Bt is target specific, depending on the particular strain used. The type used for mosquito control will have only very limited implcations for other organisms so you can consider it quite safe if the treated rain barrel water is used in the garden. Bt doesn't bother earthworms at all. Considering the problems that mosquitos can create if allowed to proliferate freely in standing water, the use of Bt mosquito dunks are by far the lesser of two evils (although I would not consider Bt much of an evil at all!).

Here is a link that might be useful: Bt fact sheet

Bt is fairly target-selective.

Dan

To add to the good comments from the others: the strain of Bt (Bacillus thuringiensis var. israelensis) you'd be using to control the mosquito larvae is specific to those insects in the Diptera order (flies, gnats, mosquitoes, etc.) and is effective only against the larvae of those insects.

Earthworms aren't insects, or larvae.

Bacillus products are a composed of different bacteria strains and are effective against those critters who are prone to infection from those bacteria.

Thanks for all the good info. Wasn't quite aware that there were different strains used in the mosquito dunks, versus products like thuricide.
Also good to note that none of them affect earthworms. In my mind if it wiggles around it's all in the same family, ;) just kidding . .

Are there any "good" insects in the Diptera family that would be harmed by watering with Bt treated water? Just curious.

a simple solution is to put screen were the bugs sre getting in i use it in my rainbarrels and never have a skeeter prob

Let me add that usu GardenGal comes in behind me and fills in the gaps in my pithy comments. This time she pre-filled in my reply - amazing anticipation. ;o)

Dan

"Considering the problems that mosquitos can create if allowed to proliferate freely in standing water, the use of Bt mosquito dunks are by far the lesser of two evils"

Let me add, that it was never a decision between, Bt and mosquito larvae. I just wanted to know all that Bt would harm and consider it as well as other options, such as screens.
I don't know what type of person would say "ah shucks I think I'll just let those mosquitoes have their day".
Silly.

When we had a bird bath I used 1/4 of a Mosquito Dunk every six months or so. After a few years I noticed that the original black gunky bottom of the concrete bird bath had become completely clean. I could see the original pores in the concrete again. I'm not sure what the mechanism was but something ate all the mold in the birdbath.

'Are there any good insects in the Diptera family that would be harmed with Bt treated water?'

Bt-I isn't effective against all Dipterans, not by a long shot. Heck, it's not even effective against all kinds of mosquitoes! Don't look for it to solve stable fly or house fly problems, for example.

I guess you could say that over-use of Bt-I could result in a loss of a food source for all of the different critters that feed upon the larvae! That might actually be important in some ecosystems. Fish, reptiles, and amphibians might go hungry. Dragonflies, bats, and certain birds, too. (See how my mind works, lol?)

But right off the bat, I can't think of a beneficial insect that we might be putting the whammy on by using the stuff.

When I was a little towheaded girl in the Adirondacks we used to be plagued by the dreaded blackfly. Mom would pretty much cover us up from head to toe with layers of clothes (the darn things could bite through a thin shirt) and slather us with (real) citronella oil. They also fogged the whole area with DDT on a regular basis. I can still remember what that stuff tasted like when it would coat our skin. Bt-I, which does control aquatic blackfly larvae, would have been great!