The U.S. Geological Survey National Water-Quality Assessment Program has posted county-level pesticide use maps covering the years 1992-2009 http://water.usgs.gov/nawqa/pnsp/usage/maps/ .
The second page at the site asks to select the chemical of interest. When you do, the 2009 map pops up with darker colors indicating more use. Beneath the map is a table revealing what crops were treated most with that chemical. Above the map is the button to move back to the previous year. Note how the change in imidacloprid use from 2003 to 2004 was followed by California’s first winter of unexpectedly high winter losses. In 2005 they were short on bees for almond pollination. Then use tapered off for a few years, picking up again in 2008 and becoming quite prevalent in 2009. It will be interesting to see the last few years when the information becomes available.