Study notes on Biological Control

Biological control may be defined as the utilization of a pest’s natural enemies in order to control that pest.

It is the control of pests and parasites by the use of other organisms, e.g., of mosquitoes by fishes which feed on their larvae.

In other words, it is a practice in which an organism is used against another organism.

Under this practice, there are four types of pest control:

(i) Classical biological control or importation, in which a natural enemy from another geographical area, often the area in which the pest originated from, is introduced to contain the pest below the economic injury level, EIL, the definition of EIL is the pest density at which the difference between ne curve showing value of the crop and the curve showing cost of achieving this pest density is neatest;

(ii) Inoculation, in which the periodic release of a control agent is required so that it is available throughout the year. Inoculation is widely practiced in the control of arthropod pests in glasshouses, where crops are removed, along with their pests and their natural enemies at the end of the growing season;

(iii) Augmentation, which involves the release of an indigenous natural enemy in order to supplement an existing population, and is therefore carried out repeatedly usually to coincide with a period of rapid growth of pest population; and

(iv) Inundation, which s the release of large numbers of natural enemy, with the aim of killing those pests present at the time. These are usually termed biological pesticides. However, insects have been main agents of biological control against both insect pests and weeds.

General Theory of Biological Control:

The classical theory of biological control based on the Nicholson-Bailey model is an equilibrium theory (Huffaker and Messenger, 1976). According to this theory, a successful biological control IS produced by the predator imposing low, stable host equilibrium (Fig. 3.9).

But a successful bio-control agent should be host-specific, synchronous with the pest, should have high intrinsic rate of increase (r), should be able to survive with few prey available, and should have high searching ability. All these properties are shown by insect parasitoids than predators. Successful bio-control agents cause density-dependent losses in the host population.

Spatial density dependence occurs when parasitoids or predators cause a higher fraction of losses in dense host patches than in sparse host patches (Hossell, 1977). If predators can aggregate in patches of high host density, then, according to this theory, biological control of the pest is much more likely. The theory has been challenged recently by Murdoch et al. (1985). They have based their view on a non-equilibrium model of predator-prey interaction.

The model assumes that a stable equilibrium of predator and prey is not necessary for satisfactory biological control. Pest populations may fluctuate wildly without pest densities exceeding the economic threshold. According to Krebs (1994), the non-equilibrium model is a meta-population model and, as such, emphasizes that population in different patches may fluctuate independently.

Biological Control by Predators and Parasitoids:

Although predators are considered poor candidates for biological control, they have been used in a number of cases. For example, a small predaceous ladybird beetle, Rodolia cardinalis, commonly called vedalia, has been used to control the cottony- cushion scale insect (Icerya purchasi), a pest of citrus trees. Adult Parasitoids (Hymenoptera) lay their eggs in or near other insects. The larval parasitoid then develops inside its host and kills it before or during the pupal stage.

Biological Control by Parasites:

Some calcid wasps control a number of major pests. The oriental fruit fly, Dacus dorsalis, a pest of ripe fruits in Hawaii has been controlled by three species of parasitic wasps of the genus Opius (O. vandenboschi, O. longicaudatus and O oophilus). This example also illustrates that several parasites of the same pest can be released without having any adverse effect on the overall control. Although the three control agents competed for the same host, the one with superior qualities displaced the others and became dominant.

In this case O. vadenboschi derived the advantage from attacking first instar larvae and thereby inhibiting the development of the eggs and larvae of O. longicaudatus, which favoured older host larvae for oviposition. Likewise, O. oophilus, which oviposits in the eggs of the host, are already present as larvae by the time hosts are suitable for attack by O. vadenboschi.

The geometrid moth Operophetera brumata or winter moth, a pest (defoliator) of hardwood forest and ornamental trees in Canada and Europe, has been controlled by a tachinid fly, Cyzenis albicans, and a wasp Agrypon flaveolatum. However, in this case there was no displacement. Instead, the two species that are compatible and complimentary to each other were able to bring about control. C. albicans was very effective at high host densities, whereas the superior searching ability of A. flaveolatum made it effective at low host densities.

(a) Bacteria:

The use of spore-forming bacteria as a means of controlling the larvae of the Japanese beetle (Popillia japonica), a serious pest of fruits and vegetables, provided the first encouragement for the application of bacteria in insect control. Bacillus popilliae and Bacillus lentimorbus that cause types A and B milky disease of Japanese beetle can both be mass- produced and are sold as a spore dust for injection into the soil. Infected larvae that die in the soil become 1 source of contamination for other larvae feeding in the vicinity. Larval population can be substantially reduced in this way and the Bacillus spores persist in the soil to infect larvae from generation to generation.

Another spore- forming bacteria Bacillus thuringiensis is a facultative pathogen that infects a variety of insects, including the larvae of lepidopterans, flies, and beetles. The bacteria can be cultured on artificial media and is therefore quite economical to produce. Commercial preparations of Bacillus thuringiensis (Biotsol, Dipel, Thuricide) containing both spores and crystals are used is a biological insecticide on a variety of crops. The rather specific nature of Bacillus thuringiensis to kill a few groups of foliage feeders and not to harm beneficial species is of great value in management programmes.

(b) Fungi:

Most entomogenous fungi are internal pathogens. They belong to all the four major taxonomic groups of true fungi, but only a few are frequently associated with insect disease outbreaks. The most commonly used in insect control are Beauvaria bassiane (white muscardine disease) and Metarrhizium anisopliae (green muscardine disease), both of which are fungi imperfect. The infective unit of an entomogenous fungi is usually a spore which germinates on the surface of the host’s integument. Once the host tissue is invaded, the fungus can complete its life cycle, but the survival and germination of spores is critical to the development of an epidemic.

