Odors emanate from plant and leave a trail to the plant. Insect perceives and homes in on source of odor. We call a factor which causes an insect to eat a phagostimulant . For many insects if the phagostimulant is not present they will not eat even if the food is highly nutritous. Will starve to death first.
Cabbage butterfly uses mustard oil as a host location factor and as a phagostimulant.
If you soak filter paper the larvae will eat it. Similarly if you put it on a non-host they will eat that host even if it kills them. If you remove from the normal host they will not eat it. Grasshoppers are paurometabolous. They lay their eggs in soil and do not take care of them.