So what is the problem Arnold? It is permited under Part 137.125 Aerial Application Operations.
(2) If an aeroplane engaged in an application operation is on the ground with the
engine running, the pilot in command must be at the controls unless:
(a) the pilot is refuelling the aeroplane in accordance with the operations
manual; or
(b) the following apply:
(i) the pilot remains near the aeroplane;
(ii) the wheel brakes are locked and, if practicable, the wheels are
chocked;
(iii) the aeroplane’s power controls are friction locked and, if possible,
the propeller is feathered;
(iv) the engine is retarded to idle and, if possible, ground idle.