USE OF FUNGI TO CONTROL INSET PEST -
As insect biocontrol agents, fungi are markedly superior to other micro-organisms because they are generally non-specific in their action & are useful against a wide range of insect pests. On the debit side, however, their use is limited by the restricted range of humidities in which they can grow,. An absolute humidity in which they can grow, an absolute humidity greater than 90% for a relatively long period is needed to enable fungi to grow & infect insect hosts. Another disadvantage is that fungi rarely, if ever, kill insects rapidly in the manner that we have come to associate with chemical insecticides.
Most of the so-called entomopathogenic fungi are phycomycetes & Deuteromycetes. Spores of these fungi attack either the external or gut cuticle of their insect hosts. They then germinate & hyphae penetrate the haemocoel. Death may result from the production by the fungus of a toxin, or following the direct utilization of the body fluids. After the insect body reserves have been used up, the mycelium then dies from lave of substrate & sporulates, the spare mass then acts as a focus of future infection.
Insecticidal toxins produced by fungi are non-enzymic, low molecular weight products, which can kill insects when present even at low concentrations. The best examples of the use of fungi to control insects are provided by species of Beauveria & Metarhizium, fungi which respectively cause white & green muscardine disease of insects.
The ideal mycoinsecticide should rapidly penetrate the cuticle of the insect, produce toxins to induce rapid paralysis, rapidly colonise the insects body & then sporulate under most environmental conditions. An obvious problem is that fungi used to control insects may be killed by fungicides sprayed on insect- infested crops.
Table – Principal Deuteromycete fungal candidates for arthropad.
|SPECIES||MAIN TARGET PESTS|
|1.||Aschersonia aleyrodis||White fly|
|2.||Beauveria bassiana||Colorado beetle|
|4.||Hirsutella thompsonii||Rust mites|
|7.||Verticillium lecanii||Aphids, Whitefly|
COMMERCIAL PRODUCTION OF FUNGAL INSECTICIES -
Fungi can be readily produced for use as fungicides, as they are easy to grow on a variety of substrates, including agar-based media, cereal grains & a variety processed & unprocessed wastes; cereal grains are generally considered the best & most economical substrates for this purpose.
Product self-life is a major limitation of fungal biocontrol agents. A period of 18 months survival at 20� C is thought to be optimal, although production methods in current use can only achieve survival rates of around 3-6 months at storage temperatures of 40C Biocontrol fungi are usually mixed or formulated with substrates to increase their survival, in some cases colours or chemicals may be added to attract the insect host to the fungal pathogen.
USE OF FUNGI TO CONTROL NEMATODES
Fungi that parasitise nematodes (nematophagus fungi) can be divided into nematode trapping fungi, endoparasitic species & fungi that parasitise nematode eggs. Nematode trapping fungi capture nematodes with specialised structures such as constructive & non-constructive rings, adhesive knobs or lastly by producing an adhesive material along the entire mycelial surface. Endoparasitic nematophagous fungi line in soils where they produce adhesive spores. These become attached to body of the nematode, on germination, a germ tube enters the body where it grows & consumes the host. Egg parasites, as their name suggests, are nematophagous fungi that parasitise the eggs of nematodes.
The following approaches have been used in an attempt of use fungi to control nematodes: 1) Exploitation of naturally supressive soils.
2) Soil amendments to encourage indigenous bio-control agents.
3) Inoculation of soils with selected strains of bacterial & fungi.
4) The use of microbal toxins & enzymes
1) Supressive soils-
Certain soils are naturally supressive to nemotodes. Fungi implicated in naturally supressive soils include Nematopthasa gynophila, Verticillium Chlamydosporium & Dactylella Oviparasitica.
2) Soil amendments -
A wide range of amendments have been found to be effective in controlling soil nematode populations. These generally involve breakdown products of added organic materials, including fatty acids & ammonia which together with an increase in natural antagonists prove effective in reducing nematode populations. Chitin amendment has been particularly extensively studied as a means of increasing populations of specific nematods antagonists.
3) Application of selected fungi -
To date, Paecilomyces lilacinus is the only fungus that has been widely tested for nematode control under field conditions.
4) Enzymes & toxins -
The direct application for enzymes, such as chitinase from egg parasitising fungi & collagenase from nematode – trapping species, to soils may also be useful means of controlling nematodes. Potentially useful nematocidal toxins are produced by fungi such as the oyster mushroom, pleurotus ostreatus.