What’s the milkweed-monarch connection?

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monarch butterfly
(Farm and Dairy file photo)

Of all the animal on the planet, insects are among the most feared and reviled. Some sting (wasps, and bees) and some bite (mosquitoes, horse flies and blackflies).

Others are agricultural pests or just annoying (gnats) or disgusting (cockroaches, fleas and lice). But many insects, such as pollinators, are beneficial. And fortunately, the vast majority of insects are benign.

Several insects, however, have worked their way into our popular culture. Honeybees, praying mantises and fireflies, for example, entertain kids of all ages.

The rock stars

Monarch butterflies, however, are the rock stars of the insect world.

School children raise them in the classroom. Citizen scientists study them. And professional entomologists monitor their epic long distance migrations. Monarchs have become so well known that the success or failure of their annual migration to the mountains of central Mexico gets attention from major news outlets.

They’re so familiar that many people know their lives are tied to milkweed plants. Female monarchs lay their eggs only on milkweed leaves; monarch caterpillars grow and survive only eating milkweed leaves.

In so doing, they ingest and incorporate into their body tissues chemicals called cardiac glycosides that are found in milkweed tissues.

Complex relationship

These chemicals are toxic to most vertebrates so monarch caterpillars and adults suffer little predation from birds, mammals, reptiles and amphibians. The milkweed-monarch relationship is a fascinating one, but like most ecological relationships, it’s more complex than it seems.

Vertebrate predators may recoil and actually vomit if they eat a monarch, but many other mortality factors affect these beautiful orange and black butterflies. A patch of milkweed includes an impressive array of other invertebrate species.

Many live on the milkweed plants and have no direct impact on monarchs, but their presence attracts a variety of invertebrate predators.

Other milkweed insects

For example, aphids are common on milkweeds, and they attract predators such as ladybug beetles, lacewings, hover flies, and a variety of tiny parasitic wasps. Some aphids secrete a sweet substance called honeydew, which attracts ants.

The ants, in return, protect the aphids from predators. Other insects that eat milkweed tissue include milkweed longhorn beetles, planthoppers, leaf beetles, milkweed weevils, and tussock moth caterpillars.

Though many milkweed-eating insects are small and inconspicuous, tussock moth caterpillars are covered by bright multicolored fuzzy tufts. When disturbed, they roll into a ball and fall to the ground. When abundant, tussock moth caterpillars can completely defoliate milkweed plants.

Other conspicuous insects that eat milkweed plant parts include soldier beetles and large and small milkweed bugs. After milkweed seedpods mature, these brightly colored black and orange true bugs are easy to spot. Sometimes hundreds of these bugs gather on individual milkweed plants.

Large milkweed bugs are about one-half inch long and have red or orange wings marked by a black band across the middle. Small milkweed bugs are similar, but their black wings are marked by a red “X.”

Assassin bugs live among flower heads and use their long piercing/sucking mouthparts to capture and subdue prey. Monarch chrysalids are among their victims.

Predators

Parasites and predators that actively pursue monarch caterpillars and chrysalids include tachinid flies and paper wasps. Tachinid flies resemble houseflies, but are larger and hairier.

They lay their eggs on the outside of monarch caterpillars. When the larva hatch, they burrow inside the caterpillar. As the tachinid fly matures, the caterpillar dies. When a tachinid fly leaves the dead caterpillar or chrysalis, it leaves behind telltale white threads hanging from the body.

Paper wasps, which often build their nest under porch roofs and eaves, actively hunt and kill a variety of caterpillars, including monarchs.

Adults drink flower nectar and sip juices from rooting fruit, but they feed their larvae protein-rich pre-chewed insects. Caterpillars are among their favorite prey.

They chew the caterpillars into small pieces, which they then deposit into the cells of their nests. Larval wasps eat the caterpillar pieces as they mature.

Though the lives of milkweeds and monarchs are ecologically intertwined, the relationship is far from exclusive. An entire community of invertebrates can be found in every milkweed patch.

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Scott Shalaway, who holds a Ph.D. in wildlife ecology from Michigan State University, writes from his home in rural West Virginia. A former faculty member at Oklahoma State University and the University of Oklahoma Biological Station, he has been writing a weekly nature column for newspapers and freelancing for magazines since 1986. He can be heard on Birds & Nature from 3-4 p.m. Sunday afternoons on 620 KHB Radio, Pittsburgh, or live online anywhere at www.khbradio.com, or on the Tune-In radio app. Visit his website at www.drshalaway.com or contact him directly at sshalaway@aol.com or 2222 Fish Ridge Road, Cameron, WV 26033.

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