Aphids are one of the most abundant groups of garden insects. This is especially true in spring and early summer, when plants are growing rapidly and there is an abundance of tender young shoots and fresh leaves. Virtually all plants are subject to attack by at least one species of aphid. At times, aphids can become abundant and cause plant damage. Despite their ability to cause damage, they are a fascinating group of insects.

Also known as plant lice, aphids are tiny, soft-bodied insects that usually feed in colonies of 20 to over 100 individuals on the stems or foliage of plants. Their mouthparts consist of specialized tube-like structures, which are inserted into the soft tissue of their host plant to extract plant juices for food. Some species of aphids inject salivary fluids into the plant to partially digest the plant juices before they are extracted.

Feeding by aphids can cause a drying and death of the foliage and shoots of host plants. In some cases, leaves infested by aphids will drop from plants. Some species can cause unsightly galls and still others can cause deformity of infested foliage. In addition, aphids produce a sweet, sticky material known as honeydew. Heavy aphid infestations produce large quantities of honeydew, which drips on cars, bicycles, chairs and other objects that may be under infested trees. Nearly every spring, the large Norway maple in front of my home has aphids on the leaves, and each morning the windshield of my truck is covered with tiny droplets of honeydew. Aphid honey can be an excellent medium for the growth of black sooty mold fungi, which discolors foliage and branches and interferes with photosynthesis.

Honeydew is also attractive to some species of ants, which feed on the sweet droplets. Some ants are known to cultivate aphids and move individuals to new locations to start colonies and, hopefully, provide additional food sources.

Aphids have varied and complex life cycles. All species mature according to a process known as simple or incomplete metamorphosis and have three life stages: egg, nymph and adult. Both the nymphs and adults feed on host plants. Some aphids have lost the capacity to reproduce sexually. All adults are females, capable of reproducing without mating. Other species have alternating sexual and asexual generations. Some aphids produce winged adults, capable of flight, whereas others produce wingless adults that depend either on air currents or ants to move them to new locations. Other aphids produce both winged and non-winged adults. Another characteristic of aphids, such as the woolly apple aphid, is that they produce conspicuous white, woolly masses in which the females lay eggs.

Heavy aphid infestations may require some type of control. Fortunately, a number of effective techniques are available to control these sometime pesky insects. A high-pressure garden hose can be used to wash aphid colonies from host plants. This approach is easy, doesn’t require purchase of insecticides and has little or no adverse effect on the aphid’s natural enemies. Aphids can also be controlled with insecticidal soaps, horticultural oils or several chemical insecticides including malathion, esfenvarelate or orthene.

Brightly colored ladybird beetles feed on aphids and are key natural enemies. Some garden stores sell packages of ladybird beetles for aphid control. Ladybird beetles can fly for long distances, and once released in a garden, often fly elsewhere in search of aphids. Usually, if aphids are in abundance, ladybird beetles are soon to follow. Both fascinating and damaging, aphids are among the most common insects of ornamental plants.
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PHOTOGRAPHS, all taken by Bill Ciesla

Examples of aphids:

1. Aphid colony on the stem of a spirea (Spirea Aphid.jpg)
2. Aphids on the leaf of a quaking aspen (Aspen Aphid.jpg)
3. Leaf curl ash aphid causes deformity of ash leaves (Aphid Leaf Curl.jpg)

The authors have received training through Colorado State University Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.

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This page updated:  August 4, 2008