The caterpillar of the monarch butterfly feeds on milkweed plants, whose toxins make the adult monarch poisonous to many predators  The viceroy butterfly, whose caterpillars do not feed on milkweed plants, is very similar in appearance to the monarch
Ttherefore, it can be concluded that the viceroy is so seldom preyed on because of its visual resemblance to the monarch
Some predators do not have a toxic reaction to insects that feed on milkweed plants.