MilkweedUSA

What else lives on milkweed

The monarch gets the attention, and it is not close to alone. About a dozen North American insects eat Asclepias and essentially nothing else, and a far larger crowd arrives for one of the richest nectar supplies on the continent.

10species on this page that eat milkweed and little else
3butterflies whose caterpillars need it: monarch, queen and soldier
2birds that build with it without eating any of it
1 in 50milkweed flowers that ever set a pod, despite all the traffic
A red milkweed beetle with long black antennae feeding on common milkweed flowers
Plant Image Library · CC BY-SA 2.0 · Wikimedia Commons
A red milkweed beetle chewing a milkweed leaf, its bright red body warning of the toxins it carries
Rbreidbrown · CC BY-SA 4.0 · Wikimedia Commons
Large milkweed bugs, orange with black bands, gathered on a milkweed seed pod
Peterwchen · CC BY-SA 4.0 · Wikimedia Commons
An adult large milkweed bug on a milkweed leaf, showing the black band across its orange back
Andrew C · CC BY 2.0 · Wikimedia Commons
A cluster of milkweed tussock moth caterpillars, tufted in orange, black and white, stripping a milkweed leaf
Jrosenberry1 · CC BY-SA 4.0 · Wikimedia Commons
A milkweed tussock moth caterpillar showing the dense tufts of hair that give it its name
Andrew C · CC BY 2.0 · Wikimedia Commons
A dense colony of bright yellow oleander aphids clustered along a milkweed stem
Юкатан · CC BY-SA 3.0 · Wikimedia Commons
Yellow oleander aphids with black legs packed onto the growing tip of a milkweed plant
Юкатан · CC BY-SA 3.0 · Wikimedia Commons

Why nearly all of them are orange and black

Line the milkweed specialists up and the resemblance is hard to miss: monarch, queen, large and small milkweed bug, red milkweed beetle, swamp milkweed leaf beetle, milkweed tussock moth, oleander aphid. Orange, red, yellow and black, over and over, on insects that are not closely related to one another at all.

They share a diet, and that diet comes with cardenolides, the heart-stopping compounds milkweed makes to protect itself. Every one of these insects has evolved a way to eat them anyway, and most go further and store the toxin in their own tissue. A bird that tries one and is sick learns to avoid what it looked like, and because they all look alike, that one bad experience protects the whole group. Each species genuinely is unpalatable, so nobody is freeloading. Biologists call this Müllerian mimicry, and a milkweed patch is one of the clearest examples of it anywhere.

The viceroy butterfly, long taught as a harmless mimic of the monarch, turns out to belong in this club too: it is unpalatable in its own right, from the salicylates in the willows its caterpillars eat.

Milkweed specialists

These insects eat Asclepias and almost nothing else. Most of them store the plant’s toxins in their own bodies and advertise it in orange, red and black, which is why a milkweed stand in August looks like it was color-matched.

Danaus plexippus Monarch butterflyThe reason most people plant milkweed, and the only butterfly whose caterpillars cannot eat anything else. Danaus gilippus Queen butterflyThe monarch’s southern cousin, common enough in Texas, Florida and Arizona that people often mistake it for a dark monarch. Danaus eresimus Soldier butterflyThe third North American Danaus, scarce, easily overlooked, and largely confined to south Texas and peninsular Florida. Euchaetes egle Milkweed tussock mothThe tufted caterpillars that turn up in a crowd in late summer and strip a leaf to its veins in a day. Oncopeltus fasciatus Large milkweed bugThe orange-and-black bugs that gather on ripening pods, drilling into the seed inside. Lygaeus kalmii Small milkweed bugSmaller, redder, and far less fussy than its large cousin: it takes seed, nectar and the occasional dead insect. Tetraopes tetrophthalmus Red milkweed beetleA red longhorn beetle whose antennae grow straight through its eyes, splitting each one in two. Labidomera clivicollis Swamp milkweed leaf beetleA rounded orange and black beetle that cuts the leaf veins before it eats, to drain the latex out of its lunch. Aphis nerii Oleander aphidThe bright yellow aphids that coat a milkweed stem, all of them female, all of them clones, and none of them native. Rhyssomatus lineaticollis Milkweed stem weevilThe reason a healthy milkweed shoot suddenly wilts from a neat row of punctures partway up the stem.

Nectar visitors

Milkweed flowers are among the richest nectar sources in the North American flora, and they open in high summer when much else has finished. These come to drink.

Bombus species Bumble beesStrong enough to pull a milkweed’s pollen sacs free, which most of its visitors are not. Sphex, Polistes and others WaspsUnglamorous, disliked, and among the most effective milkweed pollinators there are. Speyeria and others Fritillaries and other nectar butterfliesDozens of butterfly species feed at milkweed without ever laying an egg on it.

Predators

Anything that reliably concentrates insects also concentrates the things that hunt them.

Misumena, Phymata Crab spiders and ambush bugsA flower that gathers this many insects also gathers the things that eat them.

Birds

Two species take the plant apart for building material without eating a leaf of it.

Spinus tristis, Icterus galbula Goldfinches and oriolesTwo birds that never eat milkweed, and take it apart anyway.

Grazing animals

Almost everything with a backbone leaves milkweed alone, and the exceptions matter.

Odocoileus, Equus, Ovis Deer and livestockThe animals that leave milkweed alone, and the circumstances in which they do not.

What a gardener should do about any of this

Almost always, nothing. Milkweed is a host plant, and a host plant covered in insects is a host plant doing its job. The one rule worth keeping is the hard one: do not spray anything. Insecticidal soap, neem, pyrethrin and horticultural oil all kill monarch eggs and young caterpillars, and a systemic neonicotinoid keeps killing them for months. The pests guide covers the handful of genuine problems.

The second thing is to leave the dead stems standing over winter and cut them in early spring. Orioles need the fiber, insects overwinter in the stems, and nothing is gained by a tidy bed in November.