 And the easiest way to look at this is to visualize what monarchs do. Monarchs, of course, can only eat, can only develop on milkweeds. That's host plant specialization. Now, milkweeds are toxic plants. All plants are defending themselves. They don't want to be eaten. So they load their tissues with nasty-tasting chemicals. Some plants have spines and trichomes and very tough leaves. They use lots of ways to keep insects off of them, but primarily those chemical defenses.