Aphids are born pregnant and multiply quickly. They suck sugar fluids out of plants, cause sooty mold, inject plant viruses sometimes, but birds love to eat them.
Spray aphids right off your plants, leaving any survivors for birds and ladybugs to eat. Be sure to blast the bottom of the leaves as they often congregate there, and repeat every few days until numbers go down.
Encourage beneficial insects that eat aphids by not spraying pesticides and by planting native and other plants that give nectar – yarrow, seaside daisy, ocean spray, columbine, nonnative allysum, Shasta dais, parsley, calendula.
Also, dial back on fertilizers. Use none or slow release or organic so the aphids are not aided and abetted.