Whoa! Look at this critter hanging out on Asclepias curassvica ‘Silky Mix’ milkweed, an annual milkweed I’m growing in planters. For perspective, the leaves in the photo are about 5.5″ long and 1.5″ wide at the longest point. While new-to-me, Milkweed Longhorn Beetle or Red Milkweed Beetle (Tetraopes femoratus) is relatively common and found throughout Colorado.
All milkweed longhorns feed and develop only on milkweeds (Asclepias spp), some feed only on one species, while others are not as particular. This visitor showed no interest in the nearby native milkweed, Asclepias tuberosa.
It hung out on a stem for several days, did little obvious damage to the foliage or flowers and moved only slightly. They can often be seen mating on the plant, although I only saw a solitary beetle.
The female milkweed beetle lays eggs at the root crown. The larvae, called roundheaded borers, will tunnel into the root system and later emerge as adults which live about a month. According to Dr. Whitney Cranshaw of Colorado State University, longhorn beetles rarely cause serious damage to plants.
Milkweed longhorns make a squeaking sound, especially if held (I don’t know this from experience). It is believed this is a warning noise. “Purring” has also been reported as a mating call.
The distinctive color is a result of feeding on the alkaloid toxins which are contained in the milkweed sap. This is the same defensive toxin found in monarch butterflies. The flashy color screams “danger” to predators.
If you are growing milkweed, keep your eye out for these colorful, over-sized beetles.
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Text and photo by Linda McDonnell, Denver County Master Gardener