Aphids on Shade Trees and Ornamentals
Aphids are an extremely common insect in landscapes and gardens. In ornamental plants, shrubs and trees, some aphids cause wilting or leaf curling and excrete a sticky waste product, honeydew that promotes sooty mold growth and can lead to plant injury.
Description and Life Cycle
Aphids are small (0.1 inches and smaller), soft-bodied insects that may be found on leaves, stems, and sometimes branches of plants. They have an oval body, and a pair of pipe-like structures (cornicles) that can usually be seen protruding from the back of the body. Colors are widely variable among the different aphid species – ranging from very pale yellow to nearly black. Most aphids are different shades of green or orange and a few species are bright red. All aphids are small, with the larger species commonly found on stems and branches.
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 feed on only a few species of plants. Some aphids obscure their body by covering themselves with waxy threads, giving the aphid a “wooly” appearance. One such wooly aphid, Prociphilus fraxinifolii, is a pest of ash trees and causes leaf curling.
Colonies of aphids often consist of a mixture of winged and wingless forms. Most aphids 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. The most common reproduction includes parthenogenetic vivipary whereby female aphids give live birth to a genetically identical daughter through asexual reproduction. 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 activity.
At the end of the summer, different forms of aphids are produced, including sexual forms. After mating, females can lay eggs, which they typically lay in crevices around buds or on stems. The eggs are commonly the overwintering stage of sexually reproducing aphids in Colorado. Eggs hatch the following spring, shortly after bud break, and the asexual life cycle resumes.
Some aphids have even more complicated life cycles that involve alternating among host plants. With these species, eggs are laid at the end of the growing season on a tree or shrub (winter host). The eggs hatch in the spring and the aphids have several generations, produce only winged forms and then leave the winter host to move and feed on a summer host. The summer hosts of these plants include several garden plants and weeds.
Whitney Cranshaw, Colorado State University, Bugwood.org.
Prociphilus fraxinifoli aphid injury on new foliage of green ash.
Whitney Cranshaw, Colorado State University, Bugwood.org
Prociphilus fraxinifoli colony within an ash leaf.
Whitney Cranshaw, Colorado State University, Bugwood.org.
Rose aphid, Macrosiphum rosae, spring colony on a rosebud.
- 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 plants or cause wilting.
- Feeding aphids excrete honeydew, a sticky fluid that can promote sooty mold growth and 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.
Aphids often do not impact plant health but cause temporary aesthetic injury to plants. A few aphid species do cause problems, particularly if they also transmit pathogens that can cause plant diseases. Aphids use their straw-like mouthparts to suck sap from plants. When the number of aphids on a plant is very high for an extended period, their feeding can cause wilting and sometimes dieback of shoots and buds. Some aphids can cause leaf curling when the insect infests emerging leaves. Aphid injury can also include copious amounts of sticky honeydew, a waste byproduct secreted by aphids and other insects feeding on sugar-rich phloem. Honeydew may cover leaves, branches, sidewalks and anything that lies beneath an infested plant. Grayish 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. Aphids also commonly vector or transmit damaging plant viruses.
The first step in aphid management is monitoring for them on landscape and garden plants to ensure populations do not surpass the threshold of aesthetic injury. Look for signs of aphid presence such as cast white skins on plant surfaces or flecks of sticky excrement, honeydew on leaves. Many aphid species cluster on the underside of leaves or newer growth. Adult winged aphids may be found on yellow sticky traps.
Physical and Cultural Controls
On shrubs and garden plants, aphids can sometimes be managed by simply washing them off plants with a forceful jet of water. Hosing plants may 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 can be infested with aphids in the spring. Columbine, lupines and perennial asters are examples. In these plants, aphids lay eggs on the new growth in the fall. The eggs can be prevented from hatching the next spring by removing the new growth prior to spring.
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. Several species of minute wasps parasitize aphids. These parasitoid wasps insert their eggs into the body of the aphid and the larvae consume it internally. Aphids that have been killed by parasitoid wasps (“aphid mummies”) have a conspicuous appearance, becoming bloated, turning light brown or black and sticking to the plant.
Where high numbers of aphids occur and injure plants or outbreaks are not sufficiently controlled by biological controls, insecticides can be used to manage aphids. Examples of chemical controls are horticultural oils, non-persistent contact insecticide sprays, insecticidal soaps, persistent contact insecticide sprays, systemic insecticide sprays, and soil-applied systemic insecticides.