How to prevent mosquitos from breeding in my rainwater barrel

July 31, 2007 7:08 AM

We have a rainwater barrel in the back yard, for the purpose of collecting water to then water the plants with. This has worked well through the summer, but we're now finding that the mosquito population on our deck has exploded. How do I prevent the larvae from growing in our barrel, or otherwise kill them before they bite me?

Whatever way we treat the water has to be safe for plants and pets - I dont want to end up poisoning the neighbours dog, or indeed the tomato plants that the rainwater is intended for.

I did read previous threads about mosquito control, and am thinking about trying the suggestions in this one, but was wondering whether the mosquito dunks will keep the water safe for watering plants? What other non-chemical ways are there to solve this?

posted by darsh to home & garden (21 comments total)

You could try affixing a lid to the barrel to keep the mosquitos out, but you'd have to remember to open the lid when it starts raining and you want to collect more water.

My parents maintain a rainwater barrel on our small family farm, and simply dump out all the water whenever it becomes too green and nasty, clean it, and set it back in place under the gutter overhang to refill at the next rain.
posted by lohmannn at 7:15 AM on July 31, 2007

Mosquito dunks are totally safe -- we use them in our water barrels at the community garden.
posted by vetiver at 7:17 AM on July 31, 2007

Mosquito dunks. Our rainwater barrels have screened lids; we add mosquito dunks once a month, and water our veggies with the collected rainwater.
posted by MonkeyToes at 7:18 AM on July 31, 2007

From this site:

* Keep a goldfish in the barrel to eat mosquito larvae, or,
* Add a tablespoon of dish soap or ½ cup bleach to the water inside your barrel, or,
* Add anti-mosquito larvae, available at garden centres, or
* Drain your barrel water every one to two weeks.
posted by terrapin at 7:19 AM on July 31, 2007

One way is to make sure the top of the barrel is screened using no-see-um mesh. This will stop them getting to the water in the first place. You need to stretch this above the highest water level in the tank, and below the feeder pip (as they'll get in this way as well). It's a real faff but I've done this successfully by cutting it to a circle about two inches wider than I need, and then securing it with gaffer tape. I had to clean the insude of the tank and let it dry thoroughly before doing this, and if you let the mesh drop in the water you'll not be able to secure it this way, so we did it when the level was low. Putting mesh over the end of the feeder pipe won't work if there's a lid on the barrel as well.
posted by dowcrag at 7:20 AM on July 31, 2007

If you want to get all organic and stuff you could skip the dunks and go for the mosquito's natural enemy, minnows. Go to a bait store and get some. Watch out for fish kills and keep the water full and fresh in low rain periods. Also watch out when you fill your watering can that you don't suck up the little guys, though they wouldn't hurt the garden or pets.
posted by Pollomacho at 7:28 AM on July 31, 2007

I seem to remember that a small layer of oil on top of the water was used to control them in building the panama canal. Assuming your barrel empties from the bottom this shouldn't affect your watering at all.
posted by true at 7:34 AM on July 31, 2007

I wouldn't rely on bleach, because if you water the garden too soon after adding the bleach, the plants won't be happy. The dunks are totally safe for garden and pets. Personally I prefer screened water tanks, because you don't have to remember to add the dunks or chemicals; screen plus dunks is probably the best of all options. My experience has been that tanks with fish in them will still produce mosquitoes, just in smaller numbers, but with careful attention to fish species and feeding and so on that could probably be improved. If the barrel gets too hot in the sun, the fish won't do so well, either.
posted by Forktine at 7:40 AM on July 31, 2007

From the City of Toronto, who operates a rain barrel program for homeowners:

"The most effective way to discourage mosquitoes is empty the barrel as soon as possible after a rainfall so there is no standing water. The screen on the rain barrel is for debris not mosquitoes and you can place a piece of a window screen to stop the mosquitoes from entering the barrel. Placing the basket in a nylon sock to make the holes smaller or placing a 1/4 cup of vegatable oil into the barrel (it will form a film on top of the water) are also effective."
posted by mrmcsurly at 7:47 AM on July 31, 2007

