The Centers for Disease Control (CDC) has extensive webpages for bites, stings, and poisonous plants including identification and prevention. Loads of good info for cachers.
Also, your local poison help center may be able to send a rep out to your local event cache for free if you can get together at least 30-40 people to attend. UTMB runs the local Poison Help Center in the Houston area. I've already had them out to talk to my fellow employees twice and the rep has an interest in geocaching, so I have been planning an event around this very topic.
One of my favorite bits of insect safety info is the Schmidt Sting Pain Index. The descriptions are funny, but very accurate. I have been stung by everything from 1.2 to 3.0 on that list and I know of a cacher that got hit by a Tarantula Wasp (4.0) while caching with a similar description came from him. I have encountered those on numerous occasions without incident, but mannnn those suckers are freakin' scarey.
Oh man, that thing makes me feel faint. How I love my safe and boring UK where wasps are normal and spiders don't kill you.