How to get rid of mosquitos in swimming pool standing water

Question

We have an above ground pool with about 6 inches of water that does not drain. The drain is located above the bottom, to keep water in it and weigh down the pool during the off season.

However, the still warm water is the perfect breeding ground for mosquitos. This is a problem during the advent of warmer weather, before the pool is used.

It is a salt water system, and the pumps & chlorinator obviously cannot be operated with the water drained to this level. Once the pool is operational in the hot season, the conditioning and filtering prevent this mosquito problem. So this is really only a problem when the Spring weather is warm enough for breeding, but prior to starting the pool for the Summer.

Before I start-up the pool to fill it, I drain the water with a sump or water feature pump and then wet-vac it dry. This is something I do once at the beginning of the season, and takes a few hours of work. As measure to prevent mosquitos it would not be so good, since the bottom will fill with every rainfall and I'd have to start over.

What can I do to prevent mosquitos breeding until I start-up the pool for the season?

Answer 1

First point : Chemical warfare is the tool of last resort for pool maintenance.

Sometimes you end up in a crisis and you need to deal with it, but if you look after the pool correctly it should be absolutely unnecessary. Chemicals are a double-edged sword. They can get you out of a mess, but they come with headaches of their own. If you can avoid letting the pool get this bad in the first place it's altogether the better option.

This is a problem during the advent of warmer weather, before the pool is used.

The error here is to think of filling the pool as something you do when you want to swim. If a pool is of such a size that it cannot be drained of water then it is something you need to care for year-round, and treating the water is as much for the pool's sake as it is for yours. If the pool requires some water to remain in it all year around then it requires maintenance all year around.

Once the water temperature rises above 10C(50F), the water becomes a breeding ground for insects, algae, etc. You need to fill the pool and start treating the water when this happens - not simply when you want to swim. The pool needs to be opened sooner - once the water is chlorinated and is being filtered with the pump it's fine. So you need to start doing that before biology has a chance to take over.

If you really don't want to open the pool when the temperature starts rising, you need to otherwise keep it covered and chlorinated. The cover will prevent the sun from burning off the chlorine, but you will need to monitor the chlorine levels or algae begins to grow, and since this is a lot easier to do and manage with the pool filled and circulating daily it just makes more sense to open the pool and start treating it once the weather becomes favourable for algal growth and insect infestation.

In the long run, this saves you on chemicals, cost, and nasty cleanup time (scrubbing out algae and slime) and it increases the usable life of your pool.

Before I start-up the pool to fill it, I drain the water with a sump or water feature pump and then wet-vac it dry, but obviously this would only be good until the next rainfall.

If you open the pool earlier, before it grows into pond scum, you can keep that water from last year, for example, and save yourself the trouble of scrubbing all of that filth out. Same thing in the fall - keep the pool open a few weeks later, until the water temperature drops below (10C/50F) and give the leftovers a good shock before you close up. You can have clear, clean water sitting in the pool over winter and ready to go come spring.

In colder temperatures when the pool is not getting much use you don't need much circulation to keep the water clear - a minimum of chlorine and a few hours of circulation is all you need. It will depend on the size of pool and pump, but whatever your pump's throughput is you can calculate how long it takes to circulate the full pool volume. As long as you run about a pool's volume through the filter per day that's plenty. A few hours is usually enough. Use a timer.

The energy and chemicals costs of keeping the pool open longer are small compared to the work they save you in having to un-swamp and hard shock the thing every season. My pool has been open for the past seven weeks and I've spent perhaps $20 of electricity and used 2kg of chlorine (in-ground, 80,000L (20,000 gallon)) - for a smaller above-ground pool your costs will be even less.

You'll want to be very careful (ie: don't do it!) about adding additional chemicals to your pool. Soaps, oils, insecticides, metallic algecides, and other additives are not good for the pool, the pump, filter, or your chlorinator. If you add these things to the still water, you again have a big cleanup job to get all of that stuff out of there before you start up the pump and filter. If you don't clean it, it will end up in your filter and can cause damage to the pump or the other equipment.

Answer 2

Mosquito Dunks are a doughnut shaped product for exactly your problem. They contain a bacterium toxic to mosquito larva but not toxic to humans, pets or birds. They will only kill larva.

Any adult mosquito travelling by will still be available to bite you. You might also check out designs for bat or bird houses to encourage some natural predation.

Answer 3

Copper metal.

Copper discourages mosquito larvae. I used to think that the copper was directly toxic to the larvae but experiments showed this was not so - adding copper (as metal; a piece of pipe) to a fish tank where larvae were already living did not kill them.

Copper in standing water however reduces algae growth and algae is what the mosquito larvae eat. An outdoor fishtank with a piece of copper pipe in it from the start harbors few larvae or algae.

I keep a coil of thin copper pipe and put it in the aquarium if the water gets green. It works fast. If I leave it more than a couple of days the fish stop eating but they perk up a day or 2 after it is out.

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Goldfish

The other option is goldfish. I tried this at a different year from the copper experiments. I had a fish aquarium outside which had filled with rain and then with mosquito larvae and it seemed a shame to waste them. Two 8 cent goldfish made short work of those larvae and then sustained themselves on whatever else fell in the water over the rest of the summer. Two goldfish would be cheaper than a piece of copper if you don't have any pipe scraps. I used 2 because just 1 seemed lonely.

For efficiency the copper works fine and you can take it out and use it year after year. The goldfish were more fun.

Answer 4

It is a salt water system, and the pumps & chlorinator obviously cannot be operated with the water drained to this level.

Just in case you're not aware, the salt water isn't what keeps your pool clean. The chlorinator converts the salt to chlorine. You've got two problems there, in that the green stuff is algae (which is is a pain to rid yourself of once it gets going strong).

Go buy some liquid chlorine (or basic un-enhanced bleach) and start shocking the tar out of that standing water until it is no longer green. At that point, the mosquito problem will be gone as well. Liquid bleach plays nice with salt systems, just be aware that after this is done, you might need to check the pH levels before refilling the pool (as this will probably make the remaining water acidic).

Something to consider for the off-season would be a pool tablet float. It will add some chlorine slowly to the standing water when your pump isn't running.

Answer 5

Any standing water attracts mosquitos so the first step is to drain all water. If draining is difficult, add BT ( bacillus thuringensis ) , a common insecticide that only affects insects. Other insecticide could be used but may require reapplication, and may require removal before swimming.