Decision on whether more than one biological control agent should be used should be based on two primary criteria:
Multiple natural enemies should be used if the net impact is greater than that of a single natural enemy.
Research Emphasis in Classical Biological Control
The most important aspect of classical biological control is the selection of effective biological control agents that can be safely used in the release area without fear of non-target hosts being attacked. How successfully this can be done depends on appropriate host specificity tests to determine the potential host range. Major concerns are for host specificity, effectiveness, conflict of interest, and threat to endangered species.
Guidelines for Host Specificity Assessment
There are well-established guidelines for the host specificity testing of biological control agents of weeds, but guidelines for predators, parasites and pathogens of insect pests are not as well developed.
Host Specificity Studies
These studies are carried out to determine which organisms are likely to be attacked in the release area. Two tests that are involved are:
Host specificity tests can be conducted in the laboratory under quarantine in the target area or in its native habitat in the field.
The quarantine lab tests are to determine the physiological host range or maximum potential impact of the natural enemy being tested. These involve:
Field-tests in the native habitat are to determine the actual or ecological range. Only a fraction of the potential host range that is revealed in the lab test is likely to be shown in the field.
Advantages of Biological Control
Disadvantages of Biological Control
Why is Biological Control not widely practiced?
Problems associated with dependence on use of Pesticides
We cannot depend on pesticides to solve pest problems permanently due to:
Examples of Biological Control in Virginia
Biological Control Agent
|Head weevil, Rhinocyllus conicus|
|Musk thistle [ " ]||Rosette weevil,Trichosirocalus horridus|
|Canada thistle [Cirsium arvense]||Leaf beetle,Cassida rubiginosa Fly,Urophora carduii|
|Spotted knapweed [Centaurea maculosa]||
|Hedge bindweed Calystegia sepium||Moth, Bedellia somnulentella|
|Wasp, Megastigmus aculeatus|
|Horsenettle [Solanum carolinense]|
|Dandelion [Taraxacum officanale]||Beetle, Ceutorhynchus punctiger|
|Yellow nutsedge [Cyperus esculentus]||
Beetle, Taphrocerus schaefferi
Moth, Batra verutana
|Diamondback moth [Plutella xylostella]||Wasp, Diadegma insulare|
|Imported cabbageworm [Pieris rapae]||Wasp, Cotesia rubecula|
|Cross-striped cabbageworm [Evergestis rimosalis]||Wasp, Cotesia orobenae|
Wasp, Trisolcus murgantiae
Wasp, Ooencyrtus johnsoni
|Purple loosestrife [Lythrum salicaria]|
Reasons for using the Biological Control Method
Biological control should be implemented whenever possible because it does not pollute the environment. It uses natural enemies of the pests for their control. Most of the two major categories of pests in the USA, arthropods (insects and mites) and weeds, are exotic and since the use of their natural enemies has not been fully exploited, biological control should be the preferred method. Some of the reasons for use of biological control are:
Biological Control Organizations
The current important biological control organizations are:
1. International Organization of Biological Control of Noxious Animals and Plants (IOBC) formed in 1971. Its headquarters is in France.
2. International Institute of Biological Control (IIBC) formed in 1990. It is based in the United Kingdom.
3. National Biological Control Institute (NBCI), USA formed in 1990 and is based in Maryland.
Glossary of Biological Control Terms
The following are some of the more common terms used in biological control:
Abiotic refers to factors that include climate, soil, air, space, and light.
APHIS = Animal and plant Inspection Service.
Augmentation is the increase of native agents for control of native or exotic pests.
Biological Control is the use of natural enemies by man to control pest
Biotic refers to effects of other organisms
Biotic Potential is the inherent characteristics of an organism to survive and reproduce
Environmental Resistance is the sum total of all mortality factors (physical such as temperature and biotic such as natural enemies) in the environment of an organism that prevents it from expressing its full capacity to reproduce.
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.
Density dependent relationship refers to mortality factors that destroy a larger percentage of the target population as the target population increases.
Density-Independent relationship refers to mortality factors that destroy a fixed percentage of the target population regardless of changes in the target density.
Host Selection is the process in which an organism selects its host. It has four phases. First is locating the host habitat, followed by finding the host in the habitat, selecting a suitable host for laying the eggs and finally, the successful development of the parasite in the host that results in producing fertile offspring.
Hyperparasite -- a parasite that attacks another parasite.
IIBC = International Institute of Biological Control.
IOBC = International Organization of Biological Control of Noxious Animals and Plants.
Natural Control is the sum effect of natural control factors that limit the increase of an organism in an ecosystem.
NBCI = National Biological Control Institute.
Neoclassical biological control is the use of exotic biological control agents against a native pest
New Association refers to natural enemies that have not evolved with the target pest.
Old association refers to natural enemies that have evolved with the target pest in its place of origin.
Ovisorption is the process in which ripe eggs are not deposited, but are absorbed back into the body. This occurs when the female does not get enough proteins or when the female fails to locate a suitable host for laying the egg.
Parasite -- an organism that lives and feeds in or on a host.
Parasitoid -- an insect parasite that destroys its host.
Pathogen -- a microorganism that causes disease in its host.
Predator -- a living organism that feeds upon other organisms that are smaller and weaker than itself.
Preoviposition period is the interval between emergence of adult female and when it lays the first egg.
Provigenic species are those that lay their eggs in a short period of its life, but produces no more eggs during the rest of its life span.
Races or strains -- members of the same race or strain look similar and have the same characteristics, but differ from other races in function and activity.
Synovigenic species are those that lay their eggs throughout the adult stage. The female needs to feed and requires a protein source from honeydew, plant nectaries, or from feeding on the host.
USDA = United States Department of Agriculture.
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Dr. L. T. Kok, Professor of Entomology, Virginia Tech, Blacksburg, VA