no. 5.511

Aphids on Shade Trees and Ornamentals

by W.S. Cranshaw 1 (2/09)

Quick Facts...

  • Aphids are found on almost all types of plants and a few species can cause plant injury.
  • Some aphid species can curl the new leaves of some types of plant.
  • Feeding aphids excrete honeydew, a sticky fluid that can cause nuisance problems.
  • Natural enemies of aphids include lady beetles, flower fly larvae, lacewing larvae, and parasitic wasps.
  • Exposed aphids can be controlled by insecticides, insecticidal soaps and sometimes with a strong jet of water.
Figure 1: Adult aphids — winged and wingless.
Figure 1: Adult aphids — winged and wingless.
Figure 2: Black cherry aphid colony.
Figure 2: Black cherry aphid colony.

Aphids are the most common insects found on trees, shrubs, and garden ornamental plants. Over 350 different aphid species occur in the state but most can feed on only a few species of plants. However, with so many kinds of aphids, few plants grown in Colorado do not support at least one aphid. Most species rarely injure plants or even attract attention, but a few aphid species do cause problems (Table 1).

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 a infested plant material.

Gray sooty mold grows on the honeydew, further detracting from plant appearance. Ants, yellowjacket wasps, flies, and bees are usually attracted to plants that are covered with honeydew.

Table 1: Some common aphids associated with trees, shrubs and ornamentals in Colorado. Those marked with an * commonly cause leaf curling distortions in new growth.

Scientific name (Common name)
Host plant
Acyrthosiphum pisum (Pea aphid) Sweet pea, other legumes
Aphis helianthi (Sunflower aphid) Red twig dogwood, many flowering plants in summer
Aphis nerii (Yellow milkweed aphid) Milkweeds (Asclepias)
Aphis spiraecola (Spirea aphid) Spirea
Aphis viburnicola (Snowball aphid) Snowball viburnum
Brachycaudus helichrysi (Leafcurl plum aphid) Plum
Caveriella aegopodii (Willow-carrot aphid) European willows
Chaitophorus populicola Populus
Chaitophorus populifolii Populus
Chaitophorus viminalis Willow
Cindara spp. (Giant conifer aphids) Pines, juniper, spruce
Cryptomyzus ribis (Currant aphid) Currant
Dysaphis plantaginea (Rosy apple aphid) Apple
Dysaphis tulipae (Tulip bulb aphid) Dutch iris, tulip
Eriosoma lanigerum (Woolly apple aphid) Elm, apple, crabapple
Eriosoma amiercanum (Woolly elm aphid) Elm, amelanchier
Essigella spp. Pines
Eulachnus spp. Pines
Hyadaphis tataricae (honeysuckle witches’ broom aphid) Tatarian honeysuckle
Hyalopterus pruni (Mealy plum aphid) Prunus
Macrosiphum rosae (Rose aphid) Rose
Macrosiphum euphorbiae (Potato aphid) Rose, many flowers
Macrosiphum albifrons (Lupine aphid) Lupine
Monellia caryae (American walnut aphid) Walnut
Myzocallis tiliae (Linden aphid) Linden
Myzocallis alhambra (Western dusky-winged oak aphid) Bur oak
Myzocallis ulmifolii (Elm leaf aphid) Elm
Myzus ceraki (Black cherry aphid) Tart Cherry
Myzus persicae (Green peach aphid) Peach, apricot, other Prunus
Nasonovia aquilegiae (Columbine aphid) Columbine
Nearctaphis bakeri (Shortbeaked clover aphid) Hawthorn
Periphyllus lyropictus (Norway maple aphid) Norway maple
Prociphilus franxinifolii (Leafcurl ash aphid) Green ash
Pterocomma bicolor Populus
Pterocomma smithiae (Black willow aphid) Willow
Rhopalosiphum cerasifoliae (Chokecherry aphid) Chokecherry, pin cherry
Rhopalosiphum nymphaeae (Water lily aphid) Prunus, various aquatic plants
Tuberolachnus salignus (Giant willow aphid) Willow
Uroleucon sp. Many flowers

Life History and Habits

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.

