You sprayed the pyrethrin directly on the plant? If you did, if it survives, that is the cause. Cannabis cannot take much direct contact with pyrethrins. Yah it will decompose in a few weeks. I suggest you read my spider mite thread, get some neem, and cross your fingers. Pyrethrins should never be put directly on the plant, it will damage them severely.

Edit: Oh ya, don't forget to sterilize the room too.
WashougalWonder Reviewed by WashougalWonder on . spider mites, etc. (filled out the troubleshooting form) Hi gang. I've got a re-vegged plant at the end of its 5th week in flower. Last week, I sprayed pyrethin to battle a spider mite mess. This week, leaves are yellow and dropping. I know the plant is probably stressed because I re-vegged it and it's in the same old soil, but I'm hoping it will make it 2 more weeks. It does seem like the mites are gone. I don't see any new mite damage anyway. The plant looks like hell though. Also, can I really smoke this after it's been sprayed? : / Rating: 5