The bathroom windowsill faces East, so it only gets the sun (in Glasgow? Hahahah, shut up) for an hour or so in the morning, and is therefore ideal for the Salvia divinorum plants which don't like too much direct light. A few weeks ago I noticed they'd been infested with Greenhouse Whitefly, Trialeurodes vaporariorum, the wee blighters. I consulted Professor Google who suggested introducing Encarsia formosa, a tiny parasitic wasp. So I bought some from these ladybird people. You get five wee cards to hang on the stems of the plants. There's a panel in the cards with a few dozen unhatched Encarsia formosa on them. Nothing seemed to be happening, except that I did see a few very small (half a millimetre) beasts crawling upwards over the cards after a week or so. Now, after about three weeks, most of the white-ish whitefly larvae have turned black.
What's happening is really nasty, but unpleasantly satisfying if you've learned to hate the whitefly munching your precious plants. Encarsia formosa lays its eggs inside whitefly larvae. The egg hatches in there and proceeds to eat its way out. Naturally, the whitefly larvae do not survive this process, and so do not grow to adulthood and lay more whitefly eggs on my Sally Ds. The adult wasps will lay 50-100 eggs, so just one of the vicious little sods could deal with a leaf full of whitefly larvae. New adults will emerge to carry on with the job. I'm assuming their population will die out once the whitefly are done for, having nowhere to lay eggs.
So it seems to work, and I'll bear it in mind when I get a greenhouse at the allotment next year. But as for the bathroom Salvia, I'd judge it best not to say anything to family members who also use the bathroom. The beasts involved are tiny, and you really need to look closely to see anything at all, but mention of "parasitic wasps" in the same sentence as "bathroom" is unlikely to get a sympathetic hearing.