Microbes as Biocontrol Agents

Biological Control: An Alternative to Chemical Pesticides

For much of the twentieth century, protecting crops from insect pests and disease-causing pathogens relied heavily on synthetic chemical pesticides and fungicides. While often effective in the short term, these chemicals bring serious downsides: they frequently kill beneficial insects (including natural predators of the pests they are meant to control) alongside the target pest, they can accumulate in soil, water and the food chain, and repeated use encourages the evolution of pesticide-resistant pest populations. Biological control -- the deliberate use of living organisms, or substances derived from them, to suppress pest and pathogen populations -- offers an alternative that is generally more selective and environmentally sustainable. This section covers three major categories of microbial biocontrol agents: a bacterium, a fungus, and a virus.

Bacillus thuringiensis: A Bacterial Biopesticide

Bacillus thuringiensis, often abbreviated Bt, is a soil bacterium that produces protein crystals (Cry proteins, or Bt toxin) that are toxic specifically to the larvae of certain insects, particularly moths, butterflies and some beetles. When these Bt spores and crystals are sprayed onto crop plants as a biopesticide and subsequently ingested by a susceptible insect larva feeding on the plant, the crystal protein is activated in the insect's alkaline gut, where it binds to specific receptors on the gut lining, damaging the gut wall and eventually killing the insect. Because Cry proteins bind only to receptors present in the guts of particular target insect groups, Bt is highly selective and largely harmless to humans, other mammals, birds and beneficial insects like bees. This selectivity has also made the Bt toxin gene a foundational tool of agricultural genetic engineering: the gene encoding the Cry toxin has been isolated from Bacillus thuringiensis and introduced directly into the genome of crop plants such as cotton (producing what are commonly called Bt cotton varieties), enabling the plant itself to produce the insecticidal protein and resist attack by susceptible pests without external spraying.

Trichoderma: A Fungal Biocontrol Agent

Trichoderma is a genus of free-living fungi commonly found in root ecosystems, valued in agriculture for its ability to act as a biocontrol agent against several plant-disease-causing fungi. Trichoderma species protect plant roots in more than one way: they can directly parasitise and destroy the hyphae of pathogenic fungi, they compete aggressively with pathogens for the same limited nutrients and root-surface space, and some strains produce antifungal compounds that suppress pathogen growth. Trichoderma is commonly used to control soil-borne root-disease fungi belonging to genera such as Pythium and Fusarium, both of which cause serious root rot and wilt diseases in many crop species.

Baculoviruses: Species-Specific Viral Biopesticides …