Beautiful adult milkweed leaf beetles sport Mother Nature’s warning colors of orange and black.
I really don’t have anything against beetles. In fact, beetles and I go back a long way as I studied many awesome leaf beetles in graduate school. However, butterflies, especially monarch butterflies, are a real delight and like many naturalists, I anxiously anticipate their return each year and celebrate their arrival. Two weeks ago the vanguard of what I hope will be a swarm of monarchs arrived and began poking around my milkweeds. Unfortunately, in advance of the monarchs, another hungry milkweed connoisseur moved into my butterfly weed patch well in advance of the monarchs. Milkweed leaf beetles are relatives of other members of the chrysomelid clan, a large group of beetles that includes dogbane leaf beetles, Colorado potato beetles, and three-lined potato beetles we met in previous episodes. Adults and larvae of this striking insect eat leaves of common and swamp milkweeds growing wild in meadows, and also butterfly weeds running rampant in my perennial beds. Adult beetles are voracious feeders and after colonizing my butterfly weed, they quickly removed large slices of leaves. Milkweed leaf protein is translated into batches of eggs inside the ovaries of females. About a week after eggs are laid, rotund orange beetle larvae hatch and graze night and day.
Tiny orange jellybean-like eggs on the leaves of my butterfly weed soon hatch and release rotund leaf beetle larvae intent on devouring milkweeds. Adult milkweed leaf beetles are large enough and apparently scary enough to displace small monarch caterpillars as they dine on milkweed leaves. Fortunately, butterfly weeds are prolific and there should be enough to go around for all of the insects that make a meal of milkweed.
One curious and somewhat disturbing habit of milkweed leaf beetle neonates is to go cannibalistic after hatching. Yes, some early hatchers perform the ultimate act of sibling rivalry and eat their unhatched brothers and sisters. Yikes! After starting life as meat eaters, the cannibals and their surviving siblings settle in to a vegan life style, consuming milkweed leaves before moving to the soil to pupate. In a few weeks, a fresh batch of adult beetles will emerge and initiate new conquests on my beleaguered butterfly weeds. As autumn approaches, the season’s last batch of adults fatten up on milkweed leaves before finding a protected refuge somewhere in my garden to spend the winter. As you can see, milkweed leaf beetles sport the same orange and black mien worn not only by the monarch, but also by milkweed tussock moths and milkweed bugs we visited in other episodes. This cabal of milkweed feeders has evolved the ability to thrive on milkweeds despite the presence of noxious heart poisons called cardiac glycosides found in the cells and sticky white sap of the milkweed plant. In some species like monarchs, these compounds are retained during the transformation of caterpillars into adult butterflies. Cardiac glycosides found in the wings of monarchs are known to cause severe digestive distress to avian predators. These compounds help protect monarchs from disappearing down the gullets of visually gifted predators like birds that regularly prey on caterpillars, butterflies, and beetles. Conspicuous orange and black colors worn by members of the milkweed gang serve as a reminder of a potentially nasty gastronomic misadventure to experienced birds and other predators that may have attempted to make a meal of a milkweed muncher. Fortunately for the monarch, butterfly weed is prolific and I hope there will be enough for everyone if and when the monarchs arrive in force.
Acknowledgements
The delightful book “Secret Weapons” by Thomas and Maria Eisner and Melody Siegler, and the articles “Community-wide convergent evolution in insect adaptation to toxic cardenolides by substitutions in the Na,K-ATPase” by Susanne Dobler, Safaa Dalla, Vera Wagschal and Anurag A. Agrawal, and “Cannibalism and Kin Selection in Labidomera clivicollis (Coleoptera: Chrysomelidae)” by Kathleen R. Eickwort were used to prepare this episode.