The Backyard Jungle

Image Gallery A live, virgin birth of an aphid. Note the adult's proboscis penetrating the leaf on which it rests.

Here's the eleventh in a series of articles that will help you tell friend from foe in your garden.

Part XI: Aphid Biology

by Ron West

Most gardeners would be quick to name aphids as the insects that give them the most trouble. Few other pests are as widespread, persistent, and downright annoying as these small, soft-bodied "plant lice."

Aphids constitute a large family (Aphididae) of the order Homoptera, a group which also includes the cicadas, leafhoppers, whiteflies, mealybugs, and scale insects. Like their kin, aphids feed on plant juices through a tubular proboscis, which is inserted, hypodermicstyle, into the plant's tissues. In addition to causing much physical damage, this method of feeding contributes to the transmission of many plant diseases, especially when the homopteran is winged and moves readily from plant to plant.

Aphids often blend in with the color of the host plant or hide on the underside of leaves, so the damage they inflict is usually the first clue to their presence. Curled, mottled leaves are-a sign that aphids (or similar insects) are at work. In time, infested plants become stunted and sickly, and a great deal of chlorophyll may be lost. Young, tender growth is often preferred by aphids, and the damage done in such instances can be devastating.

The typical aphid hardly needs description. It has a pear-shaped body one-tenth of an inch or less in length, is winged or wingless, possesses weak legs, and can be any of a number of colors. Although these sluggish insects may seem quite defenseless, they do possess one weapon. Protruding from the rear of the abdomen are two tubular structures, the cornicles, which secrete a waxy substance that deters at least some attackers.

When ants are observed moving up and down a plant or tree, it's almost a sure bet that aphids are present. Many of these pests excrete copious amounts of a sweet fluid called honeydew, which ants relish and readily gather up. In fact, an ant will even use its antennae to milk an aphid, rubbing the homopteran's abdomen in order to stimulate the flow of additional honeydew.

Indeed, many ants regularly milk their aphid herds and, in exchange for the sweet treat, provide their six-legged cattle with a certain degree of protection. The high sugar content of honeydew also provides an ideal growth medium for sooty mold, a black fungus that often develops on heavily infested vegetation. While this fungus does not attack the leaf, it eventually covers the surface and hampers many of the plant's vital functions.

One key to the enormous success of aphids is their amazing reproductive capacity, which is potentially higher than that of any other insect. It's estimated that a single aphid could have 5 billion descendants between spring and fall! Each female produces 50 to 100 offspring, and there may be over 40 generations annually in mild climates (and even more in greenhouses). Of course, a wide variety of predators, parasites, and pathogens keep the aphid population in check, but not always well enough to prevent damage to our plants.