Aphids are one of the most wide spread and adaptable pests gardeners face. They infest a wide variety of plants and can cause considerable damage. Just as problematic is the sticky stuff they secrete, which serves as a food source for ants and encourages powdery mildew to form on your plants. This covers the plant leaves and keeps plants from being able to make enough food to survive.
Aphids are small, soft bodied insects that come in a rainbow of colors, depending on species. Different types of aphids invade different plants. All of them piece the plant with their mouths and suck fluid out. Too much of this, and the plant starves.
The life cycle of aphids is rather interesting. Aphids reproduce asexually, with the females giving birth to live offspring without mating. These offspring go through several moults on their way to adulthood. Each generation, some aphids have wings and use these to move to new plants. Most aphids, however, are born wingless and spend their entire lives on their host plant.
For the most part, aphids are more of a cosmetic problem than a serious health risk for your plants. There are viruses aphids spread from plant to plant, however, and a high enough aphid population can cause leaf curling, stunted shoots, and even death.
Fortunately, there are a lot of things that love to eat aphids. Lady beetles are famous aphid eaters. Lacewings also feed on them. Parasitic wasps lay eggs on them and when they hatch the larvae eat the aphids from the inside out. In organic gardens, these predators generally keep the aphid population in check.
Aphids are vulnerable to sprays of water that knock them off of plants. Insecticidal soap will kill them, too. If just one part of the plant is covered in aphids, pruning that part and disposing of it may solve the problem. Quarantining any new plant material until it is checked thoroughly and found to be free of aphids will help prevent their spread.
If your plants are sustaining serious injury, or the aphids are widespread, chemical controls may be all that work. The down side is that chemical controls will kill beneficial insects as well as the aphids. Try to use the chemicals only on the effected plants, not on every plant in the garden.
If you must use a synthetic chemical, malathion, permethrin and acephate (nonfood crops only).are all options. These chemicals have a residual effect so continue to kill any insects on the plant for several days or weeks, depending on the product. This will keep the aphids from returning as fast, but will kill any beneficial insects in your garden, as well. For this reason it is suggested that you target only the infested plants, not all your plants in your garden. The untreated plants provide a haven for the beneficial insects that will help keep the aphids from getting out of control again.
In general, aphids are a cosmetic problem. When they get out of control, however, predator insects, insecticidal soaps, and synthetic chemicals such as malathiona and permethrins will kill them.