Hello, Hazem. It means to use something as a reason for doing something or taking an action. With your example, non-white people might act or react as a result of being called 'invading vermin'. Here are some other sentences using 'act upon' that might help you understand its meaning better. - Ideas are useless unless you act upon them. - She did not act upon my suggestion. - The question comes down to whether we ought to act upon his advice. I hope this answer helps! If you're interested in increasing your English fluency, I am a professional English teacher on Italki and would be happy to work with you.

What a political and racist comment you are quoting. Maybe you found it in a newspaper. To 'act upon' is to behave, to act from the basis of something. So if you tell someone something derogatory (bad) about them, e.g. a parent or a boss saying bad things about the child/worker, then do not be surprised that the child/worker will behave like that, (that they are a naughty child/ a lazy worker). If you tell someone what (you think) they are, they will 'act upon' that basis; they will incorporate that information and behave as though it were true.