Establish Natural Defenses
Spot aphids early in the season before they reproduce and magnify their damage from growing numbers. This is the best defense. Aphids, like most pest insects, emerge a few weeks earlier than the predator insects that feed upon them. Cool springs often mean major aphid problems because their predators will arrive later than usual.
Attract Beneficial Insects To The Yard - Birds, spiders, and predator insects will kill many more aphids than a homeowner armed with insecticide ever will. If you avoid the use of broad-spectrum insecticides that indiscriminately kill whatever they touch, your yard will host lots of beneficial insects. Insects such as the legendary ladybug, green lacewings, braconid or chalcid parasitic wasps, spined soldier bugs, and earwigs are great aphid eaters. The three best beneficial insect families are the ants, spiders, and ground beetles, but they usually are found in organic mulch, a major reason for mulching your gardens, trees, and shrubs.
Feed Birds All Year Round - Songbirds consume enormous numbers of pest insects in your yard. Even seed-eating birds such as sparrows and finches hunt for insects to feed their several broods of young per season until they are able to digest seeds. To enlist the birds’ help in your fight against pest insects, provide food, water, and shelter for bird families.
Improve the soil in your yard by adding organic matter to it regularly so that it can nourish vigorous plants that are able to resist or repel pest insect attacks. Mulching all the bare soil under all the plants on your property is the easiest way to do this. The mulch gradually decomposes over time, earthworms pull it down into their burrows in the soil, thereby adding vital nutrients and improving soil structure and drainage. Spread 2 to 4 inches chopped leaves, grass clippings, compost, hay, shredded paper, or straw over the soil. Sphagnum peat moss makes a good mulch as long as it’s mixed with coarser materials such as chopped leaves or shredded paper. Used alone, it tends to crust over and repel water.
Encourage the many insects and animals that normally prey on insect pests to take up residence in your yard by planting a variety of trees, shrubs and groundcovers to feed and shelter them. While many are insects themselves, others are microscopic organisms, amphibians, or birds. The more of these different kinds of predators live in your yard, the fewer pest insect problems you will have. The greater the variety of plants on your property, the greater will be the diversity of natural enemies of pest insects that will be attracted to it.
Keep Trees, Shrubs, and Plants Vigorous
Unless the underlying stress that made your plants vulnerable to aphid attack is identified and corrected, the pests may return next year. Give some thought to the affected plant’s situation. Make an effort to improve its vigor by watering it during dry periods over the summer and fertilizing it in the fall, if appropriate. Prune any injured or broken branches or stems cleanly and carefully. Mulch it to discourage weeds and improve the health of the soil beneath it so it holds moisture, yet drains well. Make sure the plant is getting enough light as the surrounding plants grow and may start to block the sun.
Spray Winter Horticultural Oil
In late winter before the new buds and foliage begin to emerge on deciduous trees and shrubs, spray heavy horticultural oil (also called dormant oil or Volck oil) on their bark to suffocate any overwintering aphid eggs. Cover the entire surface of each plant. This is especially important on fruit trees. Always read and follow the instructions on the label of the bottle or container.