Milkweed is the food plant of the iconic Monarch butterfly. With drastic declines in Monarchs over the past few years, people everywhere are being encouraged to plant milkweed. But milkweed does a lot more than feed Monarchs; it has a unique flower and a sinister method of pollination that can amputate or even kill the pollinating insects that visit.
Each milkweed blossom is equipped with a trap door, called a stigmatic slit. When insects land on their droopy flowers, clinging to the petals as they feed on nectar, a foot slips into the stigmatic slit and comes in contact with a sticky ball of pollen, called a pollinium. When the insect pulls its foot out of the trap door, it brings the pollinium with it. Eventually, the insect will move on to the next flower. Should that same foot slip back into another milkweed flower’s stigmatic slit, the pollen can be transferred and pollination is completed.
Moving around pollinia from flower to flower can be a dangerous task. Sometimes insects get stuck in one of the flower’s stigmatic slits and are never able to free themselves. Other times they must tear off their own limbs to escape. Even if an insect does manage to pry its leg out of the trap door, some insects are unable s to remove the pollinia. One or two pollinia will slow an insect down, but too many can make it difficult to move. I once saw a bumblebee sitting on the leaf of a milkweed plant covered in pollinia and unable to fly.
Despite the potential harm of visiting a milkweed flower, for many insects, this is a reliable source of nectar that is worth the risk. And from the act of pollination, a lot of good can come. Each pollinated flower will become a seed pod. During the first frosty night in the fall, the seed pod will burst open, and the wind will carry each milkweed seed on a parachute of fluff. That fluff is used by mice to line their nests and keep them warm in the winter. Eventually the seed will settle on the ground, and there, the milkweed seed sprouts the next spring. Monarch butterflies will stop to lay their eggs on the fresh milkweed leaves as they journey back from Mexico, and with any luck, their decline will be slowed by the efforts of many to plant more milkweed.