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Why Do I Have to Share My Milkweed?

By Carol Allen

Many folks have enthusiastically embraced planting gardens for butterflies or as monarch way stations. Those folks understand that they are providing larval food as well as nectar sources for their intended guests and may be resigned to seeing just how much of their milkweed hungry caterpillars can consume before pupation. However, it is when other insects start to consume those precious milkweed plants that folks can become alarmed. In our perfect world, there would be sufficient habitat to provide food and cover for an entire complex network of insects and other organisms to support not only the butterflies, but also the birds we like to see. But that is not the reality we live in. Although your target “guest” might be swallowtail or monarch butterflies, there are less glamorous insects that find your garden a haven. One such unwanted guest is the milkweed tussock moth, Euchaetes egle. As any grade-school student can tell you, an insect’s black and orange color combination warns birds that it tastes

Milkweed Tussock Moth (Euchaetes egle) Mason Neck, Virginia. Photo by Judy Gallagher, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons.

bad or worse. Like the monarch, the milkweed tussock moth larva is colored black and orange, warning birds to stay away. The cardiac glycosides in the milkweed the moth feeds on can cause vomiting in the bird that unwittingly samples it. If the milkweed tussock moth larva does not support song birds, what creature eats it? Bats have been known to feed on the adult moth, although the moths give off an ultrasonic signal that can warn bats away. Beetles, stink bugs, and spiders don’t seem to be bothered by the cardiac glycosides and can provide some population balance. Perhaps the most alarming characteristic of this caterpillar is its gregarious behavior. The female moth lays her eggs in rafts and the young larvae stay together in packs through the third instar (about ¼ inch). As they grow up, they become less gregarious and can be found in fewer numbers. The larvae favor older leaves of milkweed or dogbane, completely skeletonizing the leaf. By leaving the main vein alone, they can avoid being trapped by the milkweed latex. Larger, older larvae sever the midvein to prevent the latex from entangling them as they feed. In general, monarch larvae prefer younger leaves so it is possible for one very robust plant to support both predators for a while. What do you do if you find your carefully reared milkweeds being devoured by these gate crashers? You can sweep them off with gloved fingers into a bowl of soapy water and flush them. Or you can just plant more milkweed. Oh, did I mention gloved fingers? Yup, these guys have more than one defense mechanism. There are irritating spines hidden in all that pretty fluff so do not handle them with your bare hands! o

Carol Allen describes herself as a committable plant-a-holic. She has more than 25 years’ experience in the horticulture industry, with a special interest in plant pests and diseases; is a Licensed Pesticide Applicator in the state of Maryland; and is an ISACertified Arborist. She can be contacted at carolallen@erols.com.