THE PLT SAID HE HAD COMPLETED AERIAL APPLICATION OF MOSQUITO CTL TREATMENT & WAS RTRNG TO THE LOAD SITE. THE HELICOPTER WAS IN CRUISE FLT AT 300 FT ABV THE GND WHEN THE PLT '...NOTICED IT WAS REQUIRING MORE RIGHT CYCLIC CONTROL TO KEEP THE HELICOPTER LEVEL, ALSO BEGAN TO EXPERIENCE A LOSS IN ALTITUDE.' THE PLT NOTED THE ROTOR RPM HAD DROPPED TO 90%, BUT THERE WERE NO WARNING LIGHTS OR HORNS. THE ENG OUT AUDIO WARNING WAS PLACARDED 'INOPERATIVE' & THE PLT WAS AWARE THE OIL CHIP WARNING LIGHT WAS INOP. THE PLT MADE A FORCED LANDING IN AN ALFALFA FIELD. HE STATED '...DUE TO THE RIGHT CYCLIC INPUT I HAD TO LAND WITH A DRIFT TO THE RIGHT...THE RIGHT SKID GEAR SNAPPED, ROLLING OVER TO THE RIGHT.' AN INV REVEALED THE LEFT HOPPER WAS BLOCKED & STILL CONTAINED AN ESTD 100 TO 300 LBS OF MOSQUITO CONTROL AGENT. THIS LOAD RESULTED IN AN IMBALANCE WHICH WAS COUNTERED BY EXTREME CYCLIC INPUT. EXAM OF THE ENG REVEALED 'SEVERE EROSION' OF THE COMPRESSOR BLADES, AND HORSEPOWER PRODUCTION 'SIGNIFICANTLY BELOW PRODUCTION TEST SPECIFICATIONS.'