Milkweed, fuel for some of the animal kingdom's deadly clowns, is both food and toxin. The toxin intake is limited by various strategies and exploited by various creatures.
Several insects eat milkweed. Notable among them are the larval stages of the Monarch butterfly. The less known but adorable Tussock moth caterpillar, as well as all stages of the four eyed milkweed beetle and the yclepted milkweed bug all also benefit from special relationships with milkweed. Even among aphids there seems to be a variety that leans toward wearing the garish orange/black clown suit.
After eating milkweed these insects become at best unpalatable and emetic to their predators. If swallowed and retained they may kill as well be killed. One projects an anthropomorphic wish that predators will learn from their mistakes and simply avoid eating them. Because their coloring is so "obvious" and because most of them tend to cluster in large and easy to see groups it does seem predators learn to mostly skip over them even if a few are nibbled in the process.
To emphasize a point, milkweed can nourish and can kill. It must be approached carefully. If you are a person eating milkweed several changes of cooking water are recommended. If you are a bug other methods are employed. The monarch instars 1 to 3 do a maneuver called "trenching", only eating through a few layers of the leaf, thus avoiding too much toxin. Instar 4 chews little circles out of the leaf, also reducing sap flow. The 5th instar eats everything but "they will sometimes chew a notch in the leaf's petiole, causing the leaf to hang down. This behavior is known as flagging" and also reduces sap flow to the day's dinner. 1
Other tastier insects totally avoid the toxin of milkweed but adopt the coloration of the milkweed eaters, gaining a reputation they do not deserve. The Viceroy butterfly (adult stage only) mimics the Monarch and birds learn to avoid them, those scary Halloween clowns in costumes.
The other day I noticed a swarm of milkweed bugs (harmless except to milkweed plants) on the pod I was harvesting in the field next to my work...seeds of which were destined to decorate the dry stems surrounding my front porch for my daughter's party tonight. The house is decked out as a cross between a pumpkin patch, an abandoned garden, and a graveyard.
Those seeds will later float to spots, spots where they will germinate and next year mature among the many "weeds" I coddle in my yard. They will also spread to my neighbors' yards who will in turn mostly "weed" them out but a few will remain. The seeds' silks are sparkling, pure, pure white and oh so pretty. Milkweed needs to be coddled, not only for its beauty (it has a wondrous flower) but for its entertainment value and for its niche in the life of the monarch, viceroy, and other friends.
Another year, another orange/black critter; I witnessed a posse of tussock moth caterpillars. Google image search on "milkweed tussock moth". They look like a Yorkshire Terrier, long hair dragging on the ground ... except they are orange and black. They cluster together to project their fierceness to the world but I found them adorable. That patch of milkweed is gone. Labeled as weeds and removed for hybrid zinnias. Sad.
Halloween brings ripened seed pods and thoughts of costumes. Orange and black prevails. Milkweed ties the end of Blue October2 together for me.
"It's also important to create a source of food for young pollinators and provide overwintering places for eggs and larvae. To do so, allow a corner of your backyard to naturalize with wild grasses, weeds and wildflowers (including Queen Anne's lace, thistle, burdock, borage, milkweed, evening primrose), and plant extra dill, parsley and carrots, which swallowtail butterfly larvae consume in quantity."3