Microbial insecticides

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Microbial insecticides : 

Microbial insecticides

introduction : 

introduction Insect population is the largest with more than 750000 species Negative effects of insects Chlorinated hydrocarbons,organophosphates Synthetic chemical insecticides provide many benefits to food production and human health, but they also pose some hazards. alternative methods of insect management offer adequate levels of pest control and pose fewer hazards.

Disadvantages of chemical Insecticides : 

Disadvantages of chemical Insecticides extensive pollution of the environment, serious health hazard due to the presence of their residues in food, fiber and fodder increasing cases of insects developing resistance, e.g., Helicoverpa has become resistant to most of the insecticides.

Bioinsecticides : 

Bioinsecticides Microorganisms used for insect control are often called bioinsecticide The term biopesticide is used for all biocontrol agents. Viruses, bacteria, fungi, protozoa and mites are employed to control a variety of insects attacking both plants and animals. Microbial insecticides can be Microbially produced toxic substance organism

Advantages of Microbial Insecticides : 

Advantages of Microbial Insecticides The organisms used in microbial insecticides are essentially nontoxic and nonpathogenic to wildlife, humans, and other organisms not closely related to the target pest. The toxic action of microbial insecticides is often specific to a single group or species of insects most microbial insecticides can be used in conjunction with synthetic chemical insecticides because in most cases the microbial product is not deactivated or damaged by residues of conventional insecticides.

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residues present no hazards to humans or other animals, the pathogenic microorganisms can become established in a pest population or its habitat and provide control during subsequent pest generations or seasons.

Disadvantages of Microbial Insecticides : 

Disadvantages of Microbial Insecticides they are specific to only a well defined range of target species. Heat, desiccation (drying out), or exposure to ultraviolet radiation reduces the effectiveness of several types of microbial insecticides. Special formulation and storage procedures are necessary for some microbial pesticides. Because several microbial insecticides are pest-specific, the potential market for these products may be limited

types : 

types Bacterial Viral Fungal protozoan

Bacterial insecticides : 

Bacterial insecticides Bacterial pathogens used for insect control are spore forming, rod-shaped bacteria in the genus Bacillus. They occur commonly in soils, and most insecticidal strains have been isolated from soil samples. Bacterial insecticides must be eaten to be effective; they are not contact poisons. Insecticidal products comprised of a single Bacillus species may be active against an entire order of insects, or they may be effective against only one or a few species.

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Bacillus thuringiensis var. kurstaki kill the caterpillar stage of a wide array of butterflies and moths. In contrast, Bacillus popillae (milky spore disease) kills Japanese beetle larvae but is not effective against the closely related annual white grubs that commonly infest lawns.

Bacillus thuringiensis : 

Bacillus thuringiensis Bt is a gram-positive, aerobic, endospore-forming bacterium Bt, however, is recognized by its parasporal body (known as the crystal) that is proteinaceous in nature and which possesses insecticidal properties. The parasporal body comprises of crystals varying in size, shape and morphology. The crystals are tightly packed with proteins called protoxins or-endotoxins.

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There are many subspecies and serotypes of Bt with a range of well-characterized insecticidal proteins or Bt toxins At present it has been estimated that over 60,000 isolates of Bt are being maintained in culture collections worldwide. Known Bt toxins kill subsets of insects among the Lepidoptera, Coleoptera, Diptera and nematodes.

How Bt works? : 

How Bt works? Bacillus thuringiensis Cry proteins and the bacterium itself are used as pesticides. If the plant has been genetically modified, it contains the Cry protein within itself. If a pesticide has been used, the plant is coated in the bacteria. Both accomplish much of the same effect, but the Genetically Modified method tends to work better.

Mode of action : 

Mode of action Three stage process Stage1 Stage2 stage3

Stage1 : 

Stage1 the active crystal protein is attached to the Bt spore. This spore is ingested by an unfortunate organism, and the work begins. The spore and crystal enter the digestive tract, where they make their way to the organism's gut.

Stage2 : 

Stage2 The crystal surrounding the protein breaks off from the spore and begins to dissolve. Once the crystal has been completely dissolved, the toxin is activated. In order for the crystal to be dissolved and the toxin to become activated, the pH level must be within a very limited range Thus, only specific organisms will have the correct pH balance to release the toxin.

Stage 3 : 

Stage 3 The toxin binds itself to receptors in the gut Once the toxin binds itself to the membrane, it begins to bore holes through it. It completely perforates the gut membrane. Multiple toxins will bind and follow these steps, until the creature has been paralyzed, or starves to death. The creature is paralyzed because its stomach acids leak out into the rest of the body causing serious damage (pleasant).

application : 

application Spray when caterpillars are still small. Completely cover all leaf surfaces. The insects must ingest the bacteria when they are feeding. Spray in the evening or during cloudy (but not rainy) days. There may be a need to reapply if it rains soon after application.

