At least in the United States, microorganisms (bacteria, fungi, viruses) that are natural enemies of pests are classified as biopesticides by the Environmental Protection Agency (EPA). Insects, (and other invertebrates, like spiders) and nematodes are not regulated as biopesticides. Plant extracts, some naturally-occurring chemicals (like potassium bicarbonate), and the products of genes inserted into genetically-modified plants are also regulated as biopesticides.
Microbial biopesticides work against different pests, and in different ways. Some microbe natural enemies kill insect pests while others work against . For example, products that contain the bacterium Bacillus subtilis can help control powdery mildew. Bt bacteria (and some other beneficial microbes) work by producing compounds that do the pest-fighting work for them. The fungus Beauvaria bassiana can kill insect pests outright (especially when the insect is young), while some strains of Bt bacteria will stop an insect pest from feeding. The bacterium Bacillus subtilis and the fungus Trichoderma harzianum can trigger the plant to defend itself from a variety of plant diseases. Streptomyces lydicus bacteria grow over the surface of the plant, taking up so much space that there’s no room for the disease-causing microbe. Many microbes work in multiple ways.