This year I planted a bunch of milkweed for the migrating monarch butterflies, but there’s some kind of fuzzy orange, black, and white caterpillar eating my milkweed. What should I do? PR

Good job in planting the milkweed for the monarch butterflies. Milkweed is the primary host plant for young monarchs and female monarch butterflies will search to find milkweed on which to lay their eggs. Knowing that these plants are there for monarch food, many gardeners plant their milkweed in a special spot since it will get eaten down to the stems by the young monarch caterpillars. But I can see how you might be a little protective of your milkweed since these are not the intended beneficiary of your efforts.

The insect you are asking about is the milkweed tussock moth. But first a word about the primary insect concern with milkweed - aphids. Aphids can do some damage on your milkweed but there are a couple of simple strategies you can use to fend off the aphid hoards.

The easiest thing to do when you notice aphids on your milkweed is to provide them a quick exit path from your plant by spraying your milkweed down with a strong stream of water. Aphids are pretty fragile and if the stream of water doesn’t quicken their demise, they won’t be able to make their way back to your plant after suffering through a strong dose of appropriately directed water.

If the stream of water didn’t do the trick, you can soak them with insecticidal soap. This may take a couple of applications, but aphids and insecticidal soap are not friends so you should see the aphid population diminishing quickly. However, be sure to spray early in the morning or late in the evening so you can avoid any collateral damage to beneficial pollinators. Now back to your question.

Female milkweed tussock moths begin to deposit their eggs on milkweed plants in June. You probably won’t see them unless you are looking because they like to locate them on the underside of the leaves. Since the female can deposit dozens of eggs at one time, once you see those little caterpillars all huddled together, you’ll understand why these groupings are sometimes called a mob.

One of the primary arguments for letting nature take its course with a potentially destructive insect is that they are part of the food chain for other insects etc. However, milkweed tussock moths don’t fit into this category. One of the primary predators for moths are bats since moths and bats are both nocturnal. However, since milkweed tussock moths have ingested a poison found in milkweed called cardiac glycosides (cardenolides), they are not on the menu for bats. This poison it what makes monarch butterflies distasteful to birds as well. As a further defense, adult milkweed tussock moths emit an ultrasonic click that bats recognize, letting them know to avoid this potentially toxic meal.

Milkweed tussock moths go through 5 developmental stages called instars. By the 3^rd instar, they become these beautiful little fuzzy tufts of black, orange, and white. From the looks of the caterpillar, it would be easy to expect the adult moth to be beautiful, but like many of us, they peaked at an early age… in my opinion.

Once they start to feed, these caterpillars begin to venture out on their own to adjacent milkweed plants, ultimately tending to feed alone, or perhaps in pairs.

Leaf tissue is their primary food, but they will avoid the veins. They do this because the veins have a higher concentration of the white sap found in milkweed plants and this highly concentrated sap can actually glue them to the leaf. After eating the leaf down to the veins, the caterpillar will use the vein as a bridge to another leaf.

At the end of this eating frenzy, the caterpillar forms a cocoon in which they pupate into the adult moths. In the northern part of the country there is only one generation per year but two generations per year is not unusual in our area.

Now that you know what you are dealing with, you have a decision to make. Are you going to allow these small, fuzzy little caterpillars to eat the milkweed you were hoping would provide food for visiting monarchs? There are of course ways to eliminate this insect from your milkweed, but I tend to take a live and let live approach to insects, unless of course they are eating something I was hoping to eat, and then it gets a little trickier.

If this live and let live strategy doesn’t work for you, you can start by removing them by hand. This would be the safest strategy with the least potential for damaging beneficial insects. Or you can spray the leaves the caterpillars are eating with bacillus thuringiensis (Bt). Bt is an organic pesticide that is only effective on caterpillars, so it won’t impact visiting pollinators.

As far as I am concerned, the milkweed tussock moth is welcome in our landscape. I actually don’t mind them devouring the milkweed. Since I grew milkweed for insect food in the first place - mission accomplished. See you in the garden.

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 mg@tulsamastergardeners.org. Photo by Tom Ingram