I been camping bugs and bats for almost 20 hours of game time over the course of two days and haven't seen a single drop. I think they mate it a bit too rare.
Too rare? You are lucky that it is available at all. It's HoF T4 RAID quality item, with the best overhaste clicky in the game, and it's available to group content. Why should it be easy to get??? /boggle
Seems to be about a 1-500 to 1-1000 drop..but I'm just guessing based upon the gazillion hours I've spent camping these for my main and wife's characters.
Five hours of killing in Lava Duct Crawler room with a three box team netted one. Almost constant killing, some down time between spawns and all the other groups around sniping some here and there.