It's late, but
this may be relevant. This is from the houseplants subreddit.
"I would like to share with you the method by which I was able to eradicate thrips from my entire plant collection.
I am no plant expert, but I had thrips on all my plants for months and nothing seemed to work. I had thrips on my monsteras, philodendrons and even my orchids. The thrips were eating the new leaves, and my collection was going down the drain.
I used an insecticide against thrips, but the next few days they would come back. Looking for information, I understood that there is a growth cycle, from eggs to adult, so I had to be constant during the life cycle of the thrips.
Unfortunately, nothing worked for me and they kept coming out, so I wanted to do an experiment. The initial idea was to spray the whole plant with the insecticide and cover it completely and hermetically with a transparent bag.
My intention was to make the insecticide last longer and not evaporate, but to maintain the action for a few days. The experiment was to close the plant hermetically for 5 days with transparent plastic (i.e. a bag), open it, spray it again and close it again for another 5 days.
I did this experiment with about 50 plants, the picture you see are my orchids during the experiment.
What were the results after 10 days? I would like to be humble, but I can't lie, it worked 100%, there was no trace of thrips on any plant. In fact, I was very surprised at the high success rate, so I did some research on humidity and thrips,
and found that thrips die at humidities above 70%, which I believe was the real reason for their eradication. (EDIT: I need to correct this part. It is not correct to say they die at humidities above 70%, but the closer to 100%, the better to eradicate them. Also important to maintain this level of humidity for several days. At least it worked for me. You can give it a try.)
In fact, when the plant is hermetically sealed, moistened with insecticide, the humidity levels rise to 100%. I believe that this humidity for so many days was decisive for the death of the thrips, in addition to the action of the insecticide.
Obviously, this method has its drawbacks: if you have a very large plant, it might be difficult to close it with a hermetically sealed bag, but I managed to use pieces of plastic and tape them together with adhesive tape.
I hope you liked the method and please keep in mind that this method worked for me but it will not necessarily work for you. Try it at your own risk and check daily to see if the plant is doing well.
EDIT: After reading all comments, I would like to point out a few things:
- The soil was not wet, nor dry. It was 50/50. Maybe it is not a good idea to cover hermetically the plant right after watering.
- The plants that took part in this experiment were monsteras, philodendrons and orchids. Maybe other plants like alocasias, sucullents, etc. will be stressed or damaged. Some do not like high levels of humidity.
- I have read in the comments that 10 days is not enough, and maybe it is true, but in my experience 2 months have passed after the experiment and no sign of thrips. Maybe it was luck. However I would rather share this info with you just in case it would be of any help. In case thrips appear again I will update.
- Something I failed to document was the temperature, someone point this out and definitely the temperature is a thing here. If I remember well, this experiment was in an environment of around 20 - 30 degrees Celsius.
However, as I wrote earlier, this is just my experience and so far so good after 2 months. Maybe I try the same experiment next year and the outcome is different. Maybe there were many other factors involved that I haven't considered.
I wrote all of this just in case you want to try a simple and easy method that worked from me. Try initially one or two plants and go from there, don't do it like I did with all 50 plants just in case something goes wrong."