Aphids are a worldwide insect that most gardeners should already be familiar with - some more than others. The majority of aphids are soft bodied and come in a variety of colors; green, black, brown, pink and nearly clear. Their body make up consists of several parts; antennae, stylets (for feeding), tail-like cauda, two compound eyes, and long, thin, two-jointed, two-clawed tarsi. They feed themselves by sucking through the mouthpart (stylet). In defense, aphids can release quick-hardening fluid that contains triacylglycerols called cornicle wax - pending on the exact species there may be other types of compounds.
Many aphids are monophagous, meaning that they feed only one particular type of plant, there are a few varieties that feed on hundreds of species of plants, though. Aphids passively feed on the sap of phloem vessels in plants; other scaled insects and cicadas do the same. In some circumstances they will feed on the xylem sap which has a much lower concentration of amino acids and sugars. Phloem carries nutrients to all parts of the plant so it's easy to understand how aphids can be extremely dangerous to growth and quality. Ironically, and surprisingly, aphids can't simply rely on this sap for their only nutrients largely because the diet is extremely unbalanced and lacks essential amino acids. To obtain these nutrients aphids rely on bacterial endosymbionts that live in special cells in their body. These bacteriocytes help to recycle glutamate and metabolic waste - transforming it into essential amino acids. A side effect of the aphids feeding and biological processes is the risk of plant disease transmission. Plants may show any of the following signs indicating that aphids are your problem; decreased growth rate, mottled or curled leaves, browning, wilting and even death.
School Garden Weekly has a few great tips on how to limit exposure to potential risk and even damage control if problem does arise.
Integrated Pest Management:
1) Plant Selection
A healthy plant is better able to withstand its environment than one that is stressed by improper fertilization, irrigation, or being planted out of season. Remember to plant cool-weather crops in the fall (in California) and warm weather crops in the spring.
Plant selection also includes the planting of specialized crops. Some plant varieties are more susceptible to pests than others. Choosing the right variety may be all that is necessary to ensure a healthy plant. For example tomatoes labeled with a VFN designation are better able to resist the diseases caused by Verticillium, Fusarium viruses and Nematodes (microscopic worms that feed on plant roots). This information will be readily available in most seed catalogs.
2) Physical Barriers
A good example of physical barrier is a fence. If deer or rabbits are a problem in your garden the area will need to be fenced. Bury the fence about one foot deep to keep burrowing animals out. If gophers are a problem you will need to place chicken wire below your raised bed. Floating row covers and bird netting are other examples of physical barriers.
Insect traps use pheromones, visual lures or food to attract pests and capture them. Pheromones are the substances female insects use to sexually attract males to them. Visual lures use colors and shapes to attract pests. A good example of a trap is the yellow sticky card that keeps whiteflies off your tomatoes. Aphids and white flies as well as other small flying insects are attracted to the yellow color and are then entrapped in the glue. They are not effective in a very large area but for a small school garden they are effective.
Another example of a trap is to lay a wooden board down in your pathway raised a few inches on one end to attract snails. They will try to hide there during the day. Simply turn the board over and remove the snails.
4) Biological Controls
Biological Controls rely on the use of living organisms called natural enemies or beneficials to eat or kill the pests. Two well-known beneficials are ladybugs and green lacewings.
Another biological control is BT (Bacillus thuringiensis), a microorganism that occurs naturally. It is very effective with worms and caterpillars that congregate on the underside of green leaves such as beets and chard as well as on the leaves of the entire Brassica family. BT is diluted with water and sprayed onto the underside of leaves.
Pesticides are used as a last resort. If you must use a pesticide, choose the least toxic yet most effective product that targets the pest but does not also kill natural enemies or is harmful to pets and other animals. Insecticidal soaps usually fit this bill. To learn more about pesticides see National Pesticide Information Center.
Ryan R. McKenzie (plant cross section image): https://commons.wikimedia.org/wiki/File:Labeledstemforposter_copy.jpg
Pip Courtney (October 30, 2005)."Scientist battles lettuce aphid". Landline(Australian Broadcasting Corporation). Retrieved January 1, 2007.