Pathogens are parasitic micro-organisms that cause disease in their host. They play a role in biological control of pest insects. Insects that are infested by a disease will become less active and may finally die.
Pathogens can be divided in 3 distinct groups: fungi, bacteria and viruses.
Several fungal pathogens (e.g. Verticillium, Entomophthora, Beauveria) are known to be controlling various insect pests such as thrips and aphids. To be effective most fungi require humid conditions. They are therefore usually more effective in densely planted crops.
Some of these fungi are being mass produced commercially.
The bacterium Bacillus thuringiensis is a well-known pathogen that is commercially available. It is especially effective against caterpillars. After eating leaves that are covered with the product, the caterpillars slow down and die after few days.
Viruses that cause disease in insects are usually quite specific to their host. They occur often naturally in the field. Farmers will sometimes find dead caterpillars in the crop, which were killed by a virus. These virus-infested caterpillars can be grinded up in water. Spraying this solution over the crop will further spread the disease amongst caterpillars of the same species.