Last week I had a delightful opportunity to visit a demonstration garden at the University of Maryland Extension office in pastoral Frederick, Maryland. There among beds of vegetables and flowers were luxuriant beds of milkweeds, habitat for imperiled monarchs we visited in previous episodes. In addition to attracting scads of pollinators, the bounty of blossoming milkweeds regaled me with their sweet fragrances.
Despite their subtle aromas, these herbaceous beauties are not to be taken lightly. Milkweeds contain nasty chemicals dangerous or downright deadly if consumed by humans or other vertebrates. These chemicals are called cardenolides and also known as cardiac glycosides. Chemically related to medicinal compounds like digitalis, they ward off hungry herbivores and, in a sufficient dose, they are capable of arresting the beat of a human’s heart. Ah, but the heart of a bug is sometimes stronger than the heart of a man and many members of the insect realm unabashedly consume milkweed. One of the regular connoisseurs of the milkweed is the milkweed longhorned beetle, a.k.a. red milkweed beetle. Along with other diners of this toxic plant like monarch caterpillars, milkweed bugs, and milkweed leaf beetles, milkweed longhorned beetles are seemingly undeterred by cardiac glycosides coursing through the plant’s leaves which they consume with real gusto.
By first chewing through latex laden veins, the milkweed longhorned beetle disables the plant’s sticky defense and moves to the leaf blade to devour nutritious leaf tissue.
So, in the ongoing struggle between milkweed and the beetles that eat them, it looks like the insects have the upper hand, right? Well, not exactly. Milkweeds have yet another trick up their stems. In addition to nocuous glycosides in their tissues, the sap from which milkweed gets its name is a sticky, white liquid similar in consistency to latex paint. Insects attempting to eat milkweed soon find their jaws gummed-up with a rapidly hardening gob of latex. However, the milkweed longhorned beetle has a good trick to disable the latex defense of the milkweed. Prior to eating the leaf, the beetle carefully snips the latex laden veins. This prevents the latex sap from flowing to the blade of the leaf where the beetle takes its meals. With the sticky defense disabled, the milkweed longhorned beetle is free to devour the leaf without fear of gummed-up jaws. Now is a great time to explore the meadow to enjoy these marvelous beetles and witness the on-going battle between these milkweed munchers and their chemically defended hosts.
Looks like dinner and a date for this lucky pair of milkweed longhorned beetles.
Information for this week’s episode came from the “The Love of Insects” by Thomas Eisner and “Deactivation of plant defense: correspondence between insect behavior and secretory canal architecture” by Dave Doussard and Bob Denno.