Identifying and Controlling Aphids

Aphids eating rose
Charles Krebs/The Image Bank/Getty Images

Description

Garden aphids, also known as plant lice, include many different species in the Aphidoidea insect family. Aphids are very small--roughly 1/10th of an inch long. The most common colors are green and black, though brown, reddish-brown, and gray aphids inhabit some parts of the country. They have two long, tubular appendages on the tail end of their body.

Life Cycle

Aphid eggs overwinter attached to plants, then hatch as nymphs in the spring.

These nymphs then produce eggs asexually, producing more nymphs that grow to maturity in just one week. Then, in the fall the nymphs will lay eggs that contain some male aphids. These males then mate with the nymphs to produce the eggs that will overwinter and start the next generation of aphids. Mature aphids lay three to six eggs per day. The rapid asexual reproduction cycle during the growing season is what leads to the rapid and widespread infestation so familiar to many gardeners. 

Signs of Aphid Infestation

Aphids suck the sap out of tender plant shoots and leaves using beak-like mouths, injecting the leaves with their saliva as they do so. The damage to plants is twofold:  drinking the sap can weaken the plant, and injecting the saliva can spread diseases from plant to plant. In addition, aphids excrete a sticky, clear substance called "honey dew" which commonly fosters the development sooty mold.

Sooty mold is unsightly and interferes with the plant's ability to photosynthesize.

Because aphids are so tiny, sometimes the first sign that massive infestation is pending is the sign of many ants on your plants. The honeydew secretion is much prized as a food by ants, so when you see many ants on plants, there is a very good likelihood that aphids are also present.

Effect on Garden Plants

Aphids can weaken a plant, stunt its growth, cause leaves to curl or wilt, and delay fruit or flower production. In general, an overall anemic appearance to your plants when there is not water shortage or other obvious reason will strongly hint that aphids are to blame. 

Organic Controls for Aphids

There are a number of non-chemical ways to combat or discourage aphid infestations. 

  • Sometimes, a strong blast of water from the hose will knock the aphids off of a plant and solve the problem.
  • If you attract or purchase certain beneficial insects, such as lady bugs, lacewings, parasitic wasps or damsel bugs, they will attack the aphids. For this reason, fewer chemical pesticides used in the garden can paradoxically reduce the severity of aphid infestations. A more diverse insect population generally keeps aphid attacks at bay.  Plantings mint, fennel, dill, yarrow and dandelions will attract these predators to your garden. Ants are natural enemies of predatory insects, so you may need to control ants in order to maximize the hunting ability of the beneficials. 
  • Plants can also be sprayed with insecticidal soap, or a homemade tomato leaf or garlic spray to kill aphids, but must be reapplied when the infestations reappear. 
  • Some biological insecticides based on fungi are known to work on aphids.

If You Use Chemical Solutions 

Aphids are easily killed by standard chemical pesticides, but because they are so prolific and will reinfest so readily, gardeners who try to rely on chemicals often find that their problems are intensified over the long run, as the chemicals must be reapplied often and will also destroy populations of beneficial insects and discourage other aphid predators, such as insect-eating birds. 

Many gardeners find that an adapted form of the integrated pest management (IPM) practice used by commercial agriculture is a good approach for home gardening. Under this philosophy, some degree of plant damage is deemed acceptable as the price paid for a diverse gardening culture in which the presence of many insect species tends to prevent any one pest from causing overwhelming damage.

Over the long run, minimal use of chemical pesticides tends to produce an overall healthier garden, albeit one in which small levels of insect damage may be present.