Why are crop dusting spray booms not integrated into the wing? All crop dusting aircraft seem to have an externally mounted spray boom for distributing their pesticides, I imagine this significantly increases drag &amp; therefor performance.

What gives? why not integrate the spray system into the wing?

I have read that crop dusters spray booms remain short of the wing tip to prevent interaction with the wingtip vortices, perhaps another aerodynamic interaction along the main span of the wing prevents this integration?

thanks for considering my question!
Different chemicals use different nozzles and delivery systems.  Many of the spray agents are very corrosive.  Turbulence by the sprayer can be a good thing, as it aids in mixing.

Additionally drag is not a big issue as crop dusters fly slow, and tend to have good power to weight ratios, so the drag of a spray assembly at 70 knots or 60 knots, is not a big concern.  Mixing (with air of) chemicals, reduction of airframe corrosion, and ease of removal are larger concerns.