S. Janarthanan

Associate Professor

Department of Zoology

University of Madras, Chennai - 600025, India

 e-mail : janas_09@yahoo.co.in

Introduction

Insect pest control is almost an unavoidable element of agricultural practice throughout the world. The most important problem in insect pest control is the development of resistance by the pests to chemical insecticides. Farmers are under enormous pressure to reduce the use of chemical insecticides without sacrificing yield/crop quality, but at the same time the control of pests is becoming increasingly difficult due to insecticide resistance and the decreasing availability of products. The immediate need is the use of alternate pest control methods. Many farmers are now familiar with the use of insect predators and parasitoids for biological control of pests, but it is also possible to use specific micro-organisms that kill pests of insects. These include insect pathogens (most popularly called as entomopathogens or microbial insecticides) such as bacteria, viruses and fungi. These are all widespread in the natural environment and cause infections in many insect pest species.

Entomopathogens contribute to the natural regulation of many insect pest populations. Many can be mass produced, formulated and applied to insect pest populations in a manner similar to chemical insecticides. Microbial insecticides are thus especially valuable because their toxicity to non-target animals and humans is extremely low. They are safe for both the user and consumers of treated crops when compared to other commonly used insecticides.

Types of Entomopathogens or Microbial Insecticides

1. Bacterial insecticides

              More than ninety species of naturally occurring insect-specific (entomopathogenic) bacteria have been identified but only a few have been studied most intensively. Much interest has been given to Bacillus thuringiensis (Bt), a species that has been developed as a commercial microbial insecticide.

Some commercially known Bt varieties and target insect pests

Bacillus thuringiensis

var. tenebrionis -  Colorado potato beetle and elm leaf beetle larvae

var. kurstaki   -  caterpillars

var. israelensis -  mosquito, black fly and fungus gnat larvae

var. aizawai  - wax moth larvae and various caterpillars, especially the diamondback moth caterpillar

Bacillus thuringiensis (Bt) occurs naturally in the soil and on plants. Different varieties of this bacterium produce a crystal protein that is toxic to specific groups of insects. The toxic crystal Bt protein in commercial formulations is only effective when consumed orally by insects with a specific gut pH (usually alkaline) and the specific gut membrane structures are required to bind the toxin. It is not only the insect must have the correct physiology and be at a susceptible stage of development, but the bacterium must be consumed in sufficient quantity. When ingested by a susceptible insect, the protein toxin damages the gut lining, leading to gut paralysis. Affected insects stop feeding and die from the combined effects of starvation and tissue damage. Bt spores do not usually spread to other insects or cause disease outbreaks as seen with many pathogens. Bt genes have been transferred into other microorganisms to produce more active formulations, some of which are commercially available. Additionally, researchers have genetically engineered varieties of several plant species to express the Bt toxin as part of the plant's normal development. This has led to the production of “insect-resistant” Bt-transformed varieties of tobacco, cotton, corn, tomatoes, potatoes and others.

Other bacterial insecticides

         Insecticides available in market in the generic name “milky spore disease” contain the bacterial species such as Bacillus popillae and Bacillus lentimorbus. It is very difficult to culture these bacteria in fermentation tanks but, they are obtained from laboratory-reared infected insect larvae. Insecticidal products containing B. popillae and B. lentimorbus can be applied to the soil for the control of larval / grub stage of several beetles. When a susceptible grub consumes spores of these bacteria, they proliferate within it, and the grub's internal organs are liquefied and turned milky white (most commonly called as milky spore disease). These symptoms develop slowly, often over a period of three to four weeks after initial infection. B. popillae and B. lentimorbus, unlike Bt, undergo proliferation in the environment if a substantial grub population is present at the time of application. Once grubs are killed by these bacteria, a new batch of spores is released into the soil. These spores can survive in the undisturbed soil for a period of 15 to 20 years.

2. Viral insecticides

The larvae of many insect species are vulnerable to devastating epidemics of viral diseases. The viruses that cause these outbreaks are very specific, usually acting against only a single insect genus or even a single species. Most of the viruses that are nuclear polyhedrosis viruses (NPVs), in which numerous virus particles are “packaged” together in a crystalline envelope within insect cell nuclei. Some granulosis viruses (GVs), in which one or two virus particles are surrounded by a granular or capsule-like protein crystal found in the host cell nucleus. These groups of viruses infect caterpillars and the larval stages of many dipteran insects.

Viruses, like bacteria, must be ingested to infect insect hosts. In sawfly larvae, virus infections are limited to the gut, and disease symptoms are not as obvious as they are in caterpillars. In caterpillars, virus particles pass through the insect’s gut wall and infect other body tissues. As an infection progresses, the internal organs of caterpillars are liquefied, and its cuticle become discolors and eventually ruptures. Caterpillars killed by viral infection appear limp and soggy (Fig. 1). They often remain attached to foliage or twigs of plants for several days, release viral particles that may be consumed by other larvae. The pathogen can be spread throughout an insect population in this way and by virus-contaminated eggs of infected adult females. Dissemination of viral pathogens is deterred by exposure to direct sunlight, because direct ultraviolet radiation destroys virus particles. Although naturally occurring epidemics do control certain pests, these epidemics rarely occur before pest populations have reached out peak levels.

