The larvae that it lays eats your roots eventually killing your plant so i would consider them very harmful.
At the very least there destroying the root system.
You dont have to but i find i can taste it or the weed burns badly.
I flush with water the moment i see a brown pistil but often i flush longer than 2 weeks.
Hiya.
Nothing causes them they are larvae that live in composting soil eventually crawling out flying off and laying 300 more eggs in the top of your soil.
I have had them on and off for 3 grows.
The only ways i have found to control the problem are a product like Gnat-off, Sand...an inch on top...
If you have a hot enviroment and your soils drying quickly take care in not watering using nutrients all the time.
I had this problem and watered with nuts 25% of the time the rest with just water.
I hope this is of some help.
My question still remains un-answered....Can anyone tell me if when using the silkscreen method to make hash do you prepare the weed by defrosting after you freeze or sieve whilst frozen?