Do tussock and monarch caterpillars fight over the milkweed?

Since we moved over five years ago, we have been encouraging the growth of milkweed in our garden.

I have friends who have problems with the aggressive growth of milkweed. We don't have this problem. We seem to have equally or even more aggressive growth of Monarda and various grasses that can easily hold their own against the milkweed.

But in recent years, milkweed has gained ground, and now we have some groups of maybe 30 plants where there used to be three. Like most people, we encourage the growth of milkweed for the sake of the endangered monarch butterflies.

Without meaning to, we have also promoted another, in my opinion equally charismatic caterpillar – the milkweed tiger moth (also called the milkweed tiger moth because of its black, orange and white stripes).

But I'm worried. This year, milkweed tussock moth caterpillars have eaten almost all of the leaves off the milkweed in our largest bed. We usually have an influx of monarch butterflies and caterpillars right now, and I want to make sure the caterpillars have enough to eat. Is this a tussock versus monarch caterpillar scenario? Should I remove the tussock caterpillars to save the beleaguered monarch butterflies?

The milkweed moth (Euchaetes egle) is native to this area. Its range in the U.S. extends from Maine to Minnesota and south to Florida and Texas. Two generations may occur per summer, so you may see the caterpillars in early summer and now. By definition, a tussock is a small area of ​​grass that is thicker or longer than the grass growing around it (Oxford Dictionary). And like a tussock, the milkweed caterpillar has orange “tufts” that run the length of its body and meet above the midline, as well as thick black, orange and white tufts around its head and tail end. Like the orange and black of the monarch butterfly, these colors aren't just for beauty. They're meant to warn potential predators that these animals contain toxic cardiac glycosides (which can cause potentially fatal cardiac arrhythmias) derived from their milkweed host.

A very endearing feature (in my opinion) of the milkweed moth caterpillars is that they are gregarious until the third larval stage.

Instars are developmental stages between molts (when the exoskeleton is shed to allow for growth). For example, when a caterpillar hatches from an egg, it is in its first instar. Because caterpillars are gregarious, they often congregate on a single milkweed plant and eat it bare before moving en masse to the next plant. According to my favorite source for all things caterpillar (David Wagner's “Caterpillars of Eastern North America”), “Some years, populations of milkweed clumps erupt (experience a population boom) and expose patches of milkweed.” I think that's what happened this year.