[quote="Polymerous"] I poured in a little Dawn detergent (it was that or try olive oil - those were the two things I had at hand) onto the water, and the next day there were no more wriggling larvae. (I haven't noticed any negative effect on the seedlings from this, though I should probably keep an eye on them.)
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The Dawn should not hurt the seedlings. We use Dawn as a spreader/sticker when we spray insecticides or fungicides every 2 weeks and it doesn't bother them. I have heard though to only use the Dawn Original and not the kind that is antibacterial; not sure what happens if you do.