I wasn't aware of this problem until I became a victim...spots all over my house, camper, etc.. Now I have become quite the expert. Anyway, I am now in the process of removing the mulch so I can put in river rock. The only "suggestion" I was given for handling the problem was to spread another layer of "good" mulch over the existing layer. I figured it was better in the long run to just eliminate the potential problem altogether. Needless to say, removing several inches of mulch over a LARGE area is no small task.
I was wondering if anyone else had battled this problem, and what you did to solve it. Thanks!