Mosquito Dunks and Mosquito Bits use the same active ingredient, a naturally occurring bacterium called BTI, but they’re designed for different situations. Dunks are slow-release rings that last 30 days or more. Bits are small granules that release BTI much faster, working within hours but lasting up to 14 days. Choosing between them depends on whether you need long-term prevention or a quick knockdown of larvae that are already thriving.
How BTI Kills Mosquito Larvae
Both products rely on Bacillus thuringiensis subspecies israelensis, a soil bacterium that produces multiple toxins lethal to mosquito larvae. When larvae feed in treated water, they ingest the BTI. The toxins target specific proteins in the larvae’s gut lining, essentially destroying their digestive system from the inside. One key toxin helps the others penetrate gut cells even when the larvae lack certain surface receptors, which is part of why BTI remains effective and resistance is rare.
BTI only works on larvae. It has no effect on adult mosquitoes, pupae, or eggs. This means timing matters: you need BTI in the water while larvae are actively feeding.
Mosquito Dunks: Slow, Steady Prevention
A Mosquito Dunk is a small, beige, donut-shaped disc that floats on standing water. As it slowly dissolves, it releases BTI at the water’s surface. The larvicide gradually settles through the water column, where feeding larvae ingest it. Each dunk covers up to 100 square feet of water surface, regardless of depth, and stays effective for at least 30 days.
That long duration makes dunks ideal for set-it-and-forget-it situations: ponds, rain barrels, drainage ditches, water features, livestock troughs, or any body of standing water you can’t easily drain. You drop one in and replace it monthly throughout mosquito season. For smaller containers like birdbaths or plant saucers, you can break a dunk into quarters or smaller pieces and scale the dose to fit.
The tradeoff is speed. Because dunks release BTI gradually, they take longer to reach full effectiveness. If you already have a heavy larva population wriggling in a neglected rain barrel, a dunk alone won’t deliver an immediate kill.
Mosquito Bits: Fast-Acting Granules
Mosquito Bits are corn-based granules coated with BTI at a concentration of about 2.86%. They’re much smaller than dunks and release their payload faster, making them the better choice when you need to knock out an existing larval population quickly. You sprinkle the granules directly onto standing water, and larvae begin dying within hours.
Bits work well for smaller, harder-to-reach breeding sites: tree holes, clogged gutters, flower pot saucers, old tires, bromeliads, and similar spots where water collects. They can also be scattered over low-lying areas before a rain to treat puddles as they form. The downside is duration. Bits continue releasing BTI for up to 14 days, roughly half the lifespan of a dunk, so you’ll need to reapply more often for ongoing control.
Using Both Together
Many people get the best results by combining the two products. The common strategy is to use Bits first to wipe out larvae that are already present, then drop in a Dunk for sustained, month-long prevention. This pairs the quick knockdown of granules with the extended release of the donut, covering both the immediate problem and the next wave of eggs that hatch.
For large areas like ornamental ponds or neglected swimming pools, scatter Bits across the surface while placing a Dunk (or several, based on square footage) to float. This eliminates the gap period where a Dunk alone hasn’t yet released enough BTI to be fully effective.
Mosquito Bits for Fungus Gnats
Bits have a second life indoors. Houseplant owners use them to kill fungus gnat larvae, which breed in moist potting soil. The method is simple: soak about 4 tablespoons of Bits in roughly one gallon of warm water for at least 30 minutes. Warmer water speeds up the BTI release. Strain out the granules, let the water cool to room temperature, then use it to water your plants.
Top watering works best because fungus gnat larvae tend to concentrate in the top two inches of soil. If you bottom water, leave the pot soaking long enough for moisture to wick all the way to the surface. Repeat this “mosquito bit tea” every time you water for two to three weeks to catch successive generations of larvae as they hatch. Dunks aren’t practical for this purpose because they dissolve too slowly to create a usable soil drench.
Safety for Pets, Fish, and Wildlife
BTI is one of the safest pesticides available. The EPA classifies it as practically nontoxic to mammals, birds, fish, and bees. In feeding studies, rats showed no negative effects or infection even at large doses. Bobwhite quail and mallard ducks fed BTI at 3,100 mg per kilogram of body weight daily for five days showed no toxicity. Trout, bluegill sunfish, and sheepshead minnows were similarly unaffected. Nearly all studies examining BTI’s impact on non-target aquatic animals found no harmful effects.
This means you can safely use both Dunks and Bits in birdbaths, pet water bowls, fish ponds, and horse troughs. Dogs drinking from treated water, birds bathing in it, and fish swimming in it are not at risk. BTI targets the gut biology of mosquito and fungus gnat larvae specifically, leaving other organisms alone.
Quick Comparison
- Form: Dunks are floating rings; Bits are small granules.
- Speed: Dunks take longer to reach full strength; Bits begin killing larvae within hours.
- Duration: Dunks last 30+ days; Bits last up to 14 days.
- Coverage: One Dunk covers up to 100 square feet; Bits are sprinkled to fit any size area.
- Best for prevention: Dunks, thanks to their month-long release.
- Best for active infestations: Bits, thanks to rapid BTI release.
- Indoor use for fungus gnats: Bits, soaked into a water drench.