Hello dear @tobywalter possibly the caterpillars that you are observing in the crop are of the corn earworm, whose scientific name is spodoptera frugiperda, a preventive control for this would be the application of entomopathogenic fungi such as METARHIZIUM ANISOPLIAE, is a biological control can be applied when you see low populations of the worm, if you have a lot of infestation is advisable to use pesticides.

See you later, have a great week.