Aphids are nasty little suckers.  Yep, that's right they can suck the life right out of a plant. Severe infestations of aphids can even lead to a plant's demise by weakening it and making it more susceptible to attack from other insects and more vulnerable to injury from environmental stress. Some types of aphids inject toxins into the plant as they feed, causing distorted and malformed growth. Certain aphids even spread disease as they move from plant to plant.

Appearance

Aphids are soft-bodied insects with a pear-shaped body.  Different aphids come in different colors... green, black, grey, red, purple, and yellow.  Most aphids you'll find on your garden plants don't have wings, but at certain times of the year they do develop wings.  Aphids are fairly slow moving insects so it's not hard to check them out.  Look for their Acornicles@, a pair of tail pipe-like structures projecting out from the rear of their abdomen.

Sucking Sap

While small, aphids are successful at what they do... they suck sap from plants.  They do this by pushing their tiny stylets (located in their proboscis) into stems, leaves, and even roots.  You might think the stylets are tough to be able to pierce plant tissues, but they're aren't. To protect their thin stylets the aphids secrete a fluid that forms a hard protective coating over them as they push into the plant.  Once the stylets tap into the phloem, the source of the plant sap, the aphid secretes saliva into the plant.  It's believed that the saliva is produced to counteract the effects of proteins formed by the plant in response to the wound caused by the piercing stylets.

Once the aphids tap into the phloem they have access to the sugary sap flowing through these plant conducting tissues.  Plant sap contains lots of sugar, but not much of other nutrients needed by the aphids to survive.  To get the nitrogen they require, aphids must imbibe much more sugary plant sap than needed to sustain themselves.  As a result, they excrete volumes of a sugary liquid, called honeydew, from their alimentary canal.  This excreted honeydew is deposited on the leaves and appears as shiny spots.  When aphid populations are large, honeydew can entirely coat lower leaves, making them very sticky and shiny.

Reproduction

Of the over 4000 aphid species in the world, only about 250 species are considered pests.  The presence of moderate amounts of aphids on a plant doesn't mean a plant is doomed, but it should prompt monitoring of the situation.  Not only are aphids successful at sucking, they're also very  adept at reproduction.  Several immature aphids deposited on a plant will mature in about a week.  Each of these young aphids is usually a female and able to produce 40 to 60 baby aphids or nymphs.  These babies quickly mature and then reproduce, setting off a real baby boom with the population having the potential of growing into the hundreds and thousands quite quickly.  If they become too crowded on a branch or the plant becomes weak and  Atapped out@, some of the aphids will develop wings and go off to start new colonies.

Life Cycle

The life cycle of many aphids is pretty straightforward.  Aphids go through simple metamorphosis starting out as an egg and hatching into a nymph in the spring.  The nymphs molt and change into successively larger nymphs and then into adults.  During the spring and summer, the females give birth to live babies (all female) who in turn mature and give birth and so forth.  There is usually enough time during the growing season for several or more generations.  When the aphid colony becomes too crowded or the plant starts to die, some of the aphids will develop wings so that they can fly to other feeding locations.  In the fall, some male aphids develop and mate with females.  The females lay eggs for overwintering.

That life cycle may seem simple, but some aphids are Aheteroecious@.  This means they spend the fall, winter and spring on one type of plant species and then spend the summer on an unrelated plant species. That explains the presence of aphid distorted leaves on certain plants but the absence of aphids when the plant is checked for aphids during the summer.  Examples of heteroecious aphids are the rosy apply aphid on apple and plantain and the lettuce root aphid on poplar and lettuce.  Other aphids are Aautoecious@ spending their life on one species of plant or closely related species.

Managing Aphid Infestations

While aphids are nasty little guys dedicated to sucking away at plant sap, light to moderate infestations usually cause no real harm to healthy mature plants.   Control is often desired by gardeners for aesthetic reasons or because the excreted honeydew is creating a problem.  On vegetable crops, gardeners may feel a need to control aphids because their presence diminishes their enjoyment of their produce.  The least toxic way to manage an aphid infestation is the use of a strong force of water from a garden hose to knock the aphids to the ground where most will not be able to crawl back up the plant.   Of course this method is not practical for delicate plants or those without stiff stems.

Summer oils and insecticidal soaps work well against the soft-bodies of aphids. However, be aware that these materials must come in direct contact with aphid bodies because they work by disrupting their membranes.  Soaps and oils don't have any residual effect and must be reapplied when aphid numbers increase again.  If the aphids are protected by distorted and curled leaves, these materials will not work because they don't come in contact with the aphid bodies.  It's also important to note that many aphids feed on the undersides of leaves.  Oils and soaps must be directed to the surfaces where the aphids are feeding to be effective.  Be sure to check for label directions and precautions, such as not using oils in hot weather or using soaps or oils on sensitive plants.