In 2007 and 2008, individuals in the SF Bay Area couldn't have missed this creature, the Western Tussock Caterpillar, which spent a lot of time in and around oak trees in April, May, and June.
This year, not a single soft, fuzzy bottle-brush resembling example has been observed. It's a definite caterpillar bust. However, there is a boom in the number of orange-brown soldier beetles and zillions of tiny parasitizing wasps, which are as small as tiny mayflies and no-see-ems'. Both of these critters are a powerful check on caterpillar populations.