Jerry/Noel- I built a similar type foam viaduct a few years ago and painted mine with drylok by UGL (http://www.drylok.com). It’s the stuff you normally use to waterproof basement walls. The paint has hydraulic cement in it which forms a pretty hard “shell” on the foam. So far it has kept the bugs and squirrels from eating the foam. Of course I realize that your bridge has already been painted, but it might be some useful info of the future.
-Kevin.