My money's on tossing a cheap goldfish or two in there.
posted by infinitywaltz at 7:59 AM on July 31, 2007

I agree with adding goldfish, and dont' suggest bleach.
posted by terrapin at 8:07 AM on July 31, 2007

I'm nthing goldfish, they're non-tropical fish so they should be able to handle temperature swings, they're natural (and might even add a little bit of fertilizer to the water), they're cheap (you can probably get some feeder goldfish for less than a buck) and you can even give them names (like swimmy).
posted by drezdn at 8:15 AM on July 31, 2007

Can't believe nobody has mentioned mosquitofish! We have these in our non-aerated ponds and they're pretty cute, though very fecund. They bear live young, and sometimes follow each other around the pond in lines, like ducklings. No sign of larvae since we introduced the fish, either.
posted by Scram at 9:03 AM on July 31, 2007

I seem to remember that a small layer of oil on top of the water was used to control them in building the panama canal. Assuming your barrel empties from the bottom this shouldn't affect your watering at all.

I believe they used kerosene. (Basically just dumped/sprayed it all over the swamps, since the oil slick suffocates anything on top of the water.) But they also just eliminated all sources of standing water, including things like rain barrels, in urban areas.

Probably not the recommended solution today, since there are chemicals designed specifically for the purpose that won't have quite the serious toxicity that kero or diesel would if it accidentally got poured out.

I think the mosquito dunks are the way to go.
posted by Kadin2048 at 9:05 AM on July 31, 2007

Don't use kerosene/diesel. It's a great biocide, but it's not great for your plants either. Also light petroleum products are carcinogens and evaporate easily, meaning they're an inhalation risk too.

If you must, a vegetable oil is a better choice.
posted by bonehead at 10:32 AM on July 31, 2007

You might try building some bat houses in your backyard. If mosquitos do hatch, bats do an excellent job of eating them up very quickly. Bats eat some startling amount of weight in mosquitos daily. We used them at an outdoor school I once worked in, and they kept the biting down pretty well.

Plus, they're fun to watch at twilight.
posted by Miko at 10:41 AM on July 31, 2007

Martins are nicer than bats.
posted by Bonzai at 11:46 AM on July 31, 2007

Why? Bats are awesome.
posted by agregoli at 12:41 PM on July 31, 2007

Down here in swampy mosquito-land (aka the American South) mosquitofish are commonly used.
posted by radioamy at 12:45 PM on July 31, 2007

Mosquitofish are much easier to keep alive than goldfish, you can have more of them, and they're not as messy. Like scram mentioned upthread, they bear live young -- often. But they also self-regulate their own population. If the population outgrows the food source (i.e., mosquito larvae), they, well, cannibalize each other. Bingo -- perfect population control.

My county's mosquito vector control program provides mosquitofish for free -- they even deliver. See if your county has a mosquito abatement program and ask them if they offer mosquitofish.

If you do get fish and the rain barrel is clear translucent or dark plastic, you might want to insulate it. Buy a cheap pool thermometer first and see what the average temp is so that you don't inadvertently have yourself a backyard fish boil.

In lieu of mosquitofish, mosquito dunks will control the larva. However, they're made of BT. While the BT will have no negative effect whatsoever on your plants, you might be introducing the BT-laced water into an environment where it will harm the larvae of beneficial insects. That's not really good practice from an IPM standpoint, so you're better off not doing it.

Mosquitofish are your A#1 bet, if you can get them.
posted by mudpuppie at 2:38 PM on July 31, 2007

On the family farm, Grandpa always kept a bullhead in the horse trough for chowing on mosquitos. Really any small fish that can survive higher water temps in the barrel will work.
posted by fantastic at 6:20 PM on July 31, 2007