Slugs are not insects but are capable of shredding plant leaves or defoliating an entire plant. A mature slug can grow up to 1.5 inches. They are more of a problem during cool, wet weather. Photo of slug on a corn plant by B. Christine, online courtesy Purdue University's Department of Entomology.
the obnoxious vermin were
... the slugs were warned three times
to cease from vexing the people
by corroding and consuming
the herbs of the fields and the vines,
and to depart ...
The Criminal Prosecution and Capital Punishment of Animals
(Published in London, by William Heinemann, 1906)
Today, if an animal kills a human being, it is often "put to sleep" so it doesn't injure someone else.
In medieval times, killer animals - and insects - were put on trial for similar "crimes."
Does that sound strange? It gets much more interesting.