Easy to kill. Seven if it's being grazed, Eight, Malathion, several insecticides will kill them. Depending on what insecticide is used, the speed of dead larva will vary. When sprayed with some insecticides they will take two or three days to actually die but they will quit feeding soon after they are sprayed. The trick is to catch them early. Another thing I have noticed is if they start off in a pasture and move across it and come to a cotton field, they seem to not be attracted to the cotton leaves.