With over 250 views, here is the answer. This 1928 biplane aircraft converted to a cropduster is the OLDEST crop duster registered and working in the USA, based in northern California. Subject of a Television special, N3977 goes back to the era where ordinary available and suitable production aircraft were converted to have a chemical hopper and release system to dust field crops.
Purpose-design and build crop dusters such as the Piper PA-15 Pawnee of 235 Hp and larger PA-36 Pawnee Brave of 400 Hp (of 1960 and 1973 vintage, respectively), and others like the Air Tractor line with turboprop power are mainly low wing hard-working agricultural aircraft.
Nowadays, the term aerial applicators seems to prevail in this specialized field, and Aspen Helicopters, Inc. based at KOXR locally has been doing helicopter crop-dusting for many years. Less chemical waste and better accuracy are some of the features of a helicopter dusting farm crops and orchards.