Do Mosquito Dunks and Bits work? Short answer: yes. Mosquito Dunks and similar products containing the natural insecticide Bti really do live up to their promise. They'll start killing mosquito larvae within a day after you drop them in the water. Bti is fatal when larvae eat it, but is harmless to humans and other animals, including fish and frogs.
What Dunks and Bits don't do is solve the problem of adult mosquitoes. They'll get rid of larvae wherever you put them, so they'll reduce the local mosquito problem, and that's a very good thing, but some adult mosquitoes may still hatch elsewhere and fly in, so you still may need some other protection from the remaining adults.
The biggest name in the market is Summit Chemicals in Baltimore, Maryland, which makes Mosquito Dunks and Mosquito Bits. Other companies also make similar products, like Bonide's Mosquito Beater WSP (Water Soluble Pouch) Plunks, and Microbe-Lift BMC (Biological Mosquito Control) which is a liquid formula.
Consumers generally report that all four of those work as promised. In general, the costs are comparible, also. The main reason to choose one type over the other is the slight difference in what they're designed to do.
What's the Difference between Dunks, Bits, Plunks and Liquid BMC?
Bits work fast. If you have water full of mosquito larvae, Bits will kill them quickly, before they can turn into adults, but mosquitoes keep laying eggs and new larvae keep hatching all summer, so just killing the larvae once isn't enough. Since Bits don't have a timed-release feature, you'll need to apply them again 7 to 14 days later.
Dunks dissolve slowly, releasing their Bti for 30 days, so you can add them once a month, but if there are already larvae in the water when you first add them, some may be able to grow into adults before being affected.
One plan of attack is to add Bits at the start of the season, when you already see larvae in the water, to kill them quickly. Then, a few days later, add Dunks and replace them once a month to continue killing the new larvae all summer.
Otherwise, Bits contain the same active ingredient as Dunks, and are just as safe. They just release the Bti all at once, instead of over time.
Bits are also useful to treat small areas, like flower pot trays or tree holes, since you can just sprinkle in a few. However, you can also crumble up Dunks and do the same thing, if an area is too small to need a whole Dunk.
Bonide's Mosquito Beater WSP Plunks are somewhere in between, but more like Bits. They contain Bti in a water soluble pouch. You "plunk" the pouch in the water like a Dunk and it dissolves slowly, but only lasts two weeks rather than 30 days. Plunks also have the slight disadvantage that they're not donut-shaped like Dunks, so if you need to tie them down to keep them from all drifting to one side of a pond, it's more difficult to do so.
Microbe-Lift BMC works like Bits by killing larvae within a day, but you'll need to add it again within one to two weeks, to kill the next generation. The liquid formula requires only a teaspoon for 540 square feet of water (a 23-foot by 23-foot area), so you may need to add it with an eye-dropper to smaller areas. The manufacturer recommends 6 drops for 100 gallons.
Summit Mosquito Dunks Last Longer, Safe for Wildlife, Pets
Mosquito Dunks Reviews:
"Nice because it keeps working. Crumble them up the first time to kill quick at the start of summer, then add whole ones in a couple weeks, and once a month after that."
"They say one for 100 square feet but just putting one in the middle doesn't work as well. Break it up and put it in two or three places. Easier to break after it's wet."
"This is the only thing I could find that's truly safe for my lily and fish pond. Still have fish, dragonflies, butterflies, frogs, turtles, all the other life, but no more mosquitoes."
"They're supposed to last 30 days but they don't seem to last that long."
"The neighbors didn't empty their pool when they moved out. Noticed more mosquitoes in the backyards a few weeks later. Somehow two of these dunks just happened to get tossed over the fence into it. Mosquito level slowly getting back to normal but wish we'd done it before the first batch hatched."
"We use half a dunk in each rainbarrel. The water is for the garden, and I was hoping it would kill some of the garden pests too but I guess it's just for mosquitoes. Still, the rain barrels are clear of larvae and it doesn't seem to harm the worms that are good for the garden."
"Doesn't do anything for adult mosquitoes. Good for killing larvae, but you still need repellent or something for the adults that come in. Wish all the neighbors used them."
Summit Mosquito Bits Kill Larvae Fast, But Need Reapplied In Two Weeks
Mosquito Bits Reviews:
"I use this for the birdbath and rain barrels in summer, then sprinkle a few bits on the soil around houseplants to get rid of fungus gnats when I bring them inside for the winter. Works like Gnatrol and I don't need to buy two different things."
