An insect known for turning entire mountainsides the color of rust could be making a resurgence in Colorado.  The pest is none other than the mountain pine beetle. After a roughly decade-long period of relatively lower populations, the bugs are rebuilding their numbers along the Front Range and in southwest Colorado, according to an annual forest health report published by the Colorado State Forest Service in late March. “I’m a little concerned moving in this summer because we really haven’t had any precipitation,” said Dan West, the forest entomologist for the Colorado State Forest Service. “I’m worried bark beetles are going to increase their populations in these drought-stricken trees.” Few bugs have had a more visible impact on forests across the western U.S.