Sunday, May 19, 2024

A red milkweed beetle eating a milkweed leaf. (Photo: Jeanine Farley)

As more people have become aware of the benefits of milkweed as a host plant for monarch butterflies, you will likely see more patches of it as you travel through the city. And if you look carefully, long before the monarchs arrive you may see a spotted, long red beetle with curled black antennae. This is a red milkweed beetle.

The red milkweed beetle (Tetraopes tetrophthalmus) is a long-horned beetle, sometimes called “four eyes” as its Latin name indicates, because each antenna bisects an eye, literally giving it four. There are different kinds of milkweed beetles, specializing in different types of milkweed; the red milkweed beetle prefers common milkweed (Asclepias syriaca). 

An adult eats milkweed leaves and flowers, cutting the veins of the leaf below where it eats and draining the sap from the area. The beetle feeds in the drained area, so it is less exposed to the sticky, milk-colored latex that gives milkweed its name. If a red milkweed beetle gets the latex on its mouth, it cleans itself by rubbing its mouth on the leaf, because if the latex hardens, it can gum up the beetle’s mouth.

Many species of insects try to camouflage themselves from predators, but red milkweed beetles stand out against the green leaves of milkweed plants. They can do this because milkweeds are toxic to many predators, which means milkweed beetles, as consumers of milkweed, are also toxic to many predators. This is the same strategy used by monarch butterflies, also consumers of milkweed.

The beetle is sometimes called “four eyes” because each antenna bisects an eye, literally giving it four. (Photo: Jeanine Farley)

If you were to pick up a red milkweed beetle, you might hear it make a shrill squeaking sound by rubbing together structures on the front and back of the thorax. It makes this sound when it is stuck in a milkweed blossom, is fighting, falls on its back or is in other sorts of distress. When it crawls or feeds, a red milkweed beetle may make a soft purring sound.

Life cycle

In early summer, the female beetle lays eggs at the base of a milkweed stem. The larvae/grubs travel down the stem to the soil, feeding on the milkweed’s roots until fall. They overwinter in the root and form a pupal case/chrysalis in the soil the following spring. Adult red milkweed beetles emerge in early summer when the milkweed begins to flower.

Red milkweed beetles and monarchs

Native long-horned beetles such as the red milkweed beetle have a valuable place in our ecosystem, helping to preserve the balance of nature. Red milkweed beetles, herbivores that eat only milkweed, are not harmful to monarchs or harm monarch eggs or larvae, so you do not need to remove them from your milkweed plants. The two species can and do coexist peacefully.

Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.