Fungus gnats are tiny little black flies that love living in damp organic matter (i.e. compost). They thrive in warmer weather, so at this time of year – with all the seedlings indoors – the house is full of them.
The females stay near the surface of the compost, scurrying around. Their larvae live in the soil and chomp roots – a bad infestation can kill seedlings.
The males fly further afield. They don’t fly particularly well, and are drawn to anything damp – leave the dregs of a cup of tea lying around overnight and there will probably be a couple of drowned gnats in it in the morning. They’re easily caught and squashed, although you have to do a proper job of the squishing as they are remarkably resilient.
Yellow sticky traps are the easiest way to cure a bad attack – all of the males will get stuck to it. You’re supposed to be able to keep them under control by letting the compost dry out, but I’ve never managed to make that work even with mature plants. If the weather is suitable, putting the pot outside for a couple of days certainly helps.
The other thing you can try is natural sticky traps – carnivorous plants. These two droseras live in the bathroom. They need boggy conditions and a steady supply of rain (not tap) water, but apart from that they’re low maintenance. The droseras produce sticky droplets that (as you can see) are good at trapping fungus gnats. I also have a Venus fly trap, but fungus gnats are too small to trigger the traps. If you’ve got a bluebottle problem then they’d be more use :)
martyc wrote:
...on Thu, Apr 10 '08 (1395 days ago)