Pest Corner
Those Pesky Aphids ...

Aphids, sometimes called plant lice, are one of the most common insects that can attack home, garden and landscape plants. They tend to cluster on the new growth of roses, perennials and other plants. Aphids are small, soft-bodied insects, usually 1/8 inch or less in size, with long legs and antennae and are usually wingless. Their color can vary from green, pink, yellow, tan, gray, white or black.

Aphids can be recognized by their cornicles, a pair of tubelike structures projecting from the rear of their bodies. Aphids have an extremely high rate of reproduction, which enables insect numbers to build up very rapidly. When aphid numbers are high, plants are stressed and therefore less vigorous. Aphids damage the plant by piercing the leaves, stems and flowers and sucking sap and other fluids, weakening plants and leaving them discolored and stunted. Foliage may look puckered or distorted. Aphids tend to congregate on new plant growth in the early spring and curled, distorted leaves are usually a good indicator that aphids are present. In the summer, aphids tend to congregate on veins of the lower surfaces of mature leaves. Aphids cannot digest all the plant sugars they ingest and excrete the excess in the form of honeydew, which supports the growth of an unsightly fungus called sooty mold. Sooty mold is a black or grayish-green fungus that grows on foliage where there is an accumulation of honeydew or other sugary substance. There is no effective control of sooty mold except dealing with the aphids causing the problem, which allows new growth to remain free of sooty mold. Lush plant growth is very attractive to aphids and it has been noted that high nitrogen fertilizer rates can increase aphid reproduction. One of the most common aphids in greenhouses, houseplants, and vegetable gardens is the green peach aphid which feeds on more than 200 species of plants. Rose aphids are usually associated with roses where infestations concentrate on the new shoots and flower buds. Specific garden plants that tend to have aphid infestations on an annual basis include butterfly milkweed, aster, dock and euonymus, viburnum and hydrangea.

Photo by Tammy McNiel

Aphids have many natural enemies that successfully attack them and provide effective control. They are a favorite diet for ladybird beetles. One ladybird beetle can eat up to 100 aphids per day. Parasitic wasps are very successful in attacking aphids. The wasp stings the aphid and deposits her egg inside the aphid’s body. The wasp egg hatches into a larva that grows and consumes the aphid’s internal body. The larva pupates inside the aphid and emerges as an adult wasp through a small exit hole it chews through the aphid’s outer skin. After the wasp emerges, the aphid’s corpse is referred to as a ‘mummy.’ Aphid mummies are a good indicator that parasitic wasps are at work in the garden. Other predators include lacewings, syrphid flies and assassin bugs. Encourage natural enemies of aphids by using synthetic insecticides as little as possible. Before treating any plants, thoroughly inspect the plant for presence of natural enemies. Aphids can be knocked off the plants with a strong jet spray of water. Insecticidal soaps or horticultural oils also work well to control aphids. It is best to not apply insecticidal soups or horticultural oils in direct sunlight as they can have phytotoxic effects on the plant. Heavy aphid infestations can be diffi cult to control if aphids hunker down in the folds of leaves, flower buds or new growth. Aphid populations can be controlled by pruning out and disposing of aphid infested plant material or by squishing with fingers.

It is common to observe ants and aphids on the same plants. Ants and aphids provide a fascinating relationship. Ants need sugar in their diet and aphid honeydew is particularly attractive to ants. Ants collect the honeydew from plant material and encourage aphids to produce more by guarding the aphid colonies, thus protecting their food source by driving away potential predators of the aphids. Thus, if ants are detected on plants in the garden, be sure to look more closely and you probably will find resident populations of aphids as well.

For further information on aphid control, see MU publication M145, Controlling Tree and Shrub Pests, or MU Guide G7274, Aphids, Scales and Mites on Garden and Landscape Plants.

Mary Kroening
KroeningM@missouri.edu