Tank full of wriggling mosquito larvae. How to kill them?
Can't just dump it out. Putting aside that it weighs half a ton, the whole point of having a tank is to COLLECT the water.
Scoop them out? They squirm and wriggle away. Probably doable anyway, but would take a long time, and I'd end up scooping out half the tank or more, and I still wouldn't get them all.
Strainer? Actually, this never occurred to me. (We don't have the right sort of strainer anyhow.)
Put fish in the tank? I considered it! But the goldfish in our little pond are loaners (they belong to our neighbor) and they're fine where they are. Also, I don't think two goldfish could eat several hundred larvae. Thin the herd, at best. Buy more fish? The nearest pet store is some distance away...
Oil on the water works a treat, all sources agree. The wrigglers rely on surface tension to breathe: they're so small that it resists their siphon, like plunging a knife into rubber. So they shove it in, and then they just hang for a while and breathe, suspended from the surface from below. Oil disrupts the surface tension so they can't do that and can't get a proper breath. Or so they say.
First attempt: sunflower oil from the kitchen. No good -- too thick. It formed a plump yellow disc on the surface of the water and refused to spread. Blowing on it and gently splashing just moved it around or broke it up. I'd have to use a couple of bottles. Suboptimal.
Second attempt: liquid soap. Should work to disrupt surface tension just as well as oil, right? And it did, a bit -- the wrigglers clearly didn't like it. But it had the opposite problem from the vegetable oil: within a few minutes, it dispersed and dissolved. How much liquid soap can 500 liters of water hold? Too much. No good.
Third attempt: gasoline.
Gasoline worked great. It spread out nicely over the water, shimmering with toxic rainbows: a single modest dollop from the hand-tank in the garage was enough to cover most of the surface. But it didn't dissolve, of course. It just sat there, shimmering. (And smelling very strongly of gasoline, naturally.)
And the wrigglers hated it. They would come to the surface to breathe... and then they would start flailing frantically, and dive. I think they were inhaling gasoline, and not liking it. After half an hour, I could see they were starting to die! Some of them were going limp and shriveling up and not moving any more. Yes!
Claudia, stepping out on the back patio: What's that smell? It smells like gasoline.
Me, brightly: It is! I poured gasoline in the rainwater tank!
Me: It's working great!
The small disagreements of married couples are generally not very interesting, so we'll skip over the next half hour or so. (Although I will say, for the record, that I think the whole British Petroleum thing was inappropriate and uncalled for.) Suffice it to say that this ended with me carefully scooping all the gasoline out of the rainwater tank until not a trace of rainbow shimmer remained. I had killed perhaps a fifth of the wrigglers, but the rest remained.
Gasoline, no good. Now what?
Enter Bacillus Thuringensis.
My friend Mr. Internet told me that lots of people in American use things called "mosquito dunks". Apparently these are floating discs that gradually dissolve in the water, releasing billions of these bacteria. Supposed to work pretty well.
I was vaguely aware of B. Thuringensis -- "BTh", to the cool kids. Natural insecticide, right? Some of its genes had been introduced into genetically engineered plants. But only vaguely. Could this actually work?
I know now a lot more about BTh.
It's a naturally occurring soil bacterium. Not normally found in water, and doesn't reproduce there.
It's a member of the same family as anthrax... actually, it's quite closely related to anthrax. Well, that didn't bother me. I know that very closely related organisms can behave totally differently, dog and wolf type of thing, and that goes double when dealing with bacteria. Hell, slightly different strains of the same bacterial species can be completely different: one sort of E. Coli is a lovable symbiote, its brother can be a stone killer. So, just an interesting detail, that.
It kills insect larva that swallow it. Basically it melts these huge ulcers through the lining of the larva's gut, and the insect sort of dissolves from within. Sounds unpleasant, but needs must.
(One thing that's still not clear to me: why does BTh do this? What benefit does it get? Does it multiply in the dead insect? Nobody seems to mention this.)
It's supposed to be totally and completely harmless to anything but insect larva. And even then, it only affects certain kinds -- mosquitoes and beetles, but not (for instance) dragonflies. And certain particular strains of it are specialized for killing particular insects. The dunks use BTh Israelensis, which is optimized for mosquito larvae.
Well, okay. Dunks, then. Back to Mr. Internet!
...they don't have mosquito dunks in Germany.
WTH? Germans should love this sort of thing. Organic, natural, safe. No nasty chemicals, just our special little bacterial friends doing their prokaryotic thing.
Well, to skip ahead: one German company makes a droplet solution. Why no dunks? Maybe an American holds the patent. No idea. But anyway, okay, we put in an order. And just a day later, hey, a little package arrived in the mail.
Little glass vial of brown liquid. My German just good enough to slowly read the instructions: keep cool, out of reach of children... one drop per 25 liters. Okay.
Drip, drip, drip into the tank. Twenty drops, and a couple for good luck. The brown, cloudy liquid quickly dispersed and disappeared. The wrigglers... kept wriggling. Well, of course. Germ warfare, not poison. The bottle said action within 24 hours. Be conservative and say 24 to 48. Go away and wait. Don't even look in there until tonight.
D + 6 hours: Afternoon. Look in tank.
...something is different. There's still wriggling and activity, but it's... subdued. Look closer. A lot of wrigglers are just floating, motionless. Pass hand over the water; many wrigglers flee the shadow and motion, as always, but many more just lie there. Good grief, is it working already?
D + 8 hours: Late afternoon. Look in tank.
...death. I'd say about 90% of them are dead. Obviously dead: immobile, floating. I dip the white plastic bucket, take a few inches of water, swish. It's full of tiny corpses. A few are still twitching, but the great majority are dead.
D + 9 hours: Evening. Look in tank.
Every last mosquito larvae is dead. No exceptions. Pass hand over water: nothing moves. The tank is a sea of tiny corpses.
I dipped the white bucket, peered at a few dozen dead bodies, frowned thoughtfully, started to toss the water away... then stopped. What if this stuff got loose in the yard? Just how powerful was it, anyway? I didn't want to kill off every bee and beetle.
The next day, in the bright light of morning, the concern seems excessive. BTh occurs naturally in soil; this was a specialized strain, optimized for killing mosquito larvae; it's been approved by both the American EPA and its German equivalent. So, probably okay. Oh, and also: there were some other small invertebrates in the tank -- little round guys that I think were water-bears, and the like. And they seem to be doing just fine. So it's not an indiscriminate destroyer.
I think what freaked me out a little was that the tank had been so very full of life. Disgusting, squirmy life, but life. And in just a few hours, bam. No exceptions. I am become Death, destroyer of worlds.
(Also, maybe the whole "organic" thing. Organic is such a friendly word! This nice organic thing, all-natural, will make your problem go away!
(Well. Cholera and smallpox and the Black Death, they were organic too.)
Anyway. A day later, the tank remains quiet except for a few water-bears and other invertebrate odds and ends. The bacteria don't reproduce in water, apparently, so I'll have to redose it every few weeks. There's enough in that little glass vial to keep us through the whole summer.
And I guess that's fine. I really didn't want a few hundred more mosquitoes around the house. It just... takes a little getting used to.
I guess I'm still a city kid at heart.