Fig. 1 Nuclear Polyhedrosis Viruses (NPVs) infected larvae of silkworm, Bombyx mori

The development and use of virus-based insecticides have been limited. Unlike Bt, insect viruses must be produced in live host insects. Production is therefore both expensive and time-consuming. As these viruses are genus or species specific, each viral insecticide has a limited market value. Nonetheless, though they are not well known or widely available, several insect viruses have been developed and registered for use as insecticides. Most are specific to a single species or a small group of related pests; they are not commercially available but are produced and used. Forest pests are especially good targets for viral pathogens because the permanence of the forest environment contributes to cycling of the pathogen (transmission from one generation to the next). The forest canopy also helps to protect viral particles from destruction by ultraviolet radiation.

Other insect viruses investigated for use as insecticides include those that infect the loopers, armyworms and imported cabbageworm. Although some of these viruses have been commercially formulated and applied in field tests, none has been registered or sold in the market. Most viruses are host-specific and effective only against immature stages of the target insect species.

The users must make sure to match the viral pathogen and the target pest correctly. Virus particles are killed by ultraviolet radiation and treating in the evening or on cloudy days should increase their effectiveness.

3. Fungal Insecticides

Fungi, like viruses, often act as important natural insect control agents that limit pest populations. Most of the species that cause insect diseases spread by means of asexual spores called conidia. Although conidia of different fungi vary greatly in ability to survive adverse environmental conditions, desiccation and ultraviolet radiation are important causes of mortality in many species. The viable conidia reach a susceptible insect pest host, free water or very high humidity is usually required for their germination. Unlike bacterial spores or virus particles, fungal conidia can germinate on the insect cuticle and produce specialized structures that allow the fungus to penetrate the cuticle and enter the insect’s body (Fig. 2). Fungi do not have to be ingested to cause infections. In most instances, as fungal infections progress, infected insects are killed by fungal toxins, not by the chronic effects of parasitism.

Fig. 2 Dead Hypothenemus hampei (adult Coleopteran beetle) on the application of fungus Beauveria bassiana

Many important fungal pathogens attack eggs, immature stages and adults of a variety of insect pest species. Others are more specific to immature stages or to a narrow range of insect species. Although fungal pathogens can be produced on artificial media, large-scale production of most pathogens has not yet been accomplished. Precise production and storage conditions must be established and maintained to ensure that infective spores are produced and stored without loss of viability before they are applied. Once applied, pathogenic fungi often are effective only if environmental conditions are favorable; high humidity or rainfall is usually important. In the case of fungal pathogens incorporated in soil to control soil pests, the adverse effects of ultraviolet radiation and desiccation are minimized, but other microorganisms that act as competitors or antagonists often alter pathogen effectiveness.

Fungi used as insecticides include the following:

Beauveria bassiana: This common soil fungus has a broad host range that includes many beetles. It infects both larvae and adults of many species. Understanding the interactions between B.bassiana and other soil microorganisms may be the key to successful use of this fungus.

Nomuraea rileyi: Naturally occurring epidemic infections of Nomuraea rileyi cause dramatic reductions in populations of foliage-feeding caterpillars in soybeans.

Vericillium lecanii: This fungus has been used to control aphids and whiteflies.

Lagenidium giganteum: This aquatic fungus is highly infectious to larvae of several mosquito genera. It propagates effectively in the aquatic environment, even when mosquito density is low. Its effectiveness is limited by high temperatures.

Hirsutella thompsonii: It is a pathogen of the citrus rust mite. Although preparations of this pathogen were once registered and marketed, it is no longer available commercially.

Conclusion

Microbial insecticides provide effective alternatives in an eco-friendly way for the control of many insect pests. Their greatest strength is safety, as they are importantly non-toxic and non-pathogenic to animals and humans. Although not every pest species can be controlled by the use of a microbial insecticide, their products can be used successfully in place of more toxic synthetic insecticides to control several important insect pests. Most microbial insecticides are effective against only a narrow range of pests because these insecticides are vulnerable to quick inactivation in the environment. Hence farmers must properly identify target insect pests and plan the most effective type of microbial insecticides. Moreover, they can be used without undue risks of human injury and environmental damage. Consequently, microbial insecticides are likely to become increasingly important tools in insect pest management.

References

Ferron, P. (1978) Biological control of insect pests by entomogenous fungi. Ann. Rev. Entomol., 23, 409 - 442.

Fuxa, J. R. (1987) Ecological considerations for the use of entomopathogens in IPM. Ann. Rev. Entomol., 32, 225 - 251.

Harper, J. D. (1987) Present and future status of microbial control of arthropods. Crop Protection, 6, 117 - 122.

Kurstak, E. (1982) Microbial and viral pesticides. Marcel Dekker, New York.

Lacey, L. A. and Undeen, A. H. (1986) Microbial control of black flies and mosquitoes. Ann. Rev. Entomol., 31, 265 - 296.

Ravensberg, W. J. (2011) A roadmap to the successful development and commercialization of microbial pest control products for control of arthropods. Springer, New York.