Aphids are tiny herbivorous insects in the superfamily Aphidoidea, within the order Hemiptera, which also includes cicadas, gnats, and many other "true bugs." There are ten families of aphids and about 4,000 species. They live around the world, but are most common in temperate areas. About 250 of the species are considered pests in forests and gardens.
Though aphids are only 1 to 10 millimeters (about 0.039 to 0.39 inches) long, they can present a huge problem to plant life. The insects feed on the sap of plants by piercing the phloem, the vessels through which sap flows, with a long proboscis. As they feed, they can introduce viruses to the plants, sometimes devastating ones. Even if they don't introduce disease, the bugs can destroy the appearance of plants simply by their feeding and breeding activity. Some feed only on a single plant species, while other types feed on hundreds of different plants.
Some of these insects have symbiotic relationships with other species. Some are "farmed" by ants, which protect the aphids and feed on the carbohydrate-rich honeydew they secrete. Others host Buchnera bacteria, which live in specialized cells inside the insects and synthesize amino acids that are not present in the aphids' diet.
Many species have very unusual reproductive methods. Typically, the insects mate in the autumn, and the fertilized female lays eggs. The eggs hatch in the spring, but only produce females, each of which may be winged or wingless.
The females born in the spring produce parthenogenically, without being fertilized. The daughters are therefore genetically identical to the mothers, and multiple generations may exist within an individual. Births at this point may be live, or the insects may lay eggs. In the fall, some develop into males, and the cycle begins again.
Through parthenogenesis, aphids are able to reproduce extremely quickly and in huge numbers. Those that live inside greenhouses or in areas with mild winters can continue to reproduce asexually for years. It is easy to see how they can become such a menace to their host plants.
Aphids are often kept under control by their natural predators, which may be introduced into a garden for that purpose. Aphid predators include ladybugs, lacewings, hoverfly larvae, parasitic wasps, and some parasitic fungi. These insects may also be controlled by spraying the plants down with plain or soapy water, pruning affected areas, and using aluminum foil mulch. Pesticide is also an option, though the least recommended.