By Nate Royalty, Ph.D.
What do they look like?
There are hundreds of species of aphids, and hundreds of different plants are hosts. Aphids are small, 1/16-inch to 1/8-inch long, soft, and pear-shaped. Adults can be winged or unwinged. Long, thin legs; long antennae; and two tubes called cornicles near the hind end of the body are also standard features of aphids. Due to the diversity of species, the color and size of the pest vary. The most common colors are green, yellow and black, but they also can be gray, brown, pink, red, or lavender. Often, the color depends on the host on which they feed.
Host material and lifecycle
Crapemyrtle aphids, tuliptree aphids, melon aphids, rosy apple aphids, woolly apple aphids, Asian hackberry aphids and elm aphids are just a few of the commonly occurring aphid species that are pests of ornamentals. The life cycle of aphids is highly unusual in that aphids have two different obligate host plants. The primary host is the host where both males and females are found, and sexual reproduction occurs. On the secondary host, females reproduce asexually and can give birth to between two and 20 new females per day. When aphids reach damaging numbers on a host plant, it is almost always on the secondary host. Succeeding generations become mature within a week or two and then start to procreate. The short span of the cycle can make infestations seem to occur overnight.
Aphids have slender, piercing mouth parts that suck the sap out of the leaf tissue of host plants. Heavy populations can slow the growth of the plant and cause leaf discoloration. Some species, such as rosy apple aphid, have a toxic substance in their saliva that causes leaves to curl up, while feeding by others, such as elm leaf aphids, can cause galls to form on the leaves, The galls and curled leaves provides a favorable microhabitat and protection from natural enemies. Aphid feeding also can transmit plant viruses, and many species excrete sticky material from their cornicles called honeydew. The honeydew is often food for ants; these ants can tend the aphids, protecting them from natural enemies while they harvest the honeydew. A black fungus called sooty mold also feeds on the honeydew. A heavy infestation of sooty mold will severely discolor the infested leaf, and reduce photosynthesis. If the honeydew is heavy enough, it can drip from infested trees, discoloring sidewalks and damaging cars that are parked under the trees. Healthy, mature trees can tolerate significant infestations by aphids, but large populations can threaten stressed or transplanted trees.
Because aphids are small, and often the same color as their host plants, they can be difficult to spot until honeydew or sooty mold appears. Inspect leaves regularly for honeydew and other signs of aphids. Good cultural practices, such as proper fertilizing and appropriate soil moisture, may help prevent an infestation. Foliar sprays of pyrethroids, carbamates, and organophosphates — made for control of other insect pests — can kill natural enemies that ordinarily keep aphid populations under control. If treatment for other insect pests is necessary, use a soil-applied systemic or a foliar material that is soft on beneficials.
Aphids can be easily controlled with a variety of insecticides. The most effective treatment are soil-applied systemic insecticides such as such as imidacloprid. The application should be done when the soil is moist but not saturated. A soil application will control aphids for at least a year. Aphids also can be washed off trees and plants with a strong stream of water. If done early in the day, the plant can dry rapidly in the sun and may be less susceptible to fungal diseases.
What can you do?
If colonies are found on at least five percent or more of foliage, or if sooty mold, honeydew, or distorted leaves are present, treatment is recommended. Avoid foliar sprays that kill the beneficial insects that keep aphid populations in check.
Nate Royalty, Ph.D., is product development manager — insecticides, Bayer Environmental Science.