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  • Writer's pictureBesa

This Milkweed was Planted for Monarchs


My neighbor planted a Swamp Milkweed, Asclepias incarnata, on the edge of her patio to attract monarchs, Danaus plexippus, to her garden. Swamp milkweed may be their most preferred food source of the milkweed genus, which the monarchs depend on during their caterpillar stage. This specimen grew to be about 4 foot tall and about as big around. The flowers bloomed but the monarchs didn’t come. Sometimes it can take some time for an adult monarch to find a new source of host plant. Allowing the plant to become well established before the caterpillars begin feeding is better than freshly emerged seedlings being eaten to the ground by a very hungry caterpillar that showed up early. Eventually, the monarchs will arrive, seeking out each milkweed to serve as the host plant for a new generation.


First the milkweeds leaves begin to be colonized by aphids. The orange aphids are milkweed aphids, Aphis nerii, an introduced species from the Mediterranean that clones itself instead of reproducing sexually. Milkweed aphids store the cardiac glycosides produced by the milkweed plant. Their bright orange/yellow warning coloration serves to warn predators that they are poisonous. When aphids colonize a plant, many gardeners reach for the insecticides but when you are gardening for monarchs, insecticides, even organic insecticidal soap, would kill the very species you are trying to attract. Other common solutions to aphids are to spray them off with water or manually squishing them. But my advice to my neighbor was to just leave them alone, let the food web naturally form, the predators will come. Of course, it is very hard to watch a plant you have nurtured from a seed be damaged by an invasive aphid, sucking all of its life fluids. But patience can bring exciting results.


A few days later we were having coffee on her patio with the attacked/suffering milkweed standing nearby. The aphids were still present in force but there were also a few black and orange striped lady beetle nymphs, Coccinellidae, systematically piercing aphids to suck them dry and leaving a path of dead aphids behind. On closer inspection we found golden eggs clustered under the leaves in tight formation where the lady beetles had emerged. More pale eggs nearby were suspended on threads, these are the eggs of the aphid lions, Chrysoperla sp. Born so ferocious that their parents must separate them at birth on their own threads to make sure that they don’t eat their siblings. Aphid lions look like little alligators snapping up aphids in their tiny but strong jaws and mature into delicate green lacewings. We also find flower or syrphid fly’s youngsters, Syrphidae, which are white, pointy-ended maggots that parasitize aphids causing them to turn brown. When the maggots become adults, they are important pollinators of the plant they just protected from aphids. And, jackpot, under a leaf is the tiny crystal meringue of a monarch egg.


Now that the predators have arrived on site the milkweed plant is feeling some relief from the aphids. But now our monarch babies may also be in danger as predators do not know to avoid the endangered species when they are feeding. Monarch caterpillars concentrate the cardiac glycosides from the milkweed plant in their bodies and even retain this deadly chemical in their adult butterfly form which makes them distasteful to predators. Many of the species dependent on milkweed as their host plant have learned to work around or even co-opt the chemicals the milkweed plants have developed to protect themselves from predators.


Soon the monarch caterpillars hatch. They are easy to spot by the damaged leaves. Flip one over and a tiny striped monarch baby will be busily munching underneath. The caterpillars seem to double in size overnight and we must check on them daily and marvel at their growth. The adult butterflies are also seen visiting the garden more often, laying even more eggs. My neighbor is relieved that the butterflies have found her garden a suitable place to start a family. The life of a butterfly is not easy and mortality is high out in the wild even when you are a monarch.


As the food web develops around the milkweed plant, larger predators arrive. Flower spiders, Misumena sp., stand guard on the flower head to nab unwary visitors, changing their color to match the flower for camouflage and ambush their prey. Praying mantises, Stagomantis Carolina, dangle from the stem grabbing bugs and beetles as they are distracted by the aphid harvest. Visiting pollinators may also be captured and although it is all part of the food web, it can be hard to watch a butterfly being eaten and its torn wings drift slowly to the ground. I find it amusing, in a morbidly necessary way, that the aphids pierce the plant to suck out the sap, and in turn the aphids are pierced by their predators, which suck out the aphids’ body fluids. The whole system is like one complicated straw, sucking nutrients upwards out of the air and soil from plant to insect, until a bird comes along and plucks an unwary predator and stuffs it into a chick’s mouth.


Who needs TV when you can sit near a swamp milkweed and watch battles rage from stem to stem, birth and death, eggs becoming nymphs becoming adults, so many different species doing what they do best. Plants are eaten by aphids, then eaten by beetles, who are eaten by mantes, in turn then eaten by birds, who then disperse to far corners and eventually be eaten themselves. Eggs are carefully laid on a selected host and hatch and eat and grow. The lucky few that avoid predation become adults to start the cycle over again. We could (and do) sit here for hours, fascinated by the drama in front of our eyes. This milkweed was planted to attract monarchs, but it has summoned an entire ecosystem.

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