Question
I have three quaking aspen I planted about six years ago which are not fully mature yet. Each of the last three years the leaves have been covered with small green aphids that attract yellow jackets in great numbers. Is there anything that I can do to stop this invasion by the aphids each year?

Thank you for your time.

Answer
Aphids are small (1/16-1/8 inch long), soft bodied insects commonly called plant lice or ant cows. Virtually every plant has at least one aphid species that attacks it. These small insects are masters of reproduction and are often found in great numbers on stems or leaves. Some species even feed on the roots of plants. They range in color from green to brown, red, black or purple. Some species may even have different color forms in the same colony. Most have the soft exoskeleton exposed, but some species produce waxy, cottony strands which cover the body. These are often called woolly aphids.

Aphids are identified by their sucking mouthparts, long, thin legs, long antennae, pear-shaped body and pair of tube-like structures (called cornicles) arising from the posterior of the abdomen. A hand lens may be needed to see the short cornicles of some species. These cornicles apparently are the ducts of glands that produce alarm odors. Aphids may be winged or wingless and colonies often have both forms.

Aphids excrete a sugary liquid called honeydew. This honeydew drips onto plant foliage or other structures and provides a suitable place for black sooty molds to grow. Ants often tend or care for aphids in return for the honeydew. Therefore, if ants are running over a plant, look carefully for aphids.

Each plant reacts differently to aphid attacks. Some plants show no response to the aphids, while other plants produce distorted (twisted, curled or swollen) leaves or stems.

Probably the most common disturbance caused by aphids is their never-ending production of honeydew. This sweet liquid drips onto plant foliage and stems and is soon covered with black sooty mold. Cars, sidewalks, and lawn furniture under trees with aphids are also covered with this sticky fluid. Ants, flies and wasps appreciate the sugary meal and can become a nuisance of their own. Even though plants may look bad from the growth of sooty molds, these fungi do not damage the plant tissues. Once the aphids disappear, the sooty mold often dries up and falls off the plant.

Chemical Control - Contact Insecticides Numerous contact insecticides are registered for aphid control. Since aphids are often placed under considerable pesticide pressure in field crops and greenhouses, they may be resistant to certain categories of insecticides. Therefore, if you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene), bifenthrin (Talstar), chlorpyrifos (Dursban), diazinon,  malathion, nicotine sulfate, pyrethrum, rotenone, resmethrin, and tetramethrin + sumithrin.

Chemical Control - Systemic Insecticides
These above will need to be sprayed on the foliage and branches for control. This is sometimes hard on large trees. Several systemic insecticides are useful in aphid control. Aphids have sucking mouthparts and are thus very susceptible to pesticides located in the plant vascular system. Some of the systemic insecticides also have contact activity. Systemics  applied to the ground are less harmful to beneficial insects.  Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil.
I would recommend that you use the Bayer Advanced this spring. The roots will take the insecticide to the foliage and kill  the aphids when they suck the plant juices. This should last all season. If there is not honeydew production there will be no Yellow jackets.

Check with your local nursery for these insecticides.