Pests are insects that damage crop plants and stored products. They feed on leaves and roots or suck the sap of the plants causing severe crop losses. Chemical pesticides sprayed on plants can be detrimental to ecosystem if the usage is prolonged and pests may develop resistance to the pesticides.
The term biopesticides refers to compounds that are used to manage agricultural pests by means of specific biological effects. It refers to products containing biocontrol agents like natural substances such as plants, certain minerals, animals, micro organisms including their genes or metabolites.
They are an important part of Integrated Pest Management (IPM) strategy in controlling the pest.
They are less toxic to humans and environment and they do not leave harmful residues.
They affect only the target pest.
They cause long term suppression of pest populations since they persist in the environment.
Microbial biopesticides are of three kinds
1. Bacterial biopesticide
2. Fungal biopesticide
3. Viral biopesticide
Bacteria like Bacillus thuringiensis, Bacillus papillae and Bacillus lentimorbus have the potential to kill certain insect pests and are entomopathogenic.
It is a gram positive, spore forming, rod shaped soil bacterium. During sporulation, the bacterium produces insecticidal proteins as parasporal crystals. These are called delta endotoxin also called as Cry proteins. Cry proteins are specifically toxic to insects belonging to Lepidoptera, Coleoptera and other few insect orders.
The Bt cells sprayed on the leaves have to be ingested by the larval forms of the insects in order to exert its action. This is because the Bt toxin gets activated in the insect gut at a specific pH.
· Larvae stops feeding
· Larvae becomes sluggish and static
· Water oozes out from the body
· Larvae dies and falls off the leaf
Various species of Bt are able to work against cotton boll worm, cabbage worm and gypsy moths.
These entomopathogenic fungi attack insects and cause diseases in insect body which lead to insect death. Two prominent fungi used as mycopesticide are
· Beauveria bassiana which causes white muscardine disease
· Metarhizium anisopliae which causes green muscardine disease
Beauveria bassiana, a filamentous fungus belongs to class Deuteromycetes also called imperfect fungi. It can be successfully used against Colarado potato beetle, (Figure 11.11) Codling moth and American boll worm.
This fungus invades the haemocoel of insects through spores. Once the spores attach to the cuticle, it germinates and the hyphae penetrates the insect cuticle (cuticle is the outer membrane of insects) Penetration is aided by formation of appresorium and penetration peg. The fungi secrete chitinases, lipases and proteases which can dissolve the cuticle. The hyphae enter the haemolymph and proliferate and colonise the entire insect and release blastospores. Insect death occurs due to nutrient depletion of the haemolymph or by toxaemia by secretion of toxic metabolites.
Viral insecticides are pathogens that attack insects and other arthropods. Viral pesticides are used to control Lepidopteran larvae like Helicoverpa, Spodoptera sp on Cotton, Corn, Sorghum, tomatoes. Baculoviruses are the commonly used viral biopesticide. They are extremely small and are composed of double stranded DNA. The genus Baculoviruses contains 3 subgroups.
· Nuclear Polyhedrosis viruses (NPVs)
· Granulosis viruses (GVs)
· Non occluded viruses
The virus enters the insect body via ingestion by insects and infects the midgut cells by membrane fusion. The NPV uncoat within the nucleus of cells and pass through the intestinal epithelium (Figure 11.12) and establish a systemic infection of the haemocoel.
Discoloration (larvae turns brown or yellow)
· Decomposition or softening of larvae
· Infected larvae hang upside down twigs
· Larvae become swollen with fluid containing virus and eventually die turning black in color.
NPV are mass produced in laboratory using suitable larval hosts. The fifth stage larvae are fed with food infected with
NPV. After 4-5 days, the dead larvae are collected and macerated. The liquid is centrifuged and the pellet containing the viruses is suspended in sterile distilled water. This viral suspension can be used for spraying in the fields.