Click on the green highlighted insect name to go to that information page:
     
     Pine and Spruce:
          Mountain Pine Beetle - Causing death in ponderosa pine, Scotch pine, limber pines and some spruce trees.
          Spruce Ips Bark Beetle - Causing die-back and death in spruce trees.
          Douglas-fir Tussock Moth - Primarily on spruce.
          Cooley Spruce Gall - Galls on spruce branch ends
          Aphids - What don't they get on.

     Ash:
          Aphids - What don't they get on.
          Lilac/Ash Borer - Primarily on ash trees.
          Oystershell Scale - Primarily on aspens, cottonwoods, ash, cotoneaster.

     Crabapple, Apple, Hawthorn: 
          Aphids - What don't they get on.
          Pearslug - Primarily on hawthorn trees, cotoneaster bushes.
          Woolly Apple Aphids - Apple, crabapple, mountain-ash.

     Cottonwood, Poplar, and Aspen:
          Aphids - What don't they get on. 
          Cottonwood Leaf Beetle - Primarily on cottonwood, poplar, aspen
          Oystershell Scale - Primarily on aspens, cottonwoods, ash, cotoneaster.
          Petiolegall aphid - Primarily on cottonwoods and poplars marble-like growth on leaf stalk.
          Poplar Blackmine Beetle - Affecting cottonwood leaves, showing large black patches on the leaves.

      Elm:
           Aphids - What don't they get on.
           Banded Elm Bark Beetle - Primarily on Siberian elms.
           Elm Leaf Beetle - Primarily on elms.
           Spiny Elm Caterpillar Feeding on Deciduous Tree Leaves - Primarily on elms, hackberry.

      Honeylocust:
           Aphids - What don't they get on.
           Cottony Maple Scale - Primarily on honeylocust, silver maples.
           Honeylocust plant bug - Honeylocust trees.

      Maple:
           Aphids - What don't they get on.
           Cottony Maple Scale - Primarily on honeylocust, silver maples.

      Various Shrubs:
           Aphids - What don't they get on.
           Black Vine Weevil - Primarily on lilacs.
           Leafcutter Bee - Roses and many other plants with larger leaves.
           Oystershell Scale - Primarily on aspens, cottonwoods, ash, cotoneaster.

Other Insects of Interest:
           Insects of Interest - Two-tailed swallowtail, Tussock Moth.

Colorado State University Cooperative Extension

University of Wyoming - Cooperative Extension Service:  Publication B-1035 is particularly helpful for tree care. This online publication contains information on: Aphids, Borers, Cottonwood blotch leaf miners, Cytospora canker, Fireblight, Gall makers, Aspen leaf spots, Oystershell scale, Pear slugs, Powdery mildew, and Spider mites.