Biopesticides - Fungal and Viral

Fungal Biopesticides

Entomopathogenic fungi parasitise insects, and can kill or seriously disable them. They are considered natural, environmentally-safe control agents, so used worldwide for biological control of insects and other arthropod pests. Fungal biopesticides are so diverse in nature and their means of affecting the target pest can be diverse. 

The most common modes of action are through competitive exclusion, mycoparasitism, and production of metabolites. Some fungi can exhibit all of these modes of action. 

The most common commercial fungal biopesticides are Trichoderma spp., Metarhizium anisopliae, Verticillium lecanii and Beauveria bassiana. Each are frequently used in the nursery, ornamental, vegetable, field crop, and forestry industries to control a variety of pests.

Generally, entomopathogenic fungi have a common mode of action.  When the spores of the fungus come into contact with the body of an insect host, they germinate and form an infection bulb or appressorium. From it hyphae emerge and penetrate the cuticle. The fungus then develops inside the body, forming mycelium which invades throughout the insect body thus draining the insect of its nutrients. Eventually insect is killed, after a few days. This lethal effect is aided by the production of specific insecticidal toxins-destruxins, bassianolides etc. Spores are liberated from the dead cadaver of insect and the cycle continued.

Metarhizium anisopliae formerly known as Entomophthora anisopliae, is an entomopathogenic fungus that grows naturally in soils throughout the world. It is commonly called green muscardine fungi. Mechnikov named it after the insect species it was originally isolated from, the beetle Anisoplia austriaca. The disease caused by the fungus is sometimes called green muscardine disease because of the green colour of its spores. Initially, fungal hyphae appear white, but, as conidia form and mature they often take on a characteristic olive green color.  When the spores of the fungus come into contact with the body of an insect host, they germinate and the hyphae that emerge penetrate the cuticle. The fungus then develops inside the body, draining the insect of nutrients, eventually killing it after a few days. This lethal effect is aided by the production of insecticidal cyclic peptides (destruxins). 

Metarhizium species are also known to produce compounds that are toxic to arthropods. Other insects that come in contact with infected insects also become infected with the fungus. M. Anisopliae and its related species are used to control a number of pests such as termites, thrips, etc. M. Anisopliae does not appear to infect humans or other animals and is considered safe as an insecticide. The microscopic spores are typically sprayed on affected areas. 

Beauveria bassiana, named after the Italian entomologist Agostino Bassi, who discovered it in 1815 as the cause of white muscardine disease in insects is a fungus that acts as a parasite on many insect species. B. Bassiana can attack a broad range of insects. When the microscopic spores of the fungus come into contact with the body of an insect host, they germinate, penetrate the cuticle, and grow inside the insect’s body, feeding on internal tissues and releasing insect toxins like beauvericin, beauverolides, bassianolide etc. As the insect dies, it changes color to pink or brown (due to oosporein pigment) and eventually the entire body cavity is filled with fungal mass killing the insect within a matter of days. 

Beauveria bassiana can be used as a biological insecticide to control termites, whiteflies, aphids, grasshoppers, flies, beetles, caterpillars etc. The fungus rarely infects humans or other animals, so it is generally considered safe as an insecticide. 

Verticillium lecanii

Verticillium lecanii is an entomopathogenic fungus. The mycelium of this fungus produces a toxin called bassianolide and other insecticidal toxins such as dipicolinic acid, which infect aphids, whiteflies, rust fungi, scale insects and lead to the death of the host. The spores of this fungus when come in contact with the cuticle (skin) of target insects, germinate and grow directly through the cuticle to the inner body of their host. The fungus proliferates throughout the insect’s body, draining the insect of nutrients, and eventually killing it in around 48-72 hours.

Paecilomyces species

Paecilomyces fumosoroseus, is considered a very promising biological pesticide due to its extensive host range which includes insects in over 25 different families, including moth, Russian wheat aphid, silver leaf whitefly (Bemisia argentifolii) and wide range of mites.  Paecilomyces lilacinus, is another naturally occurring fungus found in many kinds of soils throughout the world. As a pesticide active ingredient, Paecilomyces lilacinus is applied to soil to control nematodes that attack plant roots. It acts against plant root nematodes by infecting eggs, juveniles, and adult females.

Non-entemopathogenic fungi such as Trichoderma spp. (Trichoderma viride, T. harzianum) are some of the most common fungi in nature. These microbial biofungicides can out-compete pathogenic fungi for food and space, and can stimulate plant host defenses and affect root growth. Many beneficial Trichoderma have the ability to readily colonize plant roots, without harming the plant. This close relationship with the plant that make these species excellent biocontrol agents.  They are highly active on root & stem rots caused by Schlerotinia, Rhizoctonia, wilts caused by Fusarium and blights/leaf spots caused by Alternaria, Downy Mildews & powdery mildews, Trichoderma viride has been highly effective where conventional fungicides are not able to control the root diseases. 

Viral Biopesticides 

Microbial biopesticides known as baculoviruses are a family of naturally-occurring viruses known to infect and destroy a number of important plant pests only insects and some related arthropods. They are particularly effective against the lepidopterous pests of cotton, rice and vegetables. They are so specific in their action that they infect and kill only one or a few species of Lepidopteran larvae (caterpillars), making them good candidates for management of crop pests with minimal off-target effects. 

Baculoviruses used as microbial biopesticides consist of DNA surrounded by a protein coat (nucleocapsid), which is embedded in a protein “microcapsule” or occlusion body (OB) that provides some protection from degradation in the environment. Depending on the virus, OBs may contain a single nucleocapsid (granulovirus, or GV) or multiple nucleocapsids (nucleo/cytoplasmic polyhedrovirus, or PV).

Upon ingestion by a susceptible caterpillar, OBs are dissolved within the alkaline midgut, releasing nucleocapsids that infect the cells lining the midgut. The viral DNA replicates in the nuclei of the host cells and then spreads throughout the body of the larvae, essentially turning it into a “virus factory.” The infected insect stops feeding within a few days, dies and disintegrates, releasing billions of new OBs which can be ingested and cause new infection of neighbouring larvae.

 

The granulovirus of the codling moth Cydia pomonella, or CpGV, is a good example of a commercially successful viral insecticide.

Baculoviruses are of two types

1) Nuclear/Cytoplasmic Polyhedrosis                                           2) Granulosis Virus 

1. Nuclear Polyhedrosis Virus (NPV)

∙ Develop in host cell nuclei

∙ Rod shaped, double stranded; virions occluded as groups in inclusion bodies

. Highly host specific-∙ Enters through injection into insect gut through mouth &cuticle

 

∙ Symptoms in larvae are: - Discoloration (brown and yellow) -Stress -Decomposition (liquification) -Lethargy -Infected larvae hang invertedly from twigs - host will become visibly swollen with fluid containing the virus and will eventually die turning black with decay 

 

2. Cytoplasmic Polyhedrosis Virus

∙ Develop only in cytoplasm of host midgut epithelial cells

∙ Virions occluded singly in polyhedral inclusion bodies

∙ Infection confined to midgut and does not spread to other tissues

∙ Infection not always lethal but shows larval growth reduction

∙ Continuously shed infective polyhedra in faeces 

3. Granulosis Virus (GV)

∙ Develop either in the nucleus/cytoplasm/ tracheal matrix / epithelial cells of host 

∙ Virions are occluded singly in small inclusion bodies called capsules; Rod shaped virion, ds DNA

∙ They enter through ingestion, similar to NPV

∙ Fat body is the major organ invaded

∙ Diseased larvae – less active, flaccid, fragile, wilted prone to rupture in later stages,  death in 6-20 days 

 

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