Milkweed and its Fascinating Bugs

Do bugs have caring moms?
March 27, 2024
Most people know that milkweed plants are important for the survival of Monarch butterflies. But beyond that, their knowledge is a little murkier. Milkweeds are fascinating plants where you can spot a wide diversity of insects despite the plants’ interesting obstacles. I’m going to introduce you to some of the insects and their interesting relationship with milkweed by groups: the Herbivores, the Pollinators, and the Predators.

Milkweed Herbivores

Milkweed has evolved some interesting defenses to protect itself against the herbivores – insects that eat leaves, flowers, and stems. First, you’ll notice that if you break a leaf or stem, it will start “bleeding” a milky sap. This substance can glue an insect’s mouth parts or adhere to wings, killing the insect. Next, the leaves and stems have hairs that detract many species. And finally, milkweeds contain toxins – cardenolides – that can sicken or kill wildlife if ingested. It’s so potent that it can even poison grazing livestock - which is one reason milkweed has been broadly destroyed in agricultural areas over the past century.
With these defenses, it’s actually amazing that milkweeds have herbivores. But many insects have co-evolved with milkweed to the point of not only developing special chemistry to avoid the defenses, but to ingest the toxins. That in turn makes them poisonous to predators. Many of the milkweed herbivore specialists have co-evolved to have similarly bright orange/ black color markings (Mullerian mimicry) to warn potential predators that they’re all poisonous. That color pattern is understood to mean “don’t eat me!”. While monarch caterpillars lack those colors, the adult butterfly – that has inherited some of that poison – also has the color scheme. The strategy works so well, that there are many non-poisonous insects that mimic the coloration pattern just to trick predators into leaving them alone too (Batesian mimicry). 


The other big challenge for herbivores is to avoid the milky sap that is unpalatable and could glue their mouths shut. Leaf-eating specialists have learned to drain the sap by chewing holes into the stem, allowing them to munch on the leaf. So, if you want to find some of these other insects, look for plants with munched or pierced leaves.

The common Large and Small Milkweed Bugs are seed specialists. They become conspicuous later in the season when you can find little blobs of red baby bugs on milkweed pods. Large Milkweed Bugs, like Monarchs, aren’t capable of surviving Pennsylvania winters, so they migrate to southern states in the winter and repopulate each summer.

There’s no reason to be concerned that any of these specialists are harmful to monarchs. They have all co-evolved together with these plants. That said, you may also notice a yellow-orange aphid, particularly on Swamp Milkweed. This is a non-native pest, also migratory, that can weaken the plant during bad infestations. Resist the urge to use any insecticides as the teeny-tiny early instar caterpillars that are nearly impossible to see, emerge in the flower clusters with the aphids.

Milkweed Herbivore Slideshow
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Milkweed Pollinators

When it comes to providing nectar for pollinators, milkweeds are generous. This nutritious food source draws many insects to the flowers to feed – even though most of them aren’t capable of being milkweed pollinators. What do I mean by that? Milkweeds have a unique pollen structure (shared only with orchids), that form “pollinia” packages with hundreds of pollen grains. The pollinia are located on the flower, between the petals, in slits. While an insect feeds on nectar, its foot (or other body part) sometimes inadvertently slips into the slit. It takes effort for the insect to extract its foot that now has a pair of pollinia stuck to its foot! Once it’s free, the insect continues feeding, dangling the pollinium around. For pollination to finally take place, the insect must fly them to another plant (in a different colony) and place them back into an open slit. Yikes – it’s amazing they’re ever pollinated.
To accomplish that momentous task requires a right-sized, hefty insect that can pull out the pollinia and successfully handle the extra weight and awkwardness in flight. And of course, feed on enough flowers to find an open slit.
It’s the bumblebees, other large bees, soldier beetles, and a few butterflies –the Silver Spotted Skipper was noted in one study – that provide most of the pollinating services. Adult monarch butterflies feed on the nectar of milkweed, but rely on a diverse range of plants for nectar. While they can pollinate milkweeds, they’re considered a minor pollinator. 
Milkweed Pollinator Slideshow
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Predators can be found wherever pollinators congregate, but milkweeds seem to have an abundant supply. Not only will there be numerous insects coming to slurp up the nectar, but many insects, especially, medium size or dainty insects, can get a foot caught in the slit. While they’re trapped, even if only briefly, it can prevent quick escape, making them vulnerable to predation.
Typical predators include crab and jumping spiders, as well as assassin and ambush bugs. Some of the wasp pollinators are there on double duty as predators or parasitoids. Wasp larvae are meat-eaters, so the females capture and fill their nest with prey for their larvae to feed on. Then there are the parasitoids (mostly wasps and flies) that lay their eggs onto a host insect that will eventually be consumed by her young. All of this sounds awful, but it is part of nature’s population control.
Milkweed Predator Slideshow
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It’s your turn

This summer, spend some time investigating milkweed plants for the abundance of insects they attract, including the specialists. Maybe you’ll find a monarch caterpillar while you’re looking!