On recent visits to the meadow, a bounty of blossoming milkweeds and dogbanes regaled me with their sweet fragrances. Despite their subtle aromas, these herbaceous beauties are not to be taken lightly because milkweed and dogbane contain nasty chemicals dangerous or downright deadly if consumed by humans or other vertebrates. These chemicals are called cardenolides also known as cardiac glycosides. Relatives of 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 denizens of the insect realm unabashedly consume milkweed and dogbane. 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 (See August 8, 2005, The Royals are in the House), milkweed longhorned beetles are seemingly undeterred by cardiac glycosides coursing through the plant’s leaves which they consume with real gusto.
Glycoside laced dogbane faces a more serious dilemma. The dogbane leaf beetle, Chrysocus, eats dogbane and is apparently immune to the toxic cardiac glycosides in the leaves. The clever dogbane beetle turns the tables on its host by ingesting cardiac glycosides, storing them in glands, and secreting them when threatened by its own predators. It is believed that these toxic compounds confer protection from hungry animals that would otherwise devour Chrysocus.
So, in the ongoing struggle between milkweed, dogbane, and the beetles that eat them, it looks like the insects have the upper hand, right? Well, not exactly. Milkweed and dogbane have yet another trick up their stems. In addition to nocuous glycosides in their tissues, the sap from which milkweed gets its name and the sap of dogbane are sticky, white liquids similar in consistency to latex paint. Insects attempting to eat milkweed or dogbane soon find their jaws gummed-up with a rapidly hardening gob of goop.
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. The dogbane beetle has a crafty trick as well. After nibbling for a while, the dogbane beetle does a moon walk while dragging its mouthparts on the leaf to wipe-off the sticky dogbane latex. Once free of the latex, it moves to a new spot to resume its feast. Now is a great time to explore the meadow to enjoy these marvelous beetles and witness the ‘tit for tat’ relationship they have with their chemically defended hosts.
References
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.
More information on milkweed longhorned beetles and dogbane leaf beetles can be found at the following web sites:
http://www.enature.com/fieldguides/detail.asp?fotogID=724&curPageNum=4&recnum=IS0031
http://www.ipm.iastate.edu/ipm/icm/2003/7-28-2003/dogbane.html