Monarchs and the Milkweed Advantage

Monarch butterflies are again active in Eliza Howell Park. Monarchs are among the most visible and common of the roughly two dozen butterfly species found in the park and one of the few butterfly species that migrate (they head to Mexico each Fall). They have returned. To be exact, it is not the same individuals that have returned (their lifespan is not that long). But Monarchs are back.

Photo by Margaret Weber

At the same time that I started seeing the first Monarchs of the year I began to notice that Common Milkweed, the wildflower that their life history is intimately connected with, have emerged and are growing rapidly.

Monarchs use milkweeds (mostly the Common Milkweed) as the exclusive host plant for their eggs and larvae. They will soon be beginning the process. Nature walkers will be checking the underside of milkweed leaves for the Monarch caterpillar, almost as easily identified as the adult butterfly.

Photo by Margaret Weber

Monarchs nectar on many different flowers (in the first picture above, it is on a butterfly bush), but its special relationship with milkweed in reproduction gives it what I think of as the “milkweed advantage.” There is a toxicity in all parts of the milkweed plant (ranchers/farmers are warned again letting their livestock graze it) and the Monarch acquires this toxicity from ingesting the leaves as caterpillars. The result is that adult Monarchs are not preyed upon by birds, who have come to know that Monarchs are not healthy food.

It is not a surprise that Monarchs like milkweed.

Photo by Margaret Weber

The Monarch is one of the best known and best liked butterflies in this part of the country and there has been a growing concern in recent years about the decline in their numbers. They continue in good numbers in Eliza Howell Park, perhaps in part due to the limited mowing that allows milkweeds to thrive. 

The milkweed flower is followed by the seed pods, which will ripen and open to silky seeds that are dispersed by wind. 

When I saw the first Monarchs last week, I immediately looked for milkweed. The two go together that closely. 

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Postscript: There is another insect that benefits from the toxic “milkweed advantage.” It is the brightly colored Milkweed Bug, found on the plants later in the year. That may be a story for another time.