Beauveria bassiana is an entomopathogenic fungus, meaning it is a natural parasite of insects. This fungus is found in soils all over the world and is used as a biological alternative to chemical insecticides. It functions by causing a disease in insects known as white muscardine disease. The fungus does not need to be eaten by an insect to be effective; it only needs to make physical contact with its target. Different strains of B. bassiana exist, some of which can even grow inside plant tissues, offering a zone of protection against pests.
The Fungal Infection Process
The life cycle of Beauveria bassiana begins with its microscopic spores, called conidia, which are the reproductive units of the fungus. For an infection to start, these spores must land on the outer body of a susceptible insect. The spores are hydrophobic, or water-repelling, which helps them adhere to the waxy surface of an insect’s exoskeleton, also known as the cuticle.
Once attached, the spore germinates, a process dependent on favorable temperature and humidity. It produces a fine, filament-like hypha that releases specialized enzymes to break down the insect’s cuticle. This allows the fungus to penetrate the exoskeleton and invade the insect’s body cavity, or hemocoel.
Inside the host, the fungus multiplies rapidly, feeding on the nutrients available in the insect’s blood, or hemolymph. During this growth phase, the fungus also produces toxins that weaken and eventually paralyze the insect host. The infected insect is killed within days as the fungus consumes its internal structures.
After the host has died, the fungus completes its life cycle by growing back out through the softer parts of the insect’s body. It covers the carcass in a distinctive layer of white, cotton-like mold that produces millions of new conidia. These are then released into the environment to infect other hosts.
Common Pest Targets
Beauveria bassiana is effective against a wide range of pests, making it a versatile tool for agricultural and garden settings. Its broad host range covers many soft-bodied insects that are problems for gardeners and farmers. Aphids, thrips, and whiteflies, which feed on plant sap and can transmit diseases, are all susceptible.
The fungus also controls other plant pests like mealybugs and spider mites. In addition, Beauveria bassiana is used to manage various types of beetles. It is effective against the grubs of Japanese beetles in the soil, the emerald ash borer, and the Colorado potato beetle.
Beyond plant pests, research shows Beauveria bassiana is effective against certain household and nuisance pests. Studies have shown it can eliminate bed bugs that come into contact with treated fabric. Its use is also being explored for controlling malaria-transmitting mosquitoes, termites, and grasshoppers.
Commercial Use and Application Methods
Beauveria bassiana is sold commercially in several formulations, providing flexibility for various pest problems. The most common forms are wettable powders and oil dispersions, which are designed to be mixed with water for application.
The primary method of application is a foliar spray, where the solution is sprayed onto the leaves and stems of plants to target pests living on the foliage. For pests that reside in the soil, such as grubs, a soil drench is used. This involves applying the mixture directly to the soil so it can reach the target insects living below the surface.
Thorough coverage of the plant, including the undersides of leaves, is necessary for good control. The environmental conditions at the time of application also influence the success of the treatment.
High humidity and moderate temperatures are ideal for the fungal spores to germinate and infect the host. The spores can be inactivated by the ultraviolet (UV) radiation in direct sunlight, so applying the product in the evening is recommended to prolong its activity. Because the spores are living organisms, mixed solutions should be used quickly and not stored overnight.
Safety for Humans and Beneficial Insects
Beauveria bassiana is considered a reduced-risk pesticide because it is specific to insects and safe for most non-target organisms. The fungus rarely infects humans, pets, or other mammals, although individuals with suppressed immune systems may be at higher risk. When applying products containing the fungus, it is recommended to wear protective clothing, such as long sleeves and a mask, to avoid potential respiratory irritation from the spores.
A common concern is the impact of this biological insecticide on beneficial insects. While a heavy, direct application could potentially harm some beneficials like ladybug larvae, the risk to pollinators such as bees is considered low. Bees have grooming behaviors that are effective at removing spores from their bodies before an infection can take hold.
The targeted nature of Beauveria bassiana stands in contrast to broad-spectrum chemical pesticides, which can harm a wide variety of organisms. The fungus acts through direct contact, which limits its impact on organisms that do not encounter the spores. It can be used on crops up to the day of harvest without posing a risk to consumers, though washing produce is always a good practice.