Pathogens are disease-causing microorganisms that infect the pest, usually eventually killing it. Pathogen groups include:
- Fungi
- Viruses
- Bacteria
- Nematodes
Helicoverpa larva infected by Nomuraea fungus
Fungi
Fungal diseases are most common in moist environments, and may be more prevalent in wetter than average seasons. Fungal spores germinate and hyphae penetrate the insect skin (cuticle), grow in the insect’s body, and may release toxins. Infected insects become covered in hyphae (often appears as white fur) and fruiting bodies develop. The carcass typically becomes anchored to the plant surface by the hyphae.
Useful fungal genera include Beauveria, Metarhizium, Hirsutella, Entomophthora, Nomuraea and Verticilium.
Unfortunately, consistent mass production of fungi as a biopesticide is difficult as specific temperature and humidity conditions are required for successful insect control, and the formulations have a limited storage life outside the host. However, a few fungal pathogens that are being pursued as commercial preparations for application in field situations e.g. Metarhizium for black field crickets.
Viruses
Viral diseases are often highly specific. Viral proteins damage the insect’s gut lining.
They are more easily commercially produced and applied than fungal preparations. The most commonly available is the caterpillar-specific Nuclear polyhedrosis virus (NPV).
Bacteria
Bacterial cells are not usually utilised directly, but a toxin derived from bacteria (Bacillus thuringiensis (Bt)) is widely used. Generally specific to caterpillar pests, formulations are also available for some beetles and mosquito larvae. Bt toxins have also been incorporated into some genetically modified crops, dramatically reducing the number of externally-applied sprays per season.
Nematodes
Entopathogenic nematodes are microscopic worm-like organisms that actively search for insect hosts, enter the host body through natural openings and release bacteria that digest the insect. The nematode then feeds on the bacteria/insect slurry. When the dead insect’s body ruptures it releases more nematodes.
Commercially-produced nematodes are available in dehydrated cellulose mixtures (these are rehydrated before use). Research on using nematodes for the control of snails is currently underway.
