Caterpillars of the Monarch Butterfly feed on plants in the milkweed family that contain toxins called cardenolides. The caterpillars sequester the toxins from the plant. The toxins are stored in the tissues and protect the adult Monarch Butterflies from predation by birds.Cardenolides protect the caterpillars from some predators (but not others). I have personally observed brown stink bugs preying on Monarch Caterpillars. Another insect that can successfully prey on Monarch Caterpillars is the Chinese Mantid. Rafter and Colleagues* observed Chinese Mantids (Tenodera sinensis) feeding on Monarch Caterpillars using a “toxin avoidance” technique. Chinese Mantids will consume the entire tissue, including the guts, of corn borer and wax moth caterpillars. When feeding on Monarch Caterpillars, Chinese Mantids do not consume the gut but consume all other tissue.
Rafter and colleagues investigated the distribution of cardenolides in the caterpillars and found that the body and gut had similar quantities of cardenolides, but the cardenolides were qualitatively different. Monarch Caterpillars modify the cardenolides from the milkweed plant before they are sequestered. The modified forms of cardenolides are possibly less toxic to the caterpillars or more readily sequestered than the unmodified cardenolides present in the plant tissue. The modified cardenolides are possibly better tolerated by the Chinese Mantid than the toxins in the gut. By avoiding the gut, the Chinese Mantid discards the plant material inside the gut that is more toxic and benefits from the less toxic and nutritious tissues.
*Rafter, Agrawal and Preisser. Ecological Entomology, Volume 38, Issue 1, pages 76–82, February 2013