I grow milkweed in my garden for the migrating monarchs, but there are some fuzzy caterpillars that are black, orange and white devouring my milkweed. What are they and what should I do about them? MC
Monarch butterflies are often in the news. We hear about their dwindling population, and many people are planting a variety of plants to support them on their migration to and from Mexico. Milkweed is the sole source of nutrients for monarch caterpillars. However, what you are describing is another insect whose caterpillars also prefer milkweed: the milkweed tussock moth.
Female milkweed tussock moths lay their eggs in white masses on the underside of milkweed leaves starting in June. When the eggs hatch, they begin feeding and may go unnoticed for a while.
By their third instar (a phase during the developmental process), they become these unique and beautiful fuzzy caterpillars with tufts of black, orange and white. The adult moths are not nearly as stunning, but many of us have been told we were better looking when we were younger so…
If the female moth laid all her eggs in one spot, what starts as a kind of mob feeding thins out as the larger caterpillars spread out and move to other milkweed plants. At the point they are in full tufts mode, they tend to feed alone or in pairs.
Soon, they leave the milkweed plants to form a cocoon in which they pupate. Farther north, there is only one generation per year, but in our area, two generations per year are not unusual.
Bats are the primary predators of moths; however, the milkweed tussock moth tends to be immune from being fed upon by bats because they produce an ultrasonic click from what is called a tymbal organ. Bats recognize this sound and avoid them because they are not interested in a toxic meal. The moths are toxic (like monarchs) because their favorite food (milkweed) contains a poison called cardiac glycosides (cardenolides). Being toxic is a great way to get predators to leave you alone.
The caterpillars are pretty voracious eaters and can decimate your milkweed plants, so, if found, you have a decision to make. Because you said you grow milkweed for the monarchs, you might consider the milkweed tussock moth an unwelcome interloper. Or perhaps you can just embrace the idea that you were growing caterpillar food, which is serving its intended purpose, just not in the way you had planned.
If the live-and-let-live philosophy doesn’t work for you, you will want to remove the milkweed tussock moth larvae. Physical removal would be best as any chemicals you might use could work to the detriment of your monarch sanctuary.
As for me, the photo of the milkweed tussock moth seen here is from my garden. I didn’t enjoy seeing my milkweed disappear before any monarchs found it, but I decided to enjoy and appreciate the milkweed tussock moth’s beauty while hoping they leave something for the monarchs. Maybe you will, too.
You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at firstname.lastname@example.org.