As Jesus turns to the multitude he reminds them of the need for vigilance of which he has just reminded the twelve (verse 15).
Translation: The vocabulary here comes from Luke:
Luke 22:31
31 And the Lord said, Simon, Simon, behold, Satan hath desired to have you, that he may sift you as wheat:
In this case the vocabulary must come from Luke rather than the Mesoamerican context, as wheat was not a staple crop. Certainly if had there been some wheat, it would not have been a viable image.