Aphids on Trees and Shrubs

Aphids on rose bud. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Updated: October 17, 2024

Key points

Aphids (plant lice) are common plant feeding insects. Usually, they do not occur in damaging numbers.
Occasionally large populations develop that may temporarily reduce the aesthetic quality of trees and shrubs but can damage small plants like annual flowers.

Most mature aphids are about an eighth of an inch long and are yellow or light green.
A few are black, shades of red, brown, white, or grey.
Aphids may be winged or wingless.
Each aphid has 6 thin legs, 2 antennae on the head, a pair of tubes on the back, and a slender “beak” which is pushed into plants to suck sap.
Most aphids prefer to feed on buds and the underside of terminal leaves, however, some species are adapted to feed on roots.

Several species occur on needled evergreen shrubs in Maryland.
The white pine aphid, Cinara strobi feeds only on white pine.
Winged adults are 1/16 inch (4 mm) long. The body is shiny dark brown to black, with a white stripe down the middle of the back and powdery white spots on the sides. The eggs overwinter on the needles.
Heavy infestations of this aphid may kill young shrubs or branches of large shrubs. Several other species of aphids Cinara, Eulachnus, Essigella, and Schizolachnus infest other species of pine. And there are other species of aphids that attack juniper, spruce, and fir.

Aphid populations may build up quickly.
When the overwintering eggs hatch in the spring, usually April or May, the emerging insects are all females.
After that, females produce females (generally they are born alive) without male fertilization, throughout the summer.
In hot weather, a generation may be produced every 1-2 weeks.
In the fall, a generation of both males and females appears. The two sexes then mate and the females deposit oval, black eggs on the bark of shrubs and trees.
These eggs do not hatch until the next spring.

Low aphid numbers usually do not result in plant damage. However, large aphid populations can cause wilting, yellowing, and curling of leaves. This may severely damage small plants such as flowering annuals unless the aphids are controlled by beneficial insects or by human intervention.
Large shrubs and trees outgrow aphid damage during the growing season because beneficial insects reduce aphid populations below damaging numbers.
When aphids feed, they excrete “honeydew”; a sugary solution left after partial digestion of plant sap. This liquid falls on lower plant leaves, picnic tables, patios, and sidewalks. The honeydew dries into a clear, shiny spot. These spots often turn black because a black fungus called sooty mold (see photo below) grows on them. A dense sooty mold growth shades out sunlight and affected leaves may be weakened. Honeydew also has a useful purpose; it attracts predators and parasites of aphids.
Aphids also transmit many virus diseases between ornamentals. Viruses are spread from plant to plant when aphids feed on diseased plants and then move to feed on healthy plants. The viruses are carried from plant to plant inside the aphid in the insect infected plant sap.

Before considering sprays for aphids on shrubs and trees, carefully examine some infested leaves for the presence of natural enemy life stages. These include the eggs, larvae, pupae, and adults of predators such as lady beetles (ladybugs), lacewings, and flower flies.
Also, look closely for the presence of aphids containing wasp parasites. Parasitized aphids, called “mummies” are round and tan colored (refer to the photo below). When the minute wasp emerges from the aphid mummy, a small hole is left in the aphid’s back.
It only takes a few predators and parasites on a small plant or branch terminal to reduce aphid populations.

Aphid populations can be reduced by washing the leaves and stems of sturdy plants, such as shrubs and trees, with strong water sprays from a garden hose.

If no aphid predator or parasite life stages are present, and the level of plant damage is objectionable, control may be necessary.
A commercial brand of insecticidal soap is the safest insecticide to use.
Soap kills by contact, so coverage must be thorough.
Repeat applications may be necessary. Read and follow label directions.

Indoor sprays labeled for aphid infested house plants are usually pump sprays containing insecticidal soap or pyrethrum.
Be sure the chemical is labeled for indoor use and the pest and plant you want to spray is on the label.