"I like Bits better because they dissolve and animals can't eat them. I was finding Dunks on the shore of my pond half chewed the day after I'd add them. Either my dogs or raccoons or something were dragging them out."
"Don't get too excited because this won't solve all your mosquito problems. We still needed to buy a trap because they're coming from somewhere else, but we used to see a cloud of mosquitoes hatching from the drainage ditches behind our house on warm days, but we use this every two weeks when they're flooded and we don't see that anymore. Kills them before they hatch and the trap gets the rest."
"Was hoping once at the start of warm weather would do it, but you need to keep reapplying it. The larvae keep coming back. It does kill them fast though, and I like it because it's safe for the birds and frogs."
"You don't need to measure. I use the sprinkle top and just shake it in wet places. After you measure once or twice, you can guess how much you need."
"What I don't like is it take a couple days to get waterlogged and sink or disappear, so you have this "junk" on your water. That's fine for ditches or something but not so good for clear water. I switched to the liquid Micro-lift product that disappears immediately."
"I put Bits in an old bathtub used for watering horses. Next day the surface was black from all the dead larvae. Really works fast."
Bonide Mosquito Beater WSP Plunks Have Water-Soluble Pouches
Bonide Mosquito Beater "Plunks" Reviews:
"These are like Dunks except they're in a pouch. Toss them in the water and they do kill larvae. Maybe work a little faster than Dunks but don't last as long."
"Not the same as other Mosquito Beater products. No scent, no fog, just toss them in the water. They don't work on adults, but they kill the larvae."
"I thought these were so much cheaper than dunks, then realized each one only treated 50 square feet. They don't last as long, either. I noticed larvae coming back in two weeks. They clean out the larvae quick though, just need to add them more often."
"I like these better than Mosquito Bits because they're pre-measured in the pouch and you can toss it out in the middle of the pond easier. The pouch dissolves and the bits inside spread out in the water."
"The secret is to start at the beginning of the season, because as soon as the first larvae turn into pupae, it's too late and they'll become adults and the cycle will start all over again. There are always a few adults but we treat all the low areas with standing water, the drainage ditches and the watering troughs, and it makes a difference."
Microbe-Lift BMC Liquid Formula Disappears In Water Immediately
Microbe-Lift BMC Reviews:
"I don't know why more people haven't heard of this. It's like Mosquito Bits only better. I go around with an eye-dropper and add it to the birdbath, trays under potted plants, the dog's wading pool. Doesn't take much."
"You open up the jar and there's another bottle inside, plus a measuring cup. Wasn't expecting that. But it goes a long way."
"Dunks and Bits may be okay for ditches and puddles, but they just look ugly in ornamental water gardens and clear pools. What I like about this liquid is it just disappears instantly, so the water still looks clean, with nothing floating in it."
"Added it and the next day had to skim off all the dead larvae."
"This is very good for large fish ponds. No harm to the fish or wildlife."
"Only thing I wish is that the larger sizes had the same easy dispenser top as the 2-ounce size. Got the smaller size just to see if it worked and it did, but the larger ones come with a measuring cup."
How to Use Dunks and Bits
Toss a donut-shaped Dunk into the water where mosquito larvae live. It will gradually dissolve over the next 30 days. Use one Dunk for every 100 square feet of water surface.
To use Bits, measure out the suggested amount and sprinkle them evenly over the surface of the water. Repeat every one to two weeks. To use liquid BMC, measure out the right amount and pour or drop it in the water like Bits.
Toss a Plunk into the water like a Dunk, except you'll need one for every 50 square feet, and you'll need to replace them every two weeks.
See the manufacturer's labels, linked below or on the package, for more detailed information.
How Many Dunks Do I Need?
Add one Dunk for every 100 square feet of water surface. For large areas, toss each Dunk 10 feet from every other Dunk.
For smaller areas, you can break up Dunks. The manufacturer recommends:
1/4 Dunk for up to 5 square feet of water
1/2 Dunk for 5 to 25 square feet
1 Dunk for 25 to 100 square feet
Mosquito Dunks sell in packages of two, six or twenty, with the price dropping significantly per Dunk, when you buy them in the larger packs. The good news is that they keep, so you can buy more at one time and use them for several seasons. According to the manufacturer, in a test, Dunks that were eight years old worked about as well as new ones. Store them in a dry place where the temperature stays below 100 degrees, though cold or freezing temperatures won't hurt them.
How Many Mosquito Bits Do I Need?
Estimate the length and width in feet, of the water surface you want to treat, and multiply them, to calculate the number of square feet.
You'll need one teaspoon of Bits for every 25 square feet, or one tablespoon for every 75 square feet or for a large area, a half cup for every 600 square feet.
