usually in the spring time. If you are able to find a field this time of year that has milkweed and hasn't been mowed you'll likely find monarch caterpillars. Well, thats for the Bryan, Texas area anyways. But you can also se them in the fall when they are migrating.
I think it's a mixture between right time of day and luck. As far as I know, bugs are affected by the time of day, but not area. Each bug has a certain amount of odds of appearing when you enter an area. I don't think it also depends on the area. I imagine rare bugs have just 2% or 5% odds of appearing and having it depend on the area as well is sort of unfair. I mean, there must be hundreds or thousands of areas there! XD