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I've been battling thrips for awhile now. I initially thought those little black flies were fungus gnats, but thrips come in many forms and the type I have fly around as adults.
Basic life stages
These young thrips are on the tops and bottoms of leaves chewing up the leaves looking like they were covered in slime but its actually leaf damage. They also like to shit on the leaves and plant their eggs. You'll see little black spots start to show up.
I was taking care of them with some stuff based on neem oil--Fungicide3 and Azamax. It was kind of working but takes a few applications and you need to wash it off. So the guy at the store recommended Monterey Garden Insect Spray. Its a biological control using Spinosad a form of bacteria found growing under a shut down sugar factory.
I have only used it a few times but let me tell you, this stuff works. I haven't noticed any thrips or any damage on the plants that I've sprayed and the number of stray adults has dropped substantially. Usually with the other stuff its still going to be an issue after the first application.
Taken from that wikipedia link on Spinosad "It is commonly used to kill thrips and other pests on flowering marijuana plants a few weeks before harvest without harming the flowers or making them harmful if smoked." Not sure how true that is from experience, but the stuff doesn't leave a residue which makes me think it would be fine. I generally don't spray in flower, but have never had a serious enough issue that made me want to question that decision.
The directions on the Monterey's has different plants listed and their applications; some can be sprayed daily, whereas some recommend weekly. I sprayed every three days for three times and the plants are doing great. Thrips free from the first spray, but I kept at it to test tolerance.
Before I got the spray I added some diatomaceous earth to the top of the pots but that stuff is not that great when wet, they were running right across the top. They might have died later, but it didn't seem to phase them. Also the stuff turns into a barrier and its water doesn't penetrate very well. It might have helped weaken them a little bit, but the spray worked so amazing that I felt retarded for even trying DE.
Another thing that helps slow down all insects is adding Silica, which thickens and strengthens the cell walls so that insects have a harder time bitting into them. I recommend Silica to everyone because it has a lot of other benefits, pest/disease resistance, drought and heat tolerance, rigid branches, and its suppose to add weight, but I can't confirm that. It seems like it would though.
-------------------- Authorized Medical Marijuana patient. Any help given is for educational purposes only. Its your responsibility not to break any applicable laws.