Fungi in Bio-controls of Crop Pest
Entomopathogenic fungi played an important role in the early development of Insect Pathology and subsequently they were considered as beneficial organisms with potential as pest suppressive agents.
Pathogenic fungal infections are referred to as mycoses. Insect mycoses are cause by fungi in the following classes:
i) Phycomycetes: Entomophthora
ii) Ascomycetes: Cordycepes, Nectria
iii) Basidomycetes: Septobasidium
iv) Fungi imperfecti : Cephelosporium, Metarahizium, Penicillium, Verticlillum.
The Phycomycetes and Deuteromycetes contain the orders, families and genera which are most commonly considered for species of entomogenous fungi in some 35 genera, very few have been studied in details. The members of the genera Beauverina (white muscardine) against Lepidoptera, Metarahizium (green muscardine) against cockchafer beetle, and Entomophthora against aphids, jassids, housefly etc. and Coelomomyces and Aspergillus have received most of the infection. The taxonomy of entomogenous fungi is not static.
The infective unit in most fungi is a spore-usually a conidium. Invasion through the respiratory or alimentary tract has been reported. Conidia usually germinate on the cuticle and then penetrate. Enzymes and mechanical forces are involved. In most cases yeast like filaments of mycelium called hyphal bodies are produced which usually float free and apparently multiply in the haemocoel. Some strains produce sufficient toxins in this stage to cause death. After death or even before in those strains which are weak toxin producers, normal thread like mycelium remifies throughout the internal organs. This continues until insect is virtually filled with fungus. Condidiophores are then produced which erupt through the cuticle and produce spores on the outside of the insect.
In Deuteromycetes the conidiophores and conidia are not produced unless the dead insect is in a moist environment.
There is a great influence of environment condition on germination of spores. The epicuticle is of special significance in disease induction since it is bere the fungal spore must geminate before it can reach the underlying haemocoel and vital organs. At germination one or more germ tubes (short hypae) are produced from each spore.
Penetration of Cuticle:
Germ tubes may grow on the surface of the insect for short distances or may begin to penetrate the cuticle immediately. Appressoria have been found in some fungi. There are the swelling produced at the end of short germ tubes which attach to the cuticle and send infection pegs into the host. They are considered to provide firm attachment that the fungus needs to physically force its way into the host.
Insect pathogenic fungi produce toxins in two overlapping categories.:
(1) Those toxin per os and
(2) Those toxic on injection.
The former may have potential as insecticides white the later are useful in elucidating the modes of action pathogens. The normal site for toxin production is the haemolymph. Materials to do on injection are produced in vitro by several insect pathogens. Two such materials of known chemical composition from entomogenous fungi are:
(1) Destrusins A and B from Metarrhizium anisopliae and
(2) Beanvericin from Beauveria basslana.
Under field condition the disease development is mainly influenced by various aspects of three main agents pathogen, host and environment which interact and change at same time. Besides temperature and humidity as major factors are microclimate, light, air currents, host population density, stress, host activity antagonisms or synergism of other pathogens and genetic makeup of disease. The pathogen is mainly influenced by its
c) Inoculums size and
d) Virulence. Spores are widely disseminated by wind and splashing of rain. Sunlight and humidity may kill spores. As far as host insect is concerned, the host population density and susceptibility to fungi govern the disease development.
The most successful examples are with Beavaria bassiana, Metarrhiziur anisopliae and few species of Entomophthora. The entomogenous fungi can be used 3 different ways for insect control.
The fungus is introduced into as insect population where it becomes permanently established. Pest insects are killed year after year from one or few introduction.
2) Microbial Insecticides:
Applications are made repeatedly as required to control pest population like insecticides.
3) Integrated Control:
Control techniques are selected which have minimum adverse effects on natural mortality factors like parasites, predators and pathogens.