How to prevent drywood termites?

I live in Southern California. House is single story.
I recently had some infestations of drywood termites in my attic. We spotted them because they excreted these small tell-tale cylindrical pellets. The house had to be tented at considerable expense.
I noticed that the soffit vents have a quarter inch screen mesh. Would this type of screen allow any swarming dry wood termites to enter the attic?
Would it help to overlay the 1/4" screen with small mesh regular window-screening to avoid future invasions?
Is there anything else I could do to prevent future infestations of drywood termites?
Thanks
Walter The Happy Iconoclast www.rationality.net
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