Usually it's only a couple of weeks of the little sulfur-yellow biting flies, and their onset is typically mid to late May. (Like Hitch said, a noteworthy exception was two years ago when we'd had unseasonably mild mid winter weather followed by torrential late season rains, and a six week plague of the buggers).
The little face flies most every dry period visitor to the VW soon becomes intimately familiar with can occur year round with any extended warm dry spell, disappear during freezes and rainy periods, and are most associated with thickets of scrub oaks. Maddening to the uninitiated, yes-- but much more easily dealt with using nothing but a headnet, than are those little yellow bastards.
Ticks are around whenever the grass is green, peak is probably April in the little micro-climates where they're worst. Then, it's nothing to sweep a couple of dozen off your pants in a hundred yards of grassy trail in these spots, or pick half a hundred off a dog. I've even busted through snow-covered brush and picked them up. Feb-Mar I find to be worst as the poopyseed sized nymph stage ones sometimes get past my tickle sense. Ticks are typically done by late May as the grasses dry to standing hay, true in virtually all inland areas. In the fog belt a few can be found year round. (In the chaparral at the immediate coast in Big Sur they start up as early as November some years).
