Have you noticed a sticky substance on the leaves of trees and shrubs, or have you recently parked your car under the boughs of a spreading maple only to discover it splattered with a sticky material? If so, don't blame the tree. The substance is not tree sap but an indication of an infestation of insects called aphids.

Aphids, often called plant lice, seem to be infesting many plants this summer. They can be found on maples, peppers, cabbage, pine trees, bluegrass, apples and many other plants.

An aphid is usually specific to one plant or possibly a very few species. For example, the tomato aphid is found on tomatoes; the pine bark aphid is found on pine limbs and trunks; and the cabbage aphid infests cabbage, cauliflower and broccoli. However, some aphids are general feeders, such as the green peach aphid found on peach trees, peppers and potatoes.

Aphids are slow moving, winged or wingless, soft-bodied insects that vary in size and color. They multiply rapidly often producing live young that mature in a week or less to produce even more aphids. They also are very susceptible to disease and attack by parasites and predator insects. Therefore, an aphid population on a particular plant can increase rapidly or disappear just as fast.

Evidence of an aphid infestation includes curled leaves caused by aphid feeding, the presence of a sticky substance called "honeydew" with a sooty black mold growing on it and ash-colored skins shed by the aphids as they grow. Live aphids may have disappeared due to disease, lack of food or other factors. Heavy aphid infestations can generally be controlled by insecticide sprays including a few systemics. A few aphid species are, however, resistant to some of these insecticides. A heavy stream of water will knock aphids off the plants. An insecticidal soap is an organic alternative.

An infestation of aphids on healthy established shade trees generally does not need to be treated. The insignificant damage to the tree does not justify the cost of hiring a commercial sprayer who has the equipment to treat large trees. Heavy infestations on small or weak, ailing trees can be controlled with the above-mentioned insecticides.

Time and money is often better spent fertilizing and watering to ensure good recovery and health. Avoid fertilizing trees and shrubs after July 15. Late summer fertilization can often stimulate lush growth that does not have sufficient time to harden off before winter sets in.

The greatest response to fertilization is seen when it is applied in the fall or early spring.