I'd wager that some will survive by:

1) Getting very close to the crops and intertwining with them to make weeding very hard

2) Looking almost exactly like real crops (as-in, to the point where most humans can't tell the difference and the machines need to start taking genetic samples)

You're totally right though; the machines have an adaption-speed advantage, so I'd expect this to be a very effective technique overall (assuming that the robotics pan-out).