Appreciating Tent Caterpillars

A large conglomeration of Eastern Tent Caterpillars on a Black Cherry host tree.

Spring is in full swing, and with it are familiar signs of the season. One of the most drastic this year is the emergence of Eastern Tent Caterpillars. Have you noticed silky tents in cherry trees? These are the work of the tent caterpillars. Eastern Tent Caterpillars are a native species to Nantucket, with a unique life history.

Unfortunately, they are often misidentified or misunderstood. Unlike the Spongy Moth, a non-native, invasive species that wreaks havoc off-island but is not present on Nantucket, the Eastern Tent Caterpillars play a significant role in our local ecosystem. They lay their eggs on Black Cherry Trees (or occasionally Beach Plums), and when they emerge as caterpillars, they will eat the leaves of, and even defoliate, their host trees. However, the leaves almost always grow back, and the trees usually do not suffer any long-term damage.

Some years, there are huge populations of tent caterpillars, and in other years, very few. In ecology, this is known as a boom-and-bust cycle. The reason that you are seeing so many more this year is that we have just resumed the ‘boom’ part of the cycle. The last big boom was in 2015, and on average, the cycle is about 10 years long, so the timing is right for this to be a boom year.

Despite their temporary impacts on their host trees, Eastern Tent Caterpillars are an important part of local food webs. They serve as a nutritious prey source for a wide variety of birds, but are favored by cuckoos. Although both Yellow-billed and Black-billed Cuckoos can be found on the island, in most years, they are rare. I am hoping that the large abundance of caterpillars attracts cuckoos, which would be a delightful observation for birders and nature enthusiasts.

Eastern Tent Caterpillars also have a fascinating social system. Egg masses are laid in the summer, which then overwinter, and the caterpillars do not emerge until the following spring. Each egg mass usually contains around 150-200 eggs. When the caterpillars emerge, they begin to build silk tents, which are used as shelter from the elements and an aid for thermoregulation. The caterpillars live communally in the tents and engage in collaborative behavior. One such example is the trail markers they leave to signify sources of food. To forage, the caterpillars leave their tent and travel up the tree branches in search of fresh leaves. As they travel, they spin a linear trail of silk along branches that lead to good sources of food; this signifies to other caterpillars where to forage. When all the leaves along a particular branch are defoliated, the caterpillars snip the silk trail off as a sign that they need to forage elsewhere.

An Eastern Tent Caterpillar egg mass.

My personal favorite aspect of Eastern Tent Caterpillars is their predator defense mechanism. When startled, they all sway back and forth in a herky-jerky fashion to confuse and disorient whatever potentially wants to eat them. The kinetic energy of the scene is truly amazing to watch.

Beyond their life history, Eastern Tent Caterpillars can be an important indicator of climate change impacts. Their emergence each spring is temperature dependent, rather than reliant on other parameters such as day length, calendar date, or light levels. Tracking the emergence date of caterpillars each spring can help us monitor the impacts of earlier arriving warm temperatures. There is a potential for the caterpillar hatch date to be earlier than the leaf-out date of their host plants. If so, the young caterpillars will have no food when they emerge and may starve, which is known as an ecological mismatch. As warm spring temperatures advance earlier and earlier, caterpillar populations may be at risk.

A full-size Eastern Tent Caterpillar out trying to find an appropriate spot to make its cocoon.

I hope this article changed your mind about Eastern Tent Caterpillars, and they will now be something you appreciate! If not, in just about a month, they will be metamorphosing into moths, which only live for around five days. Enjoy the rest of the spring season on-island and keep an eye out for these caterpillars.