Aphids on shade trees and ornamentals
Wednesday, July 16, 2014 5:00 PM
As I was entering the post office the other day, I noticed that the sidewalk and steps appeared wet. Since the sprinklers were not on, I decided to look a little closer at the leaves on the nearby trees. As I expected ... Aphids.
Aphids feed by sucking sap from plants. When the number of aphids on a plant are very high for an extended period, their feeding can cause wilting and sometimes even dieback of shoots and buds. Some aphids can cause leaf curling when the insect infests emerging leaves.
Sometimes problems with aphids do not primarily involve plant injury but instead their production of sticky honeydew. Honeydew is the waste material excreted by aphids and certain other phloem-sucking insects (e.g., soft scales, whiteflies, some leafhoppers). It may cover leaves, branches, sidewalks and anything that lies beneath an infested plant material.
Gray sooty mold grows on the honeydew, further detracting from plant appearance. Ants, yellow jacket wasps, flies, and bees are usually attracted to plants that are covered with honeydew.
Aphids are small insects and few exceed 1/8-inch when full grown. They tend to have an oval body form and a pair of pipe-like cornicles usually can be seen protruding from the back of the body. Colors are widely variable among the different aphid species - ranging from very pale yellow to dark, nearly black. Most have shades of green or orange and a few species are even bright red. Upon close inspection, many aphids can be seen to have intricate body patterning.
Some aphids obscure their body by covering themselves with waxy threads. These are known as "woolly aphids." Woolly aphids are most commonly seen associated with pines or other conifers, lining the needles. However, the woolly apple aphid is a common woolly aphid that clusters on the limbs of apples and crabapples. Aphids that cluster within leaves that curl, such as the leafcurl ash aphid, are wax covered as are most aphids that live on plant roots.
Colonies of aphids often consist of a mixture of winged and wingless forms. The great majority of aphids usually develop into the wingless form to remain and reproduce on the plant. More winged forms tend to be produced when colonies get overcrowded, plants decline in quality, or environmental cues favor dispersal to new plants.
Essentially all aphids, regardless of their form, are females. Males, if they do occur, are present in late summer during only one of the many generations that are produced during a growing season. The normal habit of aphids is for a female to give live birth to a genetically identical daughter aphid through asexual reproduction (parthenogenesis). The newly born aphid can develop rapidly, typically becoming full-grown in about 10 to14 days. Adults usually can produce three to five young per day over the course of their lifetime, which may extend to about a month but is usually shortened by natural enemy activities.
Aphids are quite defenseless and there are numerous insects that feed on them. The best known of these natural enemies are lady beetles, with lady beetle larvae being particularly voracious predators of aphids. Other common aphid predators include the larvae of green lacewings and flower (syrphid) flies.
On shrubs and garden plants, aphids can sometimes be managed by simply washing them off of plants with a forceful jet of water. Hosing plants can lethally injure aphids and very few surviving aphids that are knocked to the ground can successfully find their way back onto their host plant.
Some flowers that are perennial, but dieback to the ground in fall, have problems with aphids in the spring. Columbine, lupines and perennial asters are examples. With these plants the eggs of the aphids are laid on the stems in fall, near the point where new shoots will emerge the following spring. Spring problems with these aphids can be prevented by removing the old top growth that contains the eggs before plants emerge in spring.
Insecticides are a useful means for controlling aphids when natural enemies are not sufficient. Some insecticides act by contact action and these must contact the body of the aphid to work. This includes insecticidal soaps (active ingredient - pyrethrins), a popular option for aphid control but one that requires sprays to cover the aphid during application. Other insecticides have some persistence on the foliage and may be able to kill aphids for a day or two if they contact the aphid (active ingredients - acephate, permethrin, bifenthrin). Contact insecticides can be effective against exposed aphids but are ineffective against species that develop within the protection of leaf curls.
A few insecticides have the ability to move within a plant, spreading in the sap. These are known as systemic insecticides and they can control aphids that occur within leaf curls (active ingredients - imidacloprid, dinotefuran). Some formulations of systemic insecticides are designed to be applied as sprays and these are absorbed by leaves and then move in the plant. Others can be applied to the soil where they are taken up by the roots and translocate to leaves, particularly young leaves.
Horticultural oils have a special place in aphid control. These act largely by smothering insects and are particularly effective for control of aphids that spend the winter as eggs on the tree or shrub, and then curl leaves the following spring. They are most widely used for aphid control on stone fruits, such as peach, apricot, and plum. Horticultural oils are applied before bud break, during the dormant season.
Source: Aphids on Shade Trees and Ornamentals, by W.S. Cranshaw, factsheet 5.511.