To protect their food source, ants ward off many predators and parasites of aphids. Management of ants is a key component of aphid management and is discussed under cultural controls. See Dreistadt et al. Usually natural enemy populations do not appear in significant numbers until aphids begin to be numerous. The skin of the parasitized aphid turns crusty and golden brown, a form called a mummy.
Many predators also feed on aphids.
Naturally occurring predators work best, especially in a small backyard situation. Commercially available lady beetles may give some temporary control when properly handled, although most of them will disperse away from your yard within a few days. Whole colonies of aphids can be killed by these pathogens when conditions are right. Weather can also impact aphids. Populations of many species are reduced by summer heat in the Central Valley and desert areas, and aphid activity is also limited during the coldest part of the year.