Our cool wet spring has apparently provided ideal conditions for that perennial garden scourge, aphids. A good crop of weeds provided early food for the pests, and now they're moving into gardens in force. The vigorous, succulent new growth of fruit trees, ornamental plants and vegetable seedlings is very attractive to the hungry aphids.

The damage caused by leaf-feeding aphids in most plants is similar. Low to moderate numbers are usually not damaging in gardens or on fruit trees. However, numbers build quite rapidly, and large populations cause curling, yellowing, and distortion of leaves and stunting of shoots. They can also produce large quantities of a sticky exudate known as honeydew, which often turns black with the growth of a sooty mold fungus. Seedlings of vegetables can be killed or severely distorted by aphids. Once seedling have 5 or 6 true leaves, they become more resistant to aphids, but full-grown squash or pepper plants can still be killed. Some aphid species inject a toxin into plants, which causes more distortion.

Some aphids even transmit virus diseases from plant to plant. Squash, cucumbers, pumpkins, melons, beans, potatoes, lettuce, beets, chard and bok choy are crops that can be affected by aphid-transmitted viruses. The viruses cause mottling, yellowing, or curling of leaves and stunting of plant growth. The diseases are difficult to control because infection occurs even when aphid numbers are very low. The best way to deal with viruses is to choose resistant varieties and remove already-infected plants before planting a new crop. Be aware that many viruses are not spread by aphids, but by whiteflies, leafhoppers and other insects.

A few aphid species attack parts of plants other than leaves and shoots. The lettuce root aphid is a soil dweller attacking lettuce roots during most of its life cycle; the woolly apple aphid infests woody parts of apple and pyracantha roots and limbs, often near pruning wounds, and can cause the plant to decline if the roots are infested for several years.

The key to preventing serious aphid damage is to check your garden regularly for the pests, at least twice weekly when plants are growing rapidly. Once aphids are high and have begun to distort and curl leaves, it is often harder to control them because the curled leaves shelter them from natural enemies or pesticide sprays. This is especially true for such fruit trees as apples or plums, which are often heavily infested by aphids in spring.

Aphids tend to be most prevalent along upwind edges of the garden and close to other sources of aphids, so be sure to check these areas carefully. Many aphid species prefer the undersides of leaves, so make a special effort to turn leaves over in your search. Be sure to check the foliage at the ends of trees and shrubs, as aphids tend to prefer new growth. Also, keep an eye out for evidence of natural enemies such as lady beetles, lacewings, syrphid fly larvae, and the mummified skins of parasitized aphids. High numbers of any of these natural controls may mean that the aphid population might be reduced rapidly, even to the point where you do not have to spray.

If natural controls do not keep pace with the aphid population, you may need to take steps to control the pests. There are a number of effective insecticide sprays, for both food crops and ornamentals, available at your retail nursery and garden store. These products range from insecticidal soaps and oils to longer lasting products such as malathion. Soil applied systemic insecticides are available for some ornamentals as well. Remember that insecticidal sprays will control natural enemies along with the aphids.

Ants are often associated with aphids, especially in fruit trees, where they protect the aphids from natural enemies and feed on the honeydew which the aphids excrete. If you see large numbers of ants climbing up your tree trunks, check for aphids on the limbs and leaves above. You can prevent ants from climbing your trees by putting a band of sticky material, available as Tanglefoot or Stickem, around the trunks. Prune out other ant routes such as branches touching the ground, buildings, or other trees.