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Caterpillars will take several days to die, but they will stop feeding soon after infection. Bt is not persistent. It may need to be reapplied. Follow label instructions. Mix only enough product needed and use immediately. This is especially important in areas with high pH water (alkaline). Use a spreader-sticker to enhance product coverage on leaves

Other bacterial insecticides : 

Other bacterial insecticides Bacillus popillae Bacillus lentimorbus Milky spore disease

Commercial Bt products : 

Commercial Bt products

Viral pesticides : 

Viral pesticides More than 450 viruses are used Epizootic Baculoviridae Pox Reo Irido Parvo Picorna rhabdo

Viruses occur as : 

Viruses occur as NPV:nuclear polyhedrosis virus develops in the host cell nuclei Virions are occluded singly or in groups in polyhedral inclusion bodies CPV: cytoplasmic polyhedrosis virus Develops only in the cytoplasm of host midgut epithelial cells Virions are occluded singly in polyhedral inclusion bodies

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GV:granulosis virus Develops in either nucleus or cytoplasm Virions are occluded in single or pairs in small occlusion bodies called capsules

General Overview : 

General Overview Insect-specific viruses can be highly effective natural controls of several caterpillar pests. Insect viruses need to be eaten by an insect to cause infection but may also spread from insect to insect during mating or egg laying No threat to humans or wildlife is posed by insect viruses. The successful commercialization of insect-pathogenic viruses has been limited

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NPV affects alfalfa looper, corn earworm, imported cabbageworm, cabbage looper, cotton bollworm, cotton leafworm, tobacco budworm, armyworms, European corn borer, almond moth, spruce budworm, Douglas fir tussock moth, pine sawfly and gypsy moth. granulosis virus have been isolated from several caterpillar species, including imported cabbageworm, cabbage looper, armyworm, fall webworm, and mosquitoes, among many others.

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Viruses invade an insect's body via the gut. They replicate in many tissues and can disrupt components of an insect's physiology, interfering with feeding, egg laying, and movement infected larvae may initially turn white and granular or very dark. "caterpillar wilt" or "tree top" disease Mass reared viruses have been successfully applied in limited areas as microbial insecticides against pests.

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Infected caterpillars have been mashed into a water solution and applied to pest populations as a form of microbial insecticide

Baculoviruses : 

Baculoviruses Rod shaped DNA viruses Include NPV and GV Pathogenic for Lepidoptera,Hymenoptera and Diptera Infection is by injestion of food Virally induced infertility was a potent control measure Effective against catterpillars,alfalfa,white butterflies,cabbage loppers,cotton bollworms,corn ear worms etc

Life Cycle : 

Life Cycle the baculovirus life cycle is divided temporally into immediate early, early, late, and very late phases. Viruses enter the cell by adsorptive endocytosis and move to the nucleus, where their DNA is released. DNA replication begins ~6 hr after infection and is followed by viral assembly in the nucleus of the infected cell.

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Two types of viral progeny are produced during the life cycle of the virus: extracellular virus particles (nonoccluded viruses) during the late phase and polyhedra-derived virus particles (occluded viruses) during the very late phase of infection. Extracellular virus is released from the cell by budding, beginning at ~12 hr postinfection,. Polyhedra-derived virus, on the other hand, appears in the nucleus at ~18 hr postinfection.

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Occluded viral particles are embedded in proteinaceous viral occlusions called polyhedra within the nucleus of infected cells. The polyhedrin protein (29 kDa) is the major protein component of the occlusion bodies. baculoviruses are lytic, they quickly kill their insect host after infection. The virus is transmitted when occlusion bodies are ingested by a new host as it feeds on a contaminated food source

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The polyhedrin protein dissolves in the alkaline environment of the new host's gut and the occluded virus is released. This virus infects the gut epithelial cells and virus replication takes place. Nonoccluded virus is then produced and budded from the infected gut cells

Baculoviruses : 

Baculoviruses Helocoverpa zea nuclear polyhedrosis virus (HzSNPV) Orgyia pseudotsugata (Ot) MNPV Lymantria dispar (Ld) MNPV Autographa californica nuclear polyhedrosis virus. Ac MNPV

Fungal insecticides : 

Fungal insecticides Entamogenous fungi was found to be effective Beauveria Metarrhizium Entomophthora Hyphomycetes Coelomomyces Deuteromycotina Zygomycotina

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Effective against coloradbeetle,gypsymoth,leaf hoppers,mosquitoes Successful sporulation and spread of pathogen requires 100% moisture Act slowly and give limited protection to crops Many fungi produce biologically active secondary metabolites, some of which are very toxic and this is a major concern with all fungal BCAs as their presence would represent a health risk.

Mode of action : 

Mode of action Formation of an Infection Structure Penetration of the Cuticle Production of Toxins

Formation of an Infection Structure : 

Formation of an Infection Structure Entomopathogenic fungi invade their hosts by direct penetration of the host exoskeleton or cuticle. conidia germinate on the host surface and often differentiate to form an appressorium. penetrates down through the host cuticle and eventually emerges into the haemocoel of the insect.

Penetration of the Cuticle : 

Penetration of the Cuticle entry into the host involves both enzymic degradation and mechanical pressure. endoproteases and aminopeptidase N-acetylglucosaminidase

Production of Toxins : 

Production of Toxins Growth of fungi in the haemolymph of insects may be as yeast-like blastospores, hyphal bodies or protoplasts Many fungi do not penetrate internal organs before the death of the host, and may kill the host by consuming nutrients in the haemolymph. A high-molecular-mass insecticidal protein toxin (>10 kDa) has been extracted from the haemolymph of infected insect eg:Destruxins

Mode of Action : 

Mode of Action Fungi invade insects by penetrating their cuticle or "skin". Once inside the insect, the fungus rapidly multiplies throughout the body. Death is caused by tissue destruction and, occasionally, by toxins produced by the fungus

Life cycle : 

Life cycle

Protozoan insecticides : 

Protozoan insecticides Not suited for short time ,quick acting microbial insecticides Must be applied before outbreak of the disease Effective against grasshoppers,mosquitoes,boll weevils they reduce host reproduction or feeding rather than killing the pest.

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must be eaten to infect an insect, but there many also be some natural transmission within a pest population, The pathogen enters the insect body via the gut wall, spreads to various tissues and organs, and multiplies, sometimes causing tissue breakdown and septicemia. Infected insects may be sluggish and smaller than normal, sometimes with reduced feeding and reproduction.

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