The mosquito larvae will certainly die out of water. You might want to use one of those biological-control "mosquito dunks" in your rain barrel, though, or put a fine mesh screen over the top to keep the mosquitoes from laying eggs in there in the first place.

when those larvae develop, well, there's more mosquitoes in your yard.

http://www.merrifieldgardencenter.com/products.php?pid=251&openparent=118

Here is a link that might be useful: mosquito dunks (one of many sources)

The mosquito larvae will die when you tip the water on to your orchids. Mosquitoes do mature quickly in warm weather so you may find that the emerge as adults before you use all of the water. Bacterial spore preparations (Bacillus thuringiensis sv israelensis) are available commercially in the US to control mosquitoes. The bacteria should have no effect on your orchids but I don't know what effect any filler added to the product may have. Adding garlic to the water will reduce larvae numbers and should have little effect on the orchids. Otherwise covering the barrels with flywire mesh should prevent the mosquitoes entering to lay their eggs in the first place.

As for mixing rainwater with well/tapwater, rainwater is reasonably low in dissolved salts. Many people will mix rainwater/RO water with tap/well water to increase levels of calcium and magnesium, which are often absent in commercial fertilizers. Unless you're growing orchids that need very soft water, adding a little tapwater isn't a bad idea. Alternatively, use a fertilizer containing calcium and magnesium or or add a little lime/dolomite to the surface of the your mix.

Mosquito larva (wrigglers) will die quickly when taken out of water. So the ones that you do scoop out will not be a problem.

However, depending on how big your barrels are, you might not be able to get them all before they complete their development cycle. Once they complete their metamorphosis then they are free flying mosquitoes.

You have a few options:
1. Use the mosquito dunk that was recommended above. I'm just not sure if the water will be safe for plants afterwards.
2. Place a fine screen mesh to cover the barrels. Make sure that the screen does not touch the water surface. If the screen is touching the water surface, then the mosquitoes from outside can still land on the water and lay their eggs on the surface. The mosquitoes will not attempt to lay eggs on the "dry" screen.
Another benefit of putting a screen cover is that the larva that were already inside will turn into mosquitoes but will not be able to exit the barrel. They will starve to death in a few days.

3. (Do not try this on cold/freezing climates) For diversity, throw in a few cheap freshwater aquarium fish in the barrels. The hardy ones like mollies and guppies will gobble up the mosquito larva like crazy. You now have a permanent mosquito control apparatus in the barrels :).
When the barrels run low on water, you can relocate the fish to a small aquarium until the rain replenishes the barrels again.
Please do not treat the fish as "disposable" pets. They are meant to be a permanent mosquito control plan.

Helga, I use goldfish in one rainbarrel and have black mollies in another. We've had a couple freezes and the fish do fine. You can get them at Walmart. I do add some well water to my rainwater, probably only a cup or so to each gallon.

The BTi will not hurt your orchids and might even be beneficial if you have a fungus gnat problem. The BTi also kills the larvae in the potting medium. I found out this year that fungus gnats will also lay eggs in sphag - grrrrr.

Brooke

Utricularia, bladderworts, eat some mosquito larvae. Try some native Utricularia vulgaris. They're perfectly hardy.

Best,
Ross

I've been using BT pellets in bromeliad cups, s/h reservoirs, carnivorous plant saucers, rain barrels, standing water at the bottom of tanks, and anywhere else that I can find standing water. It definitely does no harm to plants or animals. It only attacks "worm" stages of insect life cycles. Thus the 'wigglers' and also the fungus gnat larvae (clear worms the size of hairs), get killed...but all the cute and fun stuff, lives on. I think the government should distribute Bt pellets to every man, woman, and child in the country. I REALLY hate mosquitos.
-MB

I use the dunks no problem ,no new skeeters or knats . Gin

Screen does magic. I have two 55 gallon rain barrels that our County Extension kindly gives for free, for attending a workshop.
Here is a link of how the rain barrel is set up:
http://hillsborough_fyn.ifas.ufl.edu/FYN%20PDF%20Files/RB_directions.pdf

MB, where do you get BT pellets?! I need them...every year, I have weird caterpillar-looking things in the medium (possibly fungus gnats), and would love to spray with BT solution (it has been recommended to me)

Olya

Great info. Thank you All.
Now I have to find BTi pellets. Wonder who carries them? Walmart?
I like the fish idea. Is there a specie, that I can release into the river after the rainy season?

I think I got them at Home Depot. Its a shaker can of pellets. I bet they'll be available soon at walmart when the spring stuff really gets going. Probably anywhere that sells fishpond supplies or any big garden center.
-MB

Olya you don't spray with the BTi, you use it as a soil drench. Fungus gnat larvae are white, about 1/8" an inch long. Any place that sells pond supplies should have the mosquito dunks - pet stores, big box stores, nurseries, etc. Very cheap, effective and only harms the wiggler stage of mosquitos.

Brooke

Just wanted to add that if you buy Bt at Home Depot, make sure that it's Bti (Bacillus thuringiensis var. israelensis) since they also carry Bacillus thuringiensis var. kurstaki. There are many strains of Bacillus thuringiensis and they produce toxins that are very species specific. At home I use Bacillus thuringiensis var. kurstaki in my veggie garden for catepillar control.

Try a small amount of kerosine poured on to the surface of the water. As long as you take your water from a tap below surface, you will not have any contamination.
And more importantly NO MOSSIES

Not wild about the kerosene idea. In any case, get rid of them. Mosquitoes are more than annoying now that West Nile has spread in the US.

mosquito dunks will work fine. They also kill caterpillars that destroy waterlilies.

helga, If you want fish you can turn loose when the rainy season is over then I suggest going down to a local creek or pond and catching a few native minnows. you could also buy them by the dozen at a bait shop. The reason is that it's always a bad idea to turn loose foriegn species into your local ecosystem. Even the ones you can buy at the pet store. stick with the dunks...they are very safe and easy to use

I read a film of vegetable oil keeps down mosquito larva without damaging anything.
Barb

I think that you should go with the mosquito dunks!
NEVER, under any circumstances, release anything into your local waters!!! (If you have no other options, dispose of them first!!)
It doesn't matter if you originally caught it there or bought it.They are under enough pressure to survive.
Frequently, baitfish are hybrids and/or full of diseases! If nothing else, they're not native to the area.
Fish in captivity are able to transfer pathegenes back into the wild, even the ones that were caught there originally.
The absolute hysteria over West Nile is probably going to be the final straw for our native waters anyway.
Between the poisons and the release of Gambusia, we're killing our wetlands off!

Climbing off my soapbox...
Scott