Aphids are rarely found singly but feed in dense groups on leaves or stems. Most aphids do not move rapidly when disturbed; some species may drop from plants when disturbed.
Dense colonies of aphids can develop from a single winged female giving birth to live offspring (as many as 10-12 per day) without mating. No males are required.
Young aphids are called nymphs. During warm weather, many species of aphids can develop from newborn nymph to reproducing adult in seven to eight days. Aphids can have many generations a year. Most species molt about four times before becoming adults. Population density can increase rapidly as each adult aphid can produce up to 80 offspring in a matter of a week.
Most aphids feed on leaves, but a few aphids attack plant roots or limbs. Low to moderate numbers of leaf-feeding aphids are usually not damaging in gardens or on trees. However, large populations cause curling, yellowing and distortion of leaves and stunting of shoots; they can produce large quantities of a sticky exudate known as honeydew, which often turns black with the growth of a sooty mold fungus.
Some aphid species inject a toxin into plants, which further distorts growth. A few species cause gall formations.
Many aphids species transmit viruses from plant to plant. Virus transmission is usually from migrating winged aphids. Aphid-transmitted virus diseases are common on certain vegetable and ornamental plants. Disease symptoms may include mottling, yellowing or curling of leaves and stunting of plant growth. Aphid-transmitted virus diseases are difficult to prevent through aphid control because infection occurs even when aphid numbers are low. Migrating aphids only may take a few minutes to transmit the virus while it can take much longer to kill the aphid with an insecticide.
Honeydew droppings are a tip-off of a developing aphid problem. Ants, attracted to honeydew, often are associated with aphid populations, especially on trees. If you see large numbers of ants climbing up your tree trunks, check for aphids or mealybugs on limbs and leaves above. To protect their food source, ants ward off many predators and parasites of aphids. Ant control can promote aphid biological control from predators and parasites.
Natural enemies can be important in the control of aphids, especially in gardens not sprayed with broad-spectrum pesticides that kill natural enemy species as well as pests; natural enemy populations may not appear in significant numbers until aphids are numerous.
Tiny stingless parasitic wasps are among the most important natural enemies attacking aphids. Various species of adult female parasitic wasps lay their eggs inside aphids. The skin of the parasitized aphid swells, hardens, turns golden brown and is called a mummy. The generation time of most parasites is quite short, so once you begin to see mummies on your plants, the aphid population is likely to be reduced substantially within a week or two. Many predators feed on aphids including lady beetle adults and larvae, lacewing larvae, and syrphid fly larvae.
Weeds such as sowthistle and mustards support dense aphid colonies, so keep weeds under control. When using transplants for flower beds or vegetable gardens, check transplants for aphids and remove them before planting. Prune localized aphid populations on curled leaves or new shoots before aphids multiply and spread. Aphid habitat can be made less suitable in large trees by pruning dense inner canopy where some aphids species thrive.