Bits are sold in two sizes of containers, 8 ounce and 30 ounce.
The 8-ounce container will treat about 2,178 square feet, so it contains about 87 teaspoons or 29 tablespoons.
The 30-ounce container will treat over 8000 square feet, so it contains over 300 teaspoons or 100 tablespoons.
As usual, the larger container is a lot cheaper per application, maybe half the cost.
The good news is that you can store the Bits and use them next year too, and the year after that. Both Dunks and Bits contain the same Bti solids, spores and insecticidal toxins, so you can store them the same as Dunks, described above.
How Much Will It Cost?
To treat 100 square feet for a month, you need one Dunk, or about 3/4 to 1 1/2 ounce of Bits, or four Bonide Mosquito Beater WSP pouches. You can use those figures to compare prices of the different methods, though generally speaking, any of the three methods will cost about the same if purchased in similar quantities.
The best way to save money is to buy a larger package, even if it will last two or three seasons, since a 20-pack of Dunks or a 30-ounce container of Bits may be only one-third the price per Dunk or per ounce, compared to a smaller package.
If you need to treat a large area, such as a whole farm pond, Microbe-Lift BMC's concentrated liquid formula in a 16-ounce size treats a 5,000-gallon pond every two weeks for 17 months, or a 10,000-gallon pond every two weeks for 8 months.
Dunks Aren't Working! I Still Have Mosquitoes!
The problem with Dunks is they don't kill adult mosquitoes. They kill larvae, but adults will still fly in from elsewhere. Dunks work best if you have a few known sources for local mosquitoes, like a fish pond, small wetland area, watering trough, rain barrel, or unused swimming pool. If you live near acres of swampland, Dunks in the birdbath won't make a dent in the adult mosquito population.
If you don't see any tiny "wrigglers" (larvae) in the water where you put the Dunks, they worked, but you'll need to use a trap, repellent, spray, netting or something else to kill or repel the adults that have bred elsewhere. Either that, or convince all your neighbors to drain their standing water or use Dunks too!
Bits Aren't Working! The Larvae Are Back!
The war on mosquitoes never ends. The Bits probably killed the larvae that were in the water when you spread them, but now more mosquitoes have laid more eggs and new larvae have hatched. It'll keep happening all summer. You'll either need to apply Bits again every one to two weeks, to kill off the new generations, or use Dunks instead, which have a time-release formula which lasts for 30 days.
Can I use Dunks or Bits with Water Lilies and Fish?
Yes. Bti, the active ingredient in both, is harmless to fish, frogs, and other wildlife, as well as flowers and other plants. It only affects the larvae of mosquitoes, black flies, fungus gnats and other closely related insects, since only they have the right combination of enzymes and alkalinity in their midgut to make Bti toxic. See this page for more information. Bti is a better choice for wetlands, fish ponds or lily ponds where there's other aquatic wildlife than methoprene (sold as Altosid), since methoprene can be toxic to some wetland creatures that you may not want to affect.
Can I Add Dunks or Bits to Water Indoors?
Yes, they're safe for flooded basements, drainage areas in and around building foundations, indoor ornamental ponds, etc., and have no odor even when used indoors. If mosquitoes can get to water, they'll try to lay eggs in it, so Dunks are useful for water inside as well as outside.
If Dunks won't stay put in moving water, pound a stake in the ground and slip the donut-shaped Dunk down over it, or tie a string to the Dunk and tie the end to something on the shore.
Crumble up Dunks and spread the pieces around to make them work faster over a larger area, when first applying them for the season. After they kill the existing larvae, use whole Dunks and let them slowly dissolve.
Squirrels, raccoons, and even dogs think Dunks are tasty. Try to place them far enough out in water where animals can't get to them, or crumble them so animals can't eat them all at once. They won't hurt the animals, but you'll have to replace the missing Dunks!
My Dog Ate a Dunk!
Don't worry. Bti is harmless to animals except mosquitoes, and the other filler material in Dunks is also. The Material Safety Data Sheet (MSDS) for Summit Mosquito Dunks states "No hazardous or toxic components" and under Health Hazards, "none known." The manufacturer says on its website, "If an animal eats a Mosquito Dunk or drinks water treated with a Mosquito Dunk, it essentially gets a large dose of protein and calcium." Dunks can even be used in watering troughs for livestock, according to the approved label.
Useful Links for Reference (all pdf files):
Standing water and pond photos courtesy of the CDC. Product photos courtesy of Amazon Services LLC Associates Program.