Few insect pests are more widespread than whiteflies and aphids. They attack indoor and outdoor vegetables, flowers, herbs, shrubs, and trees. They breed quickly, and once their numbers are high, they can damage leaves, stems, fruits, and even roots by sucking plant juices. Let's look at both pests and the safest controls for them.
An aphid's life. Aphids like the low light levels and cool conditions of spring and fall. They will attack all plant parts but prefer young, succulent growth. There are many species of aphids, some named after the plants they attack, such as pea aphids and peach aphids. In general, all are small (1/16 to 1/4 inch long) and oval-shaped, and can be black, white, green, or pink. Although most aren't very mobile, some forms have wings. All reproduce quickly, and under the right conditions a small number can bloom into a major infestation in no time. By sucking plant juices from leaves and stems, they weaken the plant. More seriously, they can transmit virus diseases that gradually debilitate and kill some plants.
A whitefly's life. In cold-winter climates, whiteflies are mostly greenhouse or indoor pests, but they can be found in the garden. In mild-winter climates with no winter cold to kill them, whiteflies are serious outdoor pests. In recent years in the Southwest, pests such as the silverleaf whitefly have been among the most damaging to agriculture.
Adult whiteflies hide and feed on the undersides of leaves. They are most noticeable when you rustle the leaves and a cloud of tiny white specks emerges into the air like "plant dandruff," as some gardeners aptly describe them.
Like aphids, whiteflies reproduce quickly, laying white eggs that hatch into white crawlers on the undersides of leaves. Whiteflies thrive in sunny, warm conditions. The crawlers and adult flies suck plant juices, weakening the plants.
Aphid and whitefly controls. The first line of defense is prevention. Check plants regularly. Avoid high-nitrogen fertilizers, which create young, succulent growth which these insects love. Isolate infested plants from others and control the pests aggressively.
For whiteflies, hang yellow traps coated with a sticky substance close to the tops of plants. Whiteflies are attracted to the color yellow, and once they land will be stuck and die. Hand-crush small populations of young aphids and whiteflies. Encourage natural enemies in the garden, such as ladybugs and lacewings, by planting diversity of plants and not spraying pesticides. As a last resort, use low-toxic sprays such as insecticidal soap, horticultural oil, and neem to reduce populations before they get out of hand. Repeat treatments every few days until the problem is under control.
Aphids can easily be removed from a plant by knocking them off with a stream of water from a hose. Once off, they tend not to climb back up.
To control small populations of whiteflies on houseplants, approach them in early morning and evening when they are most sluggish. Use a vacuum cleaner set on low suction to remove the whiteflies.
Photography by Suzanne DeJohn/National Gardening Association.