One type of tree-killing beetle is attacking Colorado forests at a faster pace, but another is slowing down, partly because it’s running out of live trees to infest. State and federal forestry officials said Friday spruce beetles were active on nearly 760 square miles last year, compared with fewer than 625 square miles in 2013. Spruce beetles have attacked a total of about 2,200 square miles of Colorado forests since 1996. The mountain pine beetle was active on fewer than 25 square miles in 2014, down from almost 155 square miles in 2013. Forest managers say in many areas the pine beetle has already killed most of the larger trees that could sustain them. Pine beetles have attacked more than 5,300 square miles of Colorado trees since 1996. Mountain pine beetle activity has subsided and remains low, with just under 8 square miles affected statewide. The epidemic has ended in many areas of Colorado as mature pine trees have been depleted in the core outbreak areas. A major concern of the epidemic is the risk of falling trees that can topple without warning.