There are over four thousand species of aphids known to exist. Around 250 species are considered harmful to garden crops. They are among the most prominent and successful garden pest worldwide. These insects pierce the tender new growth of plant stems in order to suck out the nutrients from the plant. This deprives the plant of the fuel it needs to thrive. Aphids can also carry viruses that will infect the plants and cause even greater damage. Their exuded waste products attract sooty molds which can spread to coat the leaves of plants, blocking sunlight. 
Only females are present in the population at the beginning of the seasonal cycle. Eggs are produced with out mating and develop clones of the female. Aphids reproduce quickly because they give birth to live nymphs. This process continues throughout the summer prducing multiple generations that generally live 20 – 40 days. Many generations can be produced by one female. 
Fortunately there are many ways to get rid of aphids or at least keep them in check using all natural and organic means that will keep your garden healthy.
- Physical removal. If you catch the infestation early, it is possible to wipe them off plants.
- It is possible to spray aphids off of plants with your garden hose. Just give the plants a good wash down. This works best for more established plants.
- Soap and water. Use a mild household detergent diluted in a small bucket of lukewarm water. Use a sponge or spray bottle to apply the mixture where aphids have moved in. Spray the undersides of all leaves to get a good covering. The soap will dissolve the coating from aphid’s bodies, they’ll dry out and die without harming the plants. You will need to repeat any application three times to kill all the generations.
- Neem oil can be applied the same as mild detergent. The organic chemicals in Neem oil act as a repellent against aphids and a wide array of other pests as well as many types of fungus.
- Apply an Essential oils mixture of equal parts thyme, peppermint, clove and rosemary oils. 4-5 drops of each should suffice. Mix this solution into a small spray bottle filled wih water. Shake well and apply to infested plants. This mix will kill most garden pests as well as their eggs and larvae. It also works well as a general purpose outdoor/indoor insect repellent.
- Insecticidal soaps are available for purchase. Follow the instructions provided.
- Beneficial insects can be pest predators. The ladybug beetle, green lacewings and hoverfly larvae. Attract these natural predators by planting fragrant herbs such as garlic, catnip, and oregano. Their flowers will attract hoverflies. Clover, mint, dill, fennel and yarrow will attract ladybugs and lacewings.
- Bug-eating birds like wrens, chickadees, and titmice by offering free food and housing space. They prefer to nest in small trees and twiggy shrubs that provide good cover. Feeders filled with black sunflower seeds, pre-hulled sunflower seeds, nuts and nut hearts are good choices.
- Keep your plants healthy. Regular compost additions to your garden will help keep plants healthy and pest resistant.