I certainly won't pretend to be an expert in this area, however a few thoughts came to mind when I was considering why you have these issues.
1] I found this paper on the "Spectral Sensitivities of Wolf Spider Eyes" - there may be some interesting information in there that can be applied to other arachnids. For example, in one experiment it was shown that Wolf Spiders more frequently chose to rest in boxes bathed under red light. This was early on in the paper though and may not be accepted as scientific fact, so further reading would be advised! http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2225900/pdf/1.pdf
2] If we generally find that the arachnids and insects that you're shooting are averse to blasts of light during the night, can we further dim your focus-assist light with a piece of partially translucent tape?
3] I'm thinking about moonlight and assuming that most insects and arachnids aren't noticeably afraid of it. My guess is that the constant presence helps lower their sensitivity to it. There may be an analogy to be found in how you treat your duration as well as the general colour temperature of moonlight (around 4100 - 4150K, from what I just read).
I think the answer must lie somewhere in the fact that your auto-assist light is jarring to their senses in that it both comes on quickly and is relatively intense (read: bright) for the area they are hanging around in when you are shooting. I would expect someone walking with a flashlight swinging would cause a similar reaction were the beam to cross their eyes. It can be difficult to get any type of living creature to betray their natural survival instincts and reactions without conditioning, but some of these suggestions may help you ease or (in a perfect world) eliminate some of that external stimulus you are introducing.
(Again, I apologize that I'm not necessarily getting to the root of your question which is around best-practices... I'm not qualified to answer that! I simply noticed that your question was unanswered and wanted to give it a bit of thought.)