Thank you Amaranth Rose, that's a good idea about being made by robots, or at least mostly automated-- every field would need thousands of them, moving slowly like beetles to conserve energy, and small enough to easily move between the crops.

They could be used until they wear out, when they signal a problem and another one nearby carries them out to be recycled. At harvest time a universal signal would have them clear the field. Even though they might last a long time they would still need to be quite cheap.

Another advantage-- because they are machines rather than living things they would not be tasty to birds or other animals smile