We had one trip where we had a problem with mosquitos. We were stung quite a few times on the train going from Toontown to Main Street in the MK.
We bought spray at The Emporium for some ridiculous amount of money. So, we have brought it ever since just in case. Haven't had to use it but, it does not take up much room to pack.
I do the same thing with children's cold medicine, Tylenol, etc.