When the "bug season" is at its worst really depends on altitude, when the snow melts, and a lot of other conditions. Where and when are you planning to hike?

Hopefully your fellow Californians will chime in here. Here in the Pacific Northwest the mosquitoes are starting now at low altitude. Up in the mountains they start about 10 days after snow melt and are horrific until somewhere between the end of July and the middle of August. Late August and September are usually fine. Some areas in the Cascade Range consist of high plateaus with lots of little lakes and boggy patches; these places are usually nicknamed things like "Mosquito Heaven" in July. There is one area north of White Pass in Washington that is officially named "Mosquito Valley."