Question

Vanilla leave   Hello,
I have had this Bourbon Vanilla for the last 5 years, it's actually three plants into one pot and the biggest stalk is about 10 ft long. It stays indoor all the time but for summer, from June to September when it's hot and humid enough outside (Italy).
Last year as I recovered the plant indoor I noticed there was something odd on a couple of leaves but I did not pay too much attention. This year I have noticed it's very slowly spreading to other leaves, it's like something is digging a tunnel into the leaves, I tried to look for bugs or similar but couldn't find anything. I have attached a picture to show it.
Research online did not come up with anything particular related to orchids. I don't know what to do now so I'm asking for help.
Thanks in advance.

Answer
Anna, I can't say for sure what it is but the damage appears similar to that caused by a leaf miner. The adult insect deposits its eggs into or on a leaf and when the eggs hatch, the larvae burrow into the leaf between the upper and lower surface of the leaf. The larvae drop onto the surface of the potting mix and burrow into it whwere they pupate and hatch into adult flies to repeat the cycle. Insecticides are not very effevtive.  The best way to prevent the spread is to remove each so-sffected leaf and discard it.