THE AG PILOT WAS ON THE RETURN LEG OF A SPARE PARTS RUN. HE HAD NOT RECEIVED A WEATHER BRIEFING PRIOR TO DEPARTURE. PRIOR TO TAKEOFF, HE ESTIMATED THAT HE HAD ABOUT 22 GALLONS OF FUEL ON BOARD; ENOUGH FOR THE 45 MILE FLIGHT. EN ROUTE, HE ENCOUNTERED DEVELOPING THUNDERSTORM ACTIVITY, AND DEVIATED HIS COURSE TO THE WEST. WHILE DEVIATING THE THUNDERSTORM ACTIVITY, HE EXHAUSTED HIS FUEL SUPPLY. HE FORCE LANDED THE AIRPLANE IN A NEARBY PASTURE, BUT THE AIRPLANE COLLIDED WITH A TREE, RESULTING IN AIRFRAME DAMAGE.