Plant-sucking aphids are the bane of a gardener's existence. Come spring, aphids appear as if by magic and begin draining the life out of tender plants. Their ability to reproduce, both sexually and asexually, is prolific.
Aphid bodies are soft and pear-shaped. Though most often green or yellow, aphids come in a variety of colors, from red to black. Few aphids measure more than a couple of millimeters. An individual aphid would be difficult to spot, but since aphids feed in groups, their presence is usually noticeable.
Up close, aphids resemble little muscle cars with a pair of tailpipes. Entomologists believe these abdominal appendages, called cornicles, secrete waxy lipids or alarm pheromones when the aphid senses a threat. The presence of cornicles is a common characteristic of all aphids.
Antennae may have five or six segments, with the final segment ending in a thin flagellum. At their other end, aphids possess a cauda, a short, tail-like appendage centered between the cornicles. Aphids usually lack wings, though certain environment conditions may cause winged forms to develop.
Aphids feed on plant phloem tissues, sucking the sugary liquids from the host plant's vascular system. Reaching the phloem is no easy task. Aphids feed using a straw-like proboscis that contains thin, delicate stylets for piercing plant tissues. In order to protect the stylets from damage, the aphid secretes a special fluid from them, which hardens into a protective sheath. Only then can the aphid begin feeding.
Aphids need nitrogen, but phloem juices contain mostly sugars. To get adequate nutrition, aphids must consume an enormous quantity of phloem liquids. They excrete the excess sugars in the form of honeydew, a sweet residue left behind on plant surfaces. Other insects, such as ants and wasps, follow behind the aphids, licking up the honeydew.
The aphid life cycle is somewhat complex. Aphids usually reproduce asexually, with aphid mothers giving live birth to their young. Sexual reproduction occurs just once per year, if at all. Just before winter, sexual females mate with males, and then lay eggs on a perennial plant. The eggs overwinter. In warm climates or in greenhouses, sexual reproduction rarely occurs.
Special Adaptations and Defenses:
Aphids are tiny, slow-moving, and soft-bodied – in other words, easy targets. They're far from defenseless, however. Aphids use both fight and flight, and everything in between, to protect themselves.
If a predator or parasitoid approaches an aphid, it can react in a number of ways. Aphids will literally kick their attackers, with some serious aggression. In other cases, the aphid may just walk away, hoping to elude the trouble. Sometimes, the aphid does a stop, drop, and roll, and simply falls to the ground. Some aphid species employ soldier aphids to stand guard.
Aphids also arm themselves with defensive weaponry. When a pursuing predator attempts to take a bite from behind, they can excrete a waxy lipid from their cornicles to fill the attacker's mouth. Alarm pheromones broadcast the threat to other aphids, or may summon protection from bodyguards of other species. If a lady beetle attempts to feed on it, a cabbage aphid will mix toxic chemicals within its abdomen to "bomb" the offender.
Aphids also use bodyguard ants, which they pay with sweet honeydew excretions.
Range and Distribution:
Both abundant and diverse, aphids mainly inhabit temperate zones. Aphid species number over 4,000 worldwide, with about 1,350 species in North America alone.