Milkweeds have this white, sticky sap filled with an unpleasant tasting chemical. They are listed as toxic to livestock. Milkweed Tussock Moth caterpillars don’t care.
The bigger Missouri milkweeds (Common, Purple, Swamp, Butterfly Weed) are eaten by various livestock including cattle and goats with no ill effects. Deer eat them too.
A famous consumer of milkweeds is the Monarch Butterfly caterpillar. The decline in Monarch populations has led to a push to plant milkweeds even as state and local road crews continue to mow them down in ditches statewide.
Several colonies of Common Milkweed, Asclepias syriaca, grow around our house and barn. These plants do put out root runners and shoots forming colonies of a few to a forest of stalks.
The Milkweed Tussock Moth population has discovered them.
Adult moths have bland, tannish wings spreading one to two inches across. Their abdomens are yellowish with a line of black spots down the center back. These are easy to overlook unless you are a chicken.
The caterpillars are not easy to overlook for a couple of reasons.
First the caterpillars are covered with stiff tufts of yellow, black and white hairs. The tufts on the head and tail ends are long. The ones in the middle are short.
Second is the sheer numbers of caterpillars. Larger ones do tend to be more solitary. Smaller ones form armies thirty to fifty strong.
One such army attacked a Common Milkweed stalk about three feet tall, lined with large leaves. The leaves vanished quickly. The horde then surrounded the top of the stalk and began consuming it until only a few inches was left.
The army moved onto a neighboring stalk. By the next day the Milkweed Tussock armies had marched on to other places. Scattered caterpillars still worked on a few plants.
The Monarchs will be moving through again heading south in another month or so. Then the remaining leaves will vanish down the gullets of Monarch Butterfly caterpillars.