Facultative fungi such as Beauvaria and Metarrhizium can be cultured on artificial media, thereby facilitating the production of spore preparations which may be used in biological control. As with most biological control agents, fungi can be used for either persistent or short-term control. A fungus can be introduced into an area where it becomes established and kills the host year after year. Alternatively, fungal spore preparations can be used as microbial insecticide similar 10 the way Bacillus thuringiensis is used.

However, few attempts have been made to colonize entomogenous fungi. Most projects have involved the redistribution of indigenous fungi or those associated with introduced pests, rather than the importation of foreign species. The best example of attempts to establish new fungal pathogens in disease free areas involves the introduction of Coelomomyces against mosquito larvae, but so far the success has been limited.

The successful use of repeated application of fungal spores as microbial insecticides has been reported for achieving short-term reductions of pest populations. The major limiting factor in initiation of fungal disease in insect populations is the effect of the microclimate on spore survival and germination.

The optimal temperature range for the growth of entomogenous fungi is fairly narrow, and relatively high humidity is needed by most fungi to germinate and successfully penetrate their host before they can produce the new spores required to spread the disease. Sunlight also kills the spores. Consequently, the application of a spore preparation must coincide with both the presence of susceptible hosts and suitable environmental conditions. Best success can usually be obtained by applying the spores in the absence of sunlight such as on a warm evening after either rain or irrigation which provides the needed humidity.

(c) Viruses:

The insect pathogenic viruses are called inclusion viruses, as opposed to non-inclusion viruses in which the virus particles or viruses are free within the cells of the host. The virus particles first multiply in the nuclei, but later continue to replicate in the cytoplasm. The disease eventually kills the insect, leaving it hanging as a fragile sac of virus like the one which results from nuclear polyhedroses infection.

A few non-inclusion viruses also attack insects. But with the exception of Tipula Iridescent Virus (TIV) and Mosquito Iridescent Virus (MIV) that might prove useful in mosquito control, most attention has been given to the inclusion viruses. The very fact that the virus particles enclosed in a protein matrix maintain their infectivity for many years means that the inclusion viruses can be stored as concentrated preparations for later application with conventional pesticide spray equipment.

It has been shown that a nuclear polyhedroses virus is highly effective against a variety of forest sawflies and, as it persists in the environment, it provides continuous regulation of the pest in some areas. Several nuclear polyhedroses virus are being mass produced for possible use against a variety of pests, including cotton bollworm, tobacco budworm, com earworm, cabbage looper, forest tent caterpillar, and alfalfa butterfly. However, one of the problems with viruses is that there are periods when they have little effect on the pest populations. A virus may remain latent in a pest population for several generations and then develop epizootics when the pest population comes under stress.

Generally, short-term control can be achieved by frequent applications of virus preparations so that there is an active innoculum in the pest environment for a long period.

Genetic Control:

Genetic control is a type of biological control that uses two strategies to reduce pest problems. First, crop plants can be manipulated to increase their resistance to pests. Second, we can attempt to alter the genome of the pest species so that they become sterile or less harmful.

Resistant varieties of many crop plants have been developed by selective breeding (Maxwell and Jennings, 1980). However, resistant plants do not necessarily have chemical defenses. Strains of cotton plant produced with low gossypol (a chemical that occurs in green parts and seeds of cotton plant and is toxic to chickens and pigs) content are quite low in resistance to insect pests. Resistant crop plants have also been developed by genetic engineering.

Genes that produce resistance in one species can be transferred into a crop plant to make the crop genetically resistant to specific pests. Bacteria may also be used as vehicles to carry bio-pesticide genes. For example, in 1987 the first success was reported of inserting a gene (the toxin gene of Bacillus thurengiensis) into tobacco plants, conferring resistance against Lepidoptera. Bacillus thurengiensis (Bt) is the main focus at present for developing insect – resistant crops (Lambert and Peferoen 1992).

This bacteria normally lives in the soil and carries a gene for a toxic protein that kills the larvae of moths and butterflies. By splicing this gene into bacteria that normally live on crop plants, genetic engineers have produced insect- resistant crops. Insect pests would inject the bacteria while feeding on the plant and thereby is poisoned.

Alternatively, the Bt genes that produce the toxins can be transferred directly into the plant’s genome, so that the plant would protect itself As of 1992 tobacco, potato, cotton and tomato plants have been genetically engineered with Bt genes (Lambert and Peferoen, 1992). The development and use of such transgenic plants has immense potential. However, one major problem is that pest insects will become resistant to the bio-pesticide, just as they become resistant to chemical pesticides (Pimentel 1991).

The simplest genetic manipulation that can be carried out on a pest species is sterilization. A large number of pests are sterilized by radiation or by chemicals and released into the wild where they can mate with normal individuals. This technique leads to a decrease in birth rate of the pest and control can be achieved. The most notable success of this technique is the near extinction of the screw-worm fly, Cochliomyia hominivorax, which lays its eggs on fresh wounds of livestock and wild animals.

Another example of successful use of sterile-insect method was the suppression of mosquito Culex pipiens quinquefasciatus on a small island off Florida (Patterson, et.al. 1970). However, the sterile insect method cannot be used for all pest populations because it requires the rearing and sterilizing of a large number of individuals and isolation of target area so that natural males from outside the area may not be able to reach there to undrmine the programme.