Question
I have a very healthy looking beech tree (about 12 years old, about 20 feet tall) which this year has a sticky substance on the leaves that bumble bees are drawing to savour. Also nesting in a nestbox on the tree for the last 2 years is a very chirpy, rapidly expanding family of sparrows. The substance appears to be like the sticky fall out from lime trees. What is it likely to be?

Answer
this sounds like aphids. 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,and pear-shaped body. 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. Occasionally, aphids may actually kill leaves or small shoots. Since aphids may move from one plant to another, they may transmit plant diseases which contaminate the sucking mouthparts.

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 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 injected or applied to the ground are less harmful to beneficial insects. Systemic insecticides include: acephate (Orthene).

Thes above will need to be sprayed on the foliage and branches for control. This is sometimes hard on large trees.  Imidacloprid (Bayer Advanced  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. The roots transport the insecticide to the leaves killing the aphids as they suck the plant juices.

Check with your local nursery for these insecticides.