Spot Them Early
As with most pest insect problems, the sooner you spot a pest caterpillar problem the easier it will be to bring under control. While not always true, a lot of caterpillar activity occurs in the late spring and early summer.
Control by Hand
Handpicking makes a sizable dent in most populations of caterpillars and can keep them from getting out of hand if you catch the infestation right away. Drop them into a can of water with insecticidal soap in it.
For longer term control of major infestations consider using a natural insecticide.
Remember The Songbirds
Throughout this web site we push the idea that a healthy population of songbirds in and around the home landscape means fewer pest insects. Caterpillars are among the very favorite foods of songbirds. For more information see Attracting Birds