Mosquito dunks are effective against fungus gnats because the gnat larva shares a biological susceptibility with mosquito larvae. The active ingredient is a bacterium that specifically targets certain fly larvae, offering an environmentally conscious way to manage pests in houseplant soil. This method eliminates the destructive, soil-dwelling stage of the gnat’s life cycle.
Identifying the Target Fungus Gnat Life Cycle
Fungus gnats (often species of Bradysia) progress through four distinct life stages: egg, larva, pupa, and adult. The adult gnats are the small, dark flies seen fluttering around houseplants, which are primarily a nuisance and responsible for laying eggs in moist soil. Female gnats seek the damp, rich environment of potting mix to lay hundreds of tiny eggs, often near the soil surface.
The most damaging and susceptible stage is the larva, a translucent, legless grub with a shiny black head. These larvae live in the top few inches of the growing medium, feeding on decaying organic matter and fungi. They also chew on delicate plant roots and root hairs, which can lead to stunted growth and yellowing foliage, especially in young plants. After feeding for approximately two to three weeks, the larvae transition into the pupa stage before emerging as flying adults.
The Active Ingredient How Bti Works
The effectiveness of mosquito dunks against fungus gnats stems from their active ingredient, a naturally occurring bacterium called Bacillus thuringiensis israelensis, or Bti. Bti is a soil-dwelling microorganism that produces protein crystals during its spore-forming phase, which are known as delta-endotoxins. These protein crystals are harmless to nearly all non-target organisms, including humans, pets, birds, and beneficial insects.
Mechanism of Action
The mechanism of action requires a specific biological environment to activate. When the gnat larva ingests the Bti spores and crystals while feeding, the alkaline conditions of its midgut dissolve the crystalline proteins, releasing protoxins. These toxins bind to specific receptor sites on the midgut lining, creating pores in the cell membranes.
This process causes the cells to swell and rupture, leading to gut paralysis. The larva typically dies within 24 to 48 hours.
Practical Application for Soil Treatment
To control fungus gnats in potted plants, the active Bti agent must be transferred from the dunk into the soil’s moisture. This is typically done by creating a Bti-infused water solution, often called “Bti tea” or a soil drench. Soak a portion of a dunk—about a quarter of the disc—in one gallon of water for at least 30 minutes, or preferably overnight, to allow the bacteria to disperse.
Once the solution is prepared, the solid dunk material should be removed and the Bti-infused water is used to water the affected plants as usual. Applying this solution thoroughly ensures the Bti reaches the top layer of soil where the larvae are actively feeding.
Since Bti only kills the larval stage and not the eggs or flying adults, consistent application is necessary. The treatment should be repeated every time the plants are watered for several weeks to eliminate successive generations of larvae. For best results, combine this method with yellow sticky traps to capture the egg-laying adult gnats.