[Ordinary_American]

The use of aircraft (e.g. crop dusters) is one way to deploy chemical and biological weapons. But as has been correctly noted in this tread it can be complicated based on the type and quality of agent, nozzle technology (often a big problem), wind and other factors which may degrade the effect. The point is that al-Qaeda was clearly investigating this. Whether it is an effective terrorist method or whether or not it was ultimately used is another matter.

[Mitchell]

Whether it is an effective terrorist method or whether or not it was ultimately used is another matter.

As far as we know, it hasn't been used. It might have been tested, and people should be on the alert for possible tests.