Figure 3: Giant conifer aphids on juniper. Figure 4: Leafcurl plum aphid injury. Figure 5: Leafcurling produced by green peach aphid.
Figure 3: Giant conifer aphids on juniper.
Figure 4: Leafcurl plum aphid injury.
Figure 5: Leafcurling produced by green peach aphid.

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.

There is a shift in the life cycle of aphids to handle the challenge of winter, when plants are not active and cold temperatures would be lethal. At the end of the summer, different forms of aphids are produced, including special sexual form males and females. After mating, a special egg-producing aphid is produced that lays egg in protected crevices, often around buds. This egg is the stage that the aphid normally survives winter during outdoor conditions in Colorado. Eggs hatch the following spring, shortly after bud break, and the normal life cycle resumes.

Some aphids have even more complicated life cycles that involve alternating among host plants. With these species, eggs are laid on a tree or shrub.

Figure 6: Spirea aphid colony.
Figure 6: Spirea aphid colony.
Figure 7: Overwintering eggs of rose aphid.
Figure 7: Overwintering eggs of rose aphid.
Figure 8: Pea aphid colony with a syrphid fly larvae (lower left).
Figure 8: Pea aphid colony with a syrphid fly larvae (lower left).

Table 2: Some common Colorado aphids that alternate between woody and herbaceous hosts.

Aphid Overwintering Host Summer host
Black cherry aphid Woolly elm aphid Wild mustards
Currant aphid Currant Wild mustards
Green peach aphid Peach, plum, apricot Peppers, cabbage, potato, many garden plants
Leafcurl plum aphid Plum Various aster-family plants, clover, vinca, thistle
Mealy plum aphid Plum Cattail, reeds
Potato aphid Rose Potatoes, tomatoes and many other garden plants
Rosy apple aphid Apple, pear, mountain-ash Plantain
Shortbeaked clover aphid Hawthorn Legumes
Sunflower aphid Dogwood Sunflower, yucca, parsley, cilanto, pigweed, many other herbacceous plants
Water lily aphid Plum, other Prunus Water lily and many other aquatic plants
Willow-carrot aphid Willow Carrot, parsley, dill
Woolly elm aphid American elm Amelanchier (roots)

The best known 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.

Management

Natural Enemies

Figure 9: Winged aphids giving birth on aspen stem.
Figure 9: Winged aphids giving birth on aspen stem.

Aphids are quite defenseless and there are numerous insects that feed on them (Fact sheet 5.550, Beneficial Insects and Other Arthropods). 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.

Several species of minute stingless wasps parasitize aphids. These wasps insert their eggs into the body of the aphid and the larvae consume it internally. Aphids that have been killed by parasitic wasps have a conspicuous appearance, turning light brown or black and becoming bloated. Aphids killed by parasitic wasps are known as “aphid mummies.”

Table 3: Insecticides for control of aphids on shade trees and ornamentals.

_______________________________________________________________

Contact Insecticides without Residual Activity

insectical soaps-- pyrethrins

Contact Insecticides with Residual Activity

acephate-- bifenthrin-- beta-cyfluthrin
estenvalerate-- permethrin--lambda-cyhalothrin

Systemic Insecticides

acephate-- imidacloprid-- dinotefuran

________________________________________________________________

Physical and Cultural Controls

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.

Figure 10: Tulip bulb aphids. (Parasitic mummy is lower left.)
Figure 10: Tulip bulb aphids. (Parasitic mummy is lower left.)

Chemical Controls

Insecticides are a useful means for controlling aphids when natural enemies are not sufficient (Table 3). Some insecticides act by contact action and these must contact the body of the aphid to work. This includes insecticidal soaps (Fact sheet 5.547, Insect Control: Soaps and Detergents), 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. 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. 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.

Figure 11. Wooly apple aphid colony on crabapple twig.
Figure 11. Wooly apple aphid colony on crabapple twig.

Horticultural oils (Fact sheet 5.569, Insect Control: 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, then curl leaves the following spring. They are most widely used for aphid control on stone fruits (Prunus spp.), such as peach, apricot, and plum. Horticultural oils are applied before bud break, during the dormant season.

1Colorado State University Extension entomologist and professor, bioagricultural sciences and pest management. 4/96. Revised 2/09.

Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating. Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.

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