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"I wonder what happens if you put a very fine powder in this equipment, without dissolving it in a liquid medium."

Probably nothing. There are high-pressure pumps and check valves involved in spraying operations.

Many of the herbicides and pesticides used in agricultural applications come in the form of "flowable powders". Mixing them with water usually also involves the addition of surfactants/dispersants, both to aid in solubilizing the active ingredient and to keep the solution from beading on the leaf.

The nozzles also have to be replaced frequently, since they are milled to such fine tolerances (so as to control coverage) that they tend to wear out rather quickly. Especially, if the mixtures employed are somewhat corrosive or even slightly abrasive.

I suppose the enemy could develop their own rig. Something which relied simply on a venturi effect to disperse a fine powder into the air. Metering or regulating it could be difficult, but I doubt achieving a perfectly even dispersion would be high on their list of priorities.

The point is this:

1. If the material is in powder form, and would stay that way, a modification to existing equipment would be necessary.

2. If the material is going to be distributed in liquid form, the operation would require considerable pre-testing to establish proper specifications. Then, the mixing and calibration stages would require some precision. They couldn't just steal a plane, load it up and start spraying death over the landscape. Pump pressure, nozzle size and type, flying speed and altitude, etc. would all have to enter into the calculation.

I know that, if I were in charge of such an operation, I'd want to already own a crop-dusting service and be able to modify the equipment to my particular specifications. I would not want to attempt such an operation "off the cuff", with stolen equipment, jury-rigged modifications and a rushed timetable.

Thus, were I the FBI, I would be investigating the ownership/operators of all registered cropdusting operations in the country. And I wouldn't be much concerned with the opportunistic "snatch & spray" operation.