You can always plant more milkweed to help feed the cats, but almost always even more eggs get layed. Spray the milkweed with a hard stream of water or squish them by hand, I wear gloves because my fingers turn completely orange. To keep the butterflies from laying eggs in the first place, cover the plant with netting or mesh fabric to allow air, sunlight and water in. You can also protect the cats (with the netting or mesh fabric cover) from wasps and some other predators.

I ended up planting over 35 milkweed plants in my front and back yards! Believe it or not, it sometimes is not enough food for all the caterpillars!

Mechelle

I grew about 2 dozen Asclepias curassavica (tropical) plants from seed last year, and all but about 4-5 got cut down to be used for cat food. Even the plants with intact flowers get leaves trimmed and cut for food. If you have numerous caterpillars perhaps you could try putting a sleeve on a plant or two to preserve some flowers to collect seed.

Controlling aphids on milkweed is pretty easy - IF you catch the oleander aphids before they become more numerous. I regularly patrol all the Asclepias species for insects, and remove them, to keep the leaves clean and remove predators. If you check the most tender growth tips, that is where the aphids start. Just rub them gently off the leaves smushing them as you go (but not too hard), and keep at it, they never get a chance to colonize.