Hmmm. That is an interesting thought, but they have to have
somewhere to take off from and it seems that we have a good presence. And while I understand that ultralights can take off in a fairly short distance, a M-16 round in the head of the pilot would seem to be a proper defense.
An interesting though nonetheless.
Can we defend the skies 24/7 from chemical laden ultralights in a town of 4-5 million people? (size of Chicago)