If you were to spot the ailanthus webworm moth (Atteva aurea) crawling on goldenrod, you might think it is an orange-and-white beetle. It keeps its wings folded tightly around its long body, so it does not have a typical moth profile; however, it is a moth and it does fly.
As its name indicates, the host plant for its caterpillars is the ailanthus tree, also known as the tree of heaven because of how quickly it grows skyward. Ailanthus trees (Ailanthus altissima) are invasive but popular in cities because they are extremely hardy and survive in conditions that are inhospitable to most other trees. It is the tree that grew out of a crack in the sidewalk in the novel “A Tree Grows in Brookline.”
Ailanthus trees are native to Asia, but around 1784 a gardener introduced the tree to woodlands on his estate in West Philadelphia. In the 1820s, New York City planted them along many urban streets. By the mid-1800s, the tree had spread to southern Florida, part of the native range of this moth. The moth’s native host tree is the paradise tree, which grows from Central America to Florida. The moth caterpillars feast on it leaves. It just so happens that ailanthus trees are members of the same plant family as paradise trees, so as ailanthus trees became prevalent in the south, the moths jumped onboard. Today ailanthus webworm moths can be found wherever ailanthus trees grow – which is basically everywhere from Florida to southern Canada.
One ailanthus tree can produce 325,000 seeds per year, and these seeds have a very high germination rate, so these invasive trees have spread throughout much of the United States and southern Canada, including right here in the Northeast. Besides hosting the ailanthus webworm moth, this tree also harbors the invasive spotted lantern fly.
Today garden supply houses still sell these trees to people who want fast-growing, pollution-tolerant trees that thrive in poor soil. They reproduce uncontrollably, grow five feet per year, are difficult to kill, crowd out native trees and release chemicals that suppress the growth of nearby plants. (The result of decomposing leaves releasing toxic compounds.)
The leaves of this tree release a foul odor that some say smells like rotten peanut butter. White-tailed deer don’t like it any more: They will eat many types of leaves, but not the leaves of an ailanthus tree.
It attracts pollinating insects, though.
Ailanthus webworm caterpillars make webby nests that enclose the leaves of ailanthus trees. Inside the web, the caterpillars can grow to an inch or more in length. It is difficult for predators to penetrate the network of threads to eat the caterpillars when they are inside the web. During the day, the caterpillars rest motionless inside. At night, they leave it to feed on ailanthus leaves. The web encloses eggs, caterpillars and pupae. Throughout the summer, eggs progress to adult moths in as little as a month.
For many years, scientists observed this moth but had never seen the caterpillars and did not know on what plant the caterpillars fed. Then in 1911, an entomologist named Carl Ilg submitted this observation to a scientific journal:
It was at the later part of August when I was out collecting, that my attention was called to a web which looked to me like a spider’s nest, on a small ailanthus bush. By investigating more closely, I saw a chrysalis suspended in the web. Not knowing what it was, I took it home, and several days after, a small moth emerged and proved to be Atteva aurea. As I knew the food plant now, I looked in the same neighborhood and found several similar webs containing newly hatched, as well as full grown, larvae and also chrysalids in them. The full-grown larva is about 1¼ inches long, blackish, with a distinct brown stripe all along its back, while the sides are dotted with fine white spots.
This moth’s scientific name is Atteva aurea. Aurea is Latin for “golden,” which seems appropriate given its orangish coloring. Ilg’s discovery of the host plant led to its common name: ailanthus webworm moth.
Ailanthus webworm moths, being tropical, cannot survive our cold winter temperatures. When our weather freezes, the moths die. Nests fall out of the trees as the leaves they are attached to fall.
Moths to our south start moving north early in the spring. Entomologist Teá Montagna of the University of Massachusetts at Boston has spotted the adult moths in Massachusetts from March through November. Ailanthus trees are not yet blooming in March, though. According to Montagna, the earliest moth arrivals probably die. But the moths keep moving north relentlessly until eventually some arrive when conditions are favorable for breeding.
This moth is different from most other: It visits flowers during the day and is brightly colored – often a warning to predators that a species has some type of protection. But this moth does not sting or bite. It does not have sharp spines. Its protection is most likely chemical. Its orange markings are a warning: Don’t eat me. I may be toxic or distasteful! Because the ailanthus tree releases toxins that kill nearby plants, it is not unreasonable to expect that insects who eat its leaves may become toxic or unpalatable to predators.
Although people introduced ailanthus trees to the United States in the 1700s and 1800s, scientists have found ailanthus fossils from about 38 million years ago in North America. Why ailanthus trees disappeared is unknown. Because ailanthus webworm moths have so rapidly recolonized North America, it is possible that these moths are just reclaiming lands that their ancestors once thrived in.
Seen nearby
Doug Ferrell spotted this praying mantis in Carson City, Nevada, in August and named it Dewie.
Have you taken photos of our urban wild things? Send your images to Cambridge Day and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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