You mean kind of like animals that can smell particular chemicals like the way dogs sniff out drugs?. Yea, I think that herbivores are already doing that in the way they sometimes avoid poisonous plants. with training, I think dogs can fit the task
Yes, I think so. The relation between insects and plants is known.
Especially with volatiles compounds, but also with constitutive compounds. Monitoring, however, requires a profound understanding of the relation and of the affecting factors.