One of the most well known associations between an animal and plant species is the relationship between Monarch butterflies and Milkweed. Monarch butterflies may certainly be seen nectaring at various species of milkweeds…
but this isn’t unique – they also drink at a wide variety of other flower species.
It’s the dependency that Monarchs have on Milkweeds as the only food source for their caterpillars that makes this relationship so noteworthy. Monarchs, like many species of insects, have evolved to specialize in their larval (in this case caterpillar) food source in order to gain protection from predators through the chemicals they ingest from the plants they eat. Milkweeds contain cardiac glycosides, which are toxic to many species of birds and mammals. Plants have evolved these chemicals to protect themselves from being eaten, a strategy that has largely been successful for the plants. Plants are all about surviving and reproducing, to further the continued existence of their species.
Such a plan for protection is never completely foolproof, however. Monarchs, along with some other insect species, have evolved to be able to digest these plants and sequester the toxins in their bodies, making the insect unpalatable at best and toxic at worst to anyone inexperienced enough to attempt to eat them. As a reminder to bird or mammal predators who sample such an insect and survive to eat another meal, insects with these toxins have also evolved to have bright warning colors, an easy to remember signal to predators to beware before attempting such a meal again. In exchange for this protection obtained from eating Milkweeds, Monarchs are gambling that this food source will continue to be available. Without it, Monarchs won’t survive.
Monarchs are not alone in their use of Milkweeds. Their copious nectar offerings attract a broad range of butterflies to drink at their flowers, from Eastern Tiger Swallowtails,
to the smallest skippers.
Swamp Milkweed (Asclepias incarnata)
and Butterflyweed (Asclepias tuberosa)
as well as other milkweed species are also favorites of butterflies, bees, and other insects that are nectar feeders, for their reliable, sweet, high energy food.
Butterflies benefit from the food offered by milkweeds, and in return they do help the plants with pollination, but they are not the most successful pollinators of milkweeds. Milkweeds have bundles of pollen, called pollinia, that are linked in pairs by a thin filament. This connector snags an insect appendage that is inserted in just the right spot in a flower. An insect has to approach the flower in a way that will engage the filament connecting the pollinia, and it must also be robust enough to remove the pollinia from the flower in order to assist the plant in cross pollination. The pollinia is carried by the insect to another flower, and inserted by the same mechanism.
Take a look at the Eastern Comma below. It’s perched on top of a flower, using its straw-like proboscis to sip nectar from a Common Milkweed (Asclepias syriaca) flower. With this approach, a butterfly isn’t that likely to be helping this Milkweed out with pollination.
This Bumble Bee, on the other hand, is facing the flower, with its left front leg inserted in the very location where the pollinia are stored. This bee is engaging the pollen sacs, and has the heft to be able to escape from the flower with them clinging to its leg.
If you look carefully at the bee’s left front leg in the photo below, you can see the yellow pollinia attached to it.
Large bees, such as Bumble Bees and Carpenter Bees, are among the most successful intermediaries in Milkweed pollination. Common Milkweed flowers release a potent fragrance to attract bees to assist them in their reproduction.
Many other insects take advantage of the nectar bounty offered by Milkweeds, including the Snowberry Clearwing Moth (Hemaris diffinis).
There’s another insect species dining on this Swamp Milkweed. Do you see the little yellow critters on the stem? You might be thinking, “Eeuuw! Aphids!” If so, you would be half right. These are Oleander Aphids (Aphis nerii), a species frequently found on Milkweeds. But you might want to re-think the “Eeuuw!” It turns out that aphids are an important part of the food chain.
Aphids rarely really harm a plant. And they offer a sustainable food source in the form of honeydew, a sweet excrement that ants love. The ants protect aphids in exchange for this tasty meal. Ants are essential for aerating soil, decomposing plant matter, disbursing seeds, and in some cases protecting plants from other predators.
Aphids are also a food source for other insects. In the photo below, this Oleander Aphid is being parasitized by two predators at once! It’s being bitten in the butt (abdomen) by a Convergent Lady Beetle (Hippodamia convergens) larva, who plans to consume the aphid. Notice also the bulge on the lower left side of the aphid. That is likely the result of a braconid wasp parasitizing the aphid. The adult female wasp lays an egg inside the aphid, with its resulting offspring consuming the aphid from the inside, leaving an empty husk. (There’s a white squiggly thing on the lady beetle larva that I’m guessing may also be a predator, but so far I haven’t identified it. If you know what it is, let me know!)
If you’re really observant you may have noticed that this scene was taking place on half of a Common Milkweed leaf, with the right side of the leaf missing. Wondering how that happened? It’s the way Milkweed Tussock Moth (Euchaetes egle) caterpillars feed, neatly chewing side by side, stopping at the midrib.
Another insect that feeds on milkweed leaves is the Red Milkweed Beetle (Tetraopes tetraophthalmus). This clever creature bites through the midrib of the leaf in a few spots near the leaf tip. This stops the milky latex-like sap from flowing to that part of the leaf, making it possible for the beetle to eat it without having its mouthparts glued together by the sticky substance.
Even Milkweed seeds are a source of food for insects like the Small (Lygaeus kalmii) and Large (Oncopeltus fasciatus) Milkweed Bugs. Adults may also consume nectar.
All of these insects sport bright colors that warn birds and mammals to avoid eating them. Insect and arthropod predators including the Lady Beetles, wasps, assassin bugs, spiders and Praying Mantises (or Mantids) are not put off, however. They may consume not just nectar feeders, but foliage and seed feeders, too.
Many of these predators are safe for birds and other predators to eat. A large percentage of a bird’s diet consists of insects, especially when they are raising their young.
Birds also benefit from Milkweeds by using them as nesting material. The fluffy hairs attached to the seeds can make a soft lining for a Goldfinch nest.
Most Milkweeds also have strong fibers in their stems that birds use to weave nests, including Northern Orioles
and Yellow Warblers.
Milkweeds are essential to the continued survival of the Monarch butterfly. They are a copious nectar source for our beleaguered bee populations, and offer food to many other beneficial insects. They’re a source of insect protein and nest material for birds and other animals. Milkweeds – they’re vital for Monarchs, and a whole host of other species, too.
Eastman, John. The Book of Field and Roadside. 2003.
Eastman, John. The Book of Swamp and Bog. 1995.
Eaton, Eric R.; Kauffman, Ken. Kaufman Field Guide to Insects of North America. 2007.
Harrison, Hal H. Eastern Birds’ Nests. 1975
Tallamy, Douglas W. Bringing Nature Home, 2007,