Known as greenflies in the UK, there are over 5,000 species of aphids worldwide. Fewer than 10% of known species are agricultural pests, but they are a genuine nuisance. Like whiteflies, aphids are classed in the order Hemiptera because of their sucking mouthparts. They range in size from 1 to 10 mm long and may be green, grey, or black in colour. Local species usually occur in large numbers along the stems and flower buds of a wide range of plants.
Aphids feed by puncturing the outer cells of plants and sucking out nutrients. This robs the plant of the food it needs to develop properly, and may cause yellow blotches to appear on leaves, and even total collapse of the host plant. The punctured cells become open to viruses and other disease. Aphids also secrete honeydew, which creates large sticky areas on plants and attracts sooty moulds that can then spread quickly over the rest of the plant.
For the best organic control of aphids, please read Companion Planting with Umbelifers.
A quick blast with the hose will dislodge aphids, but biological controls are recommended. Ladybugs, lacewings, crab spiders, and parasitic wasps are all natural predators of aphids. The larvae of hoverflies and the aphid midge are particularly voracious predators. Aphid eggs account for over 60% of the winter diet of chickadees, so look for specialty chickadee houses and hope that a pair move in. If aphids become a serious problem (as they sometimes do on Nasturtiums, for instance), consider avoiding the plant that is attracting them.
Dispose of infested plant parts in the garbage, not the compost.