I don't have a definitive answer, but having lived in California for 1/3 of a century I have made some observations. Everybody knows that it never rains in California (in the summer, anyway). The rain is a winter/spring thing. And indeed, there are plenty of bugs in the spring, while the vegetation is still green. They seem to disappear as things dry out. My theory is that the lack of water kills them off. I sure don't miss them - it sure is nice to go camping in summer and not have to worry about mosquitos!