Crop dusters take off and land so frequently because they're dumping a lot of chemicals. Even though some of the bigger ones carry up to 500 gallons of chemicals, they can go through all of that in a few minutes. Their fuel supply is usually good for 2-3 hours.

It really isn't all that easy to read a plane's N number, especially if you've just been sprayed. People tend to look away when stuff is raining down on them, especially with all the anthrax scares going around.

About all they have to go on is a vague discription - a white plane, perhaps a cropduster. Okay, time to try and narrow that down. Was it a biplane or monoplane (there are still some biplane crop dusters out there)? Was it all white or were there other colors? Use the process of elimination and narrow down the list of suspects. Even though that's farming country, there probably aren't that many white crop dusters out there. Work your way down the list to account for all possible matching planes. Crop dusters have to maintain all sorts of records and they need at least one person on the ground to help them refill between runs. It shouldn't be all that hard to account for the planes.

I still say there's no way a professional cropduster pilot would throw away his livelihood by pulling such a stupid stunt. The FAA would pull his license in a heartbeat and he may never get it back. It'd be like a lawyer getting disbarred or a doctor losing his license - years of hard work and expense to reach a professional level thrown down the drain.