Like clockwork, every fall they want in. The best way to identify boxelder trees is to examine their leaves and seeds on female trees. (Nancy Downs, Colorado Master Gardener, Denver County, Special to The Denver Post) I’m not referring to our four-legged furry friends after their morning patrol of the yard. “Boxelder bugs are a little creepy and seem to appear out of nowhere” says Alison O’Connor, Colorado State University Extension specialist in Larimer County.