This is the time of the year when the menagerie of insects that feed on members of the dogbane family (Apocynaceae), including common milkweed (Asclepias syriaca), seem to arrive en masse to the consternation of monarchists. Some well-meaning gardeners aim to reserve milkweeds exclusively for the pleasure of monarchs. What about other native insects that feed on milkweeds; let them eat cake?
Monarch butterflies (Danaus plexippus) are beautiful insects and their plight remains uncertain. Elevated multi-national concerns over their future have raised awareness of the rewards of gardening beyond aesthetics. Monarchs have served as excellent ambassadors for the environmental benefits of expanding our plant selection palettes to paint more diverse landscapes. Milkweeds are no longer weeds.
However, efforts to save monarchs may sometimes morph into crusades to serve the aristocracy at the expense of the proletariat. Good intentions can be taken too far. Royal sovereignty should not rule the distribution of host plant food!
The Milkweed Banquet
Members of the dogbane family try to ward off herbivores by loading their milky, sticky sap with alkaloid toxins, called cardiac glycosides (cardenolides). However, some specialized herbivores, including monarchs, evolved a capacity to handle these toxins. The downside is that these herbivores are so specialized milkweeds are the only food they eat. This includes a diverse range of native insects beyond monarchs.
Part of their dependency on milkweeds involves mounting a successful defense against predators. Like monarch caterpillars, many of these native insects accumulate alkaloid toxins in their flesh that are acquired from feeding on milkweeds. Presumably, this makes them taste bad, but I've never tested this theory.
Most members of the milkweed menagerie advertise their toxic character through bright coloration, usually involving an orange-on-black motif. Employing splashy colors and color patterns to defend against being eaten is called "aposematism" from the Greek for "away" and "sign." Of course, as illustrated by monarchs, gaudy aposematic coloration can be beautiful.
Liberty, Equality, Fraternity
Notable native insects that depend on milkweeds for their livelihood include beautifully colored Milkweed Tussock Moth caterpillars (Euchaetes egle); Swamp Milkweed Leaf Beetles (Labidomera clivicollis); Red Milkweed Beetles (Tetraopes tetrophthalmus); Large Milkweed Bugs (Oncopeltus fasciatus); and Small Milkweed Bugs (Lygaeus kalmii). These native insects have the same inalienable food rights as monarchs.
The small, yellow-green first instar milkweed tussock moth caterpillars have black head capsules and are covered in wispy white hairs. Later instars sport rows of black and orange hairs punctuated by tufts of long white and black hairs; these colorful hairs are the feature most often associated with these caterpillars and are responsible for the alternate common name of "milkweed tiger moth."
All instars feed side-by-side in groups sometimes called "colonies" and can consume entire leaves leaving only the veins. In fact, the caterpillars can appear to be serious competitors to monarchs. However, tussock moth caterpillars tend to be found on only a few milkweed plants and they focus their attention on one leaf at a time. Plenty of food remains for monarchs.
The milkweed bugs are seed feeders. They use their piercing-sucking mouthparts to skewer seeds, inject enzymes to dissolve the internal tissue, and suck-up the resulting slurry. The bugs may also feed on developing seed pods but seldom cause enough damage to kill the entire pod. They will reduce the number of viable seeds; however, milkweeds are prolific seed producers. How many seeds are enough to carry milkweeds to next season? There's plenty of seed to go around.
However, I recently observed what happens when large milkweed bugs arrive before their seed-food matures: they become cannibals. They use their digestive enzymes on one another to suck-up the essence of insect. It's a jungle out there.
Brightly colored swamp milkweed leaf beetles (Labidomera clivicollis, family Chrysomelidae) are one of my favorites; partly because they are somewhat rare. Despite their common name, both the adults and larvae feed on the leaves of several members of the dogbane family.
Although swamp milkweed leaf beetles use milkweed toxins as a chemical defense, there can be too much of a good thing. The latex in the sticky sap can clog their chewing mouthparts. To avoid this, both the adults and larvae will chew through veins "upstream" from their feeding site to reduce sap flow. Evidence of beetle damage includes deep leaf notching usually towards the leaf tip.
The tubular-shaped red milkweed beetles are a type of longhorned beetle (family Cerambycidae). Although the "longhorned" name of the family refers to long antennae, milkweed beetles have relatively short antennae. However, a close examination of their prominent black antennae will reveal another family trait. Their antennae bisect their compound eyes creating two sets of eyes with one set located above the antennae and one set below. The name of the genus, Tetraopes, and specific epithet, tetrophthalmus, describes this unusual feature; both are derived from the Latin for "four eyes."
Red milkweed beetles feed on entire plants with the adults feeding on the leaves and the larvae boring into the roots and stems. However, I've never seen enough damage to present a serious challenge to monarchs.
I support monarch recovery. However, we should embrace all native insects with equal affection. I mean this figuratively since tussock moth caterpillars have defensive hairs and milkweed bugs can drill fingers with their piercing-sucking mouthparts. Members of our native milkweed menagerie should enjoy the same natural born rights to milkweeds as monarchs. The bottom line: liberty, equality, and fraternity for all insects that you may find sharing milkweeds with monarchs.