Life history and habits

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.

Injury

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.

Management

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. 

Cultural control

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 old growth prior to the emergence of new growth in spring.

Biological control

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. 

Chemical control

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. Insecticidal soaps are a popular chemical control for aphid management but must come in direct contact with aphids during the application to be effective. Other insecticides have a longer residual activity and can kill aphids on contact for one or two days after an application. However, contact insecticides are ineffective against aphid species that develop within curled leaves. One group of insecticides, called horticultural oils, act by smothering insects and are of particular importance in aphid control on stone fruits. These horticultural oils are often applied during the dormant season before bud break during, which targets aphids that overwinter as eggs on trees and shrubs.

Systemic insecticides work by moving through the plant where the toxin accumulates in plant tissues and fluids. Some formulations are designed to be applied as foliar sprays, while others are applied in soil where they are taken up by roots and transported to young leaves.

Additional reading

University of Nebraska-Lincoln. (n.d.). Aphids on Shade Trees and Ornamentals. University of Nebraska-Lincoln: Extension. Available https://lancaster.unl.edu/aphids-in-trees-ornamentals

Utah State University (n.d.). Aphids. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/ornamental-pest-guide/arthopods/aphids-adelgids/other-aphids