Use a screen over your barrels. You only have to install it once.

Thuricide is a form of Bacillus Thuringensis (BT), which targets the
larvae of lepidoptera (caterpillars). There is a form of BT which is
used against diptera, but is is not the one that is generally sold in
hardware stores unless you see mosquitoes specifically on the label. If
you use the other stuff it has no effect on the mosquitoes. Using a
screen to deny mosquito access to the barrel will keep the adults from
laying eggs there.

The mosquito larvae can't live in pure water: they need some organic
material in the water to live on. If you keep the rain barrels clean,
that will help. This means you have to prevent the growth of algae in
the barrels. Bleach works great for that, but since you want water
without chlorine in it it's not the solution for you. You might try
hydrogen peroxide for that purpose, although it would probably take a
quart of the stuff from the grocery store to kill the algae (I've never
tried this so I don't know if it really kills algae at moderate
concentrations). However you would probably have to add a quart every
couple of weeks, at around $1/qt, so this appears to be an economically
unsound solution.

The algae won't grow if it doesn't get light, so using opaque barrels
with lids will also help. The lid will help keep out the mosquitoes also.

The screen seems the cheapest way to me.

Chlorine in water will evaporate with time. If you take a bucket of tap
water and set it outside the chlorine will disappear into the air. This
might take a week.

wrote:
I use several rain barrels to have water for my plants
and garden that doesn't have the clorine, etc. added to
tap water supplies.

To avoid supplying mosquittoes a birthing place I add a
few drops of Black Leaf Thurcide microbial insecticide
now and again. This is felt safe to apply to grape and
vegatable growths.

Is there a more suitable protector to use?

Ciao, Ack.

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