Aphids

In Cheyenne, trees most commonly affected by aphids are willows, boxelder maples, cottonwoods, aspens, pines, spruce, and junipers. Aphids, scale insects and leafhoppers produce honeydew, a sticky substance that covers anything below an infested tree or shrub.

Aphids feeding on the sap produced by leaves, on the underside of a leaf.  A drop of "honeydew" is on the middle, extreme left side of this photo.   Honeydew is the "tree sap" dripping on your car.    
Photo by John A. Weidhass, Virginia Tech through Forestry Images

Aphids are common plant pests affecting all types of plants including trees and shrubs. Aphids with sucking mouth parts, feed on the plant sap produced by the plant leaves. They excrete a sugary, sticky, "honeydew" which covers leaves, tree limbs and trunks, and anything under the infested tree or shrub. If you park your car under a tree and it becomes covered with sticky droplets, these droplets are not tree sap. They are droplets of "honeydew" or to put it bluntly, sap-feeding insect poop. Trees and shrubs with a yearly high infestation of aphids or other sap feeding insects can have a black colored bark. The black coloring is due to sooty mold growth on the honeydew. The sooty mold will also grow on concrete, rock, wood, and other things under the tree.

Ants are a common sight on aphid infested trees. The aphid is a "milk cow" for the ants. The ants do not injure the tree, they are feeding on the honeydew from the aphids.

The aphids usually do not cause death on larger trees. Smaller trees with a high aphid infestation can be severely weakened causing the tree to succumb to other insect or disease infestation. Aphids are primarily a nuisance problem.

Aphids produce primarily female insects in the spring and summer reproducing asexually. Aphids can have wings or they can be wingless. Winged insects are produced as a result of overcrowding, changes in day length, or other environmental clues.

On shrubs and smaller trees, aphids can be knocked off the plant by water. Many aphids cannot fly, which leaves them susceptible to natural predators. The best control used for aphids should not injure natural predator insects. A mild dish soap diluted with water can be sprayed on an aphid infested tree without causing significant predatory insect death. Test spray a few leaves prior to spraying the entire plant to see if the soap will damage the leaves. The soapy water will quickly kill the aphids or at least knock them off of the tree. On larger trees and shrub areas, a soil injection of Imidacloprid (Merit®) will be taken up by the root system and move systemically throughout the tree providing a control for insects feeding on the tree. Acephate (Orthene®), a systemic chemical, can also control aphids and other tree feeding insects, but should not be used on or near cottonwoods, aspens, or other poplar trees.

 
Aphids feeding on new growth in a pine tree.  
 Photo by Clemson Univ. USDA Cooperative Extension through Forestry Images.

A predatory wasp laying an egg in an aphid.  When the egg hatches the wasp larvae will eat the aphid.    
Photo by: Alton N. Sparks Jr. - Univ. of Georgia through Forestry Images.

Links:

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.