Death in the rainwater tank (1)

So we have a rainwater tank.

Claudia had been wanting one for a while, for watering the garden.  (She's been getting more ambitious with the garden.)   So we finally got it a few weeks ago.  Sits by a back corner of the house.  The rain downspout had a flip-switch on it, waiting and ready.  

And for a couple of weeks, all was well.  It rained!  Water poured in!  The tank holds 500 liters; soon it was mostly full.  Claudia turned the flip-switch back, filled her water can, and marched off happily to douse the tomatoes.  Yes, all was well...

...until the mosquito larvae showed up.

The first mystery is, why us?  Claudia's mother has an even bigger rainwater tank, but no wigglers.  Why did we get them?  My tentative guess is that the tank is situated just right -- it's under a bushy corner, so it gets a perfect mixture of light (which encourages algae to grow) and shade (which the larvae like).  Also, leaves and twigs drop in, which adds biomass to the water -- the leaves are food for bacteria, who in turn are food for protists, whose wastes support the algae.  Also, maybe we just have more mosquitos around.

But anyway: all unaware, we seem to have created some truly, truly awesome mosquito habitat.  By the end of a month the tank was crawling, wriggling, squirming with life.  There were hundreds, maybe thousands of them.  Peering closely, you could see various stages -- "instars" is the technical term -- from little freshly-hatched ones just barely big enough to see, to final-instar larvae long as a fingernail clipping.  There were little egg rafts floating on the surface of the water.  Dip out a bucket and peer closely, and you could see shed skins -- they molt between instars -- looking just like the larvae, but transparent and motionless. 

Mosquito larvae are not motionless.  They rest sometimes, usually in clumps along the edge of the tank. But some are always wiggling and swimming around, and if you approach quickly, your shadow or vibration will panic the whole tank into frenzied motion.  They swim down into the murky depths to escape the predator.  Stand still for a minute or two, though, and they'll come twitching and jerking back to the surface: they're air-breathers, and they have to come up every few minutes, or die.

Once the initial disgust passed... well, it didn't really pass, actually.  I try to keep an open mind, but you can't look at several hundred mosquito larvae without thinking: scratch scratch itch itch scratch.  Also, in our family?  Claudia and Alan are the sweet-blooded mosquito attractors; the whining little parasites absolutely adore them.  Poor Alan can't resist yet, and so scratches his bites bloody.  So, I wasn't really sympathetic to these little guys.

I did get pretty interested, though.  They are interesting.  They live on algae and protists and whatnot, filtering the water with featherlike mouthparts.  In a normal pond they're a big part of the food chain -- fish eat them, and amphibians, and dragonfly larvae.  They breath through their butts, more or less -- they have a a hoselike "siphon" near the posterior end of their bodies, and stand on their heads to stick it above water.

But that didn't weaken my resolve to kill them.  Kill them all.

The question was, how?

Comments

[Bernard Guerrero]

Go on, how?! (We had quite a few in our pond in NY, but I introduce goldfish and the equilibrium level came way down. I suspect that wouldn't be practical for you, though.)

[Christine]

Surely a simple strainer to scoop them out and let them die in the air, would be simple and "organic"? Or not that easy?

So, what did you do?

[Kirsten]

Nuke 'em from orbit. It's the only way to be sure.

Seriously, start by getting the top cleared off and apply some bio-warfare. There's a bacteria that preys on mosquito larvae--bacillus wossname--comes in little rings or tablets at the hardware shop.

Once clear, prevent future infestations with a squirt of detergent to create a slight barrier film against future egg laying.

[Bernard Guerrero]

"bacillus wossname"

Thuringensis. I considered an introduction to my pond, but if it takes it can have an impact on butterflies & moths.

[Will Baird]

What about an oil layer?

[RS]

I lived in Houston, TX as a kid...it's a big swamp. The mosquitoes would grow in all sorts of places that held water, like the in-ground plastic boxes that held the water meters in every yard. We always used a layer of vegetable oil and it worked...but I like the goldfish idea!

[Doug (not Muir)]

"Thuringensis"

As in Thüringen? Like, right next door to chez Muir?