How to stop bud worms/caterpillars?!?

In a dry CA climate I have gotten more than my fair share of bud worms the last two years. These bastards just appear and all of a sudden there is like 25 of em on one plant. Crop looks like fire then in the last couple of weeks this happens. Sucks. Any suggestions on how to prevent these caterpillars from showing up on an outdoor crop in the first place?
 
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