Mosquito season is coming up—and maybe it’s already here if you live in a warmer place. We’ve already explained how to keep mosquitoes from biting you (DEET or other effective spray for your skin and a box fan for your porch or deck) but let’s talk about preventing them from buzzing around in the first place. You need mosquito dunks.

Mosquitoes need stagnant water to breed. Their babies start out as little worm-like creatures that breathe through their butts and eat algae and bacteria until they are ready to become the biting, flying nemeses we all know so well. This is why you’re supposed to cover or dump out any containers that collect water weekly. But what about puddles, potholes, and containers like rain barrels that can’t easily be covered or dumped? That’s what mosquito dunks are for.

What are mosquito dunks?

Mosquito dunks are doughnut-shaped pellets that you can float in standing water. (You can also run a string or stake through the hole, if you need a way to keep it in place.) As the dunk dissolves, it releases bacteria that mosquito larvae will eat. So far, that sounds great for the mosquitoes.

But the bacteria in the dunks are Bacillus thuringiensis israelensis (Bti), which are harmless to people and most animals, but poisonous to baby mosquitoes. You can safely use them in fish ponds or birdbaths, and the EPA notes that while Bti will also kill the larvae of black flies and gnats, it does not affect honeybees or most other insect species.

Here are some mosquito dunks to consider:

These dunks last 30 days and treat 100 square feet of water surface. (You can break a dunk into pieces to treat smaller areas.)

Mosquito bits are the same idea as dunks, but in smaller pellet form.

Have a lot of mosquito-breeding areas? Buy dunks in bulk.

Do mosquito dunks actually work?

Unlike a lot of mosquito-repelling yard hacks, mosquito dunks are legit. Both the CDC and EPA recommend them for standing water you’re unable to cover or drain. This could include ornamental features of your yard like koi ponds and bird baths, or places that collect water by accident, like gutters or ditches that fill up with water, or unused swimming pools. (For swimming pools that are actually in use, the chlorine is enough to keep mosquitoes out.)

Note that dunks prevent mosquitoes from breeding, but they don’t kill existing mosquitoes. This means you shouldn’t expect an immediate reduction in their numbers from using a dunk. But their life cycle is short, running from a few days to a few weeks, depending on the species and the climate. That means if you eliminate the places that larvae can thrive, you’ll likely notice reduced numbers of mosquitoes later on this summer.

If you’d like to speed up the process, you can try using a dunk to create a “Bucket of Doom.” This is a trap for mosquito mothers to lay their eggs in a nice, swampy bucket—which you happen to have treated with a dunk.