Native moth feasts on invasive tree

image adult ailanthus webworm
Adult ailanthus webworm looks up from a 
leaflet of tree of heaven.

ailanthus webworm larvae photos
Ailanthus webworm caterpillars defoliate a
sapling.

image all sprouts defoliated
Every ailanthus sprout in this part of the arboretum understory has been defoliated.

Large trees seem little affected.
Unfortunately, large trees seem little affected.
(All photos by T'ai Roulston).
You may have first mistook the tree of heaven (Ailanthus altissima) for black walnut. Its leaves are similar. But then you noticed the bark was different and the leaves smelled awful. You also couldn't get rid of it. Each time you lopped the trunk, it sprouted multifold, Hydra-like, and spread throughout the yard. Hard to believe it was introduced intentionally to North America as an ornamental. It now occurs in 44 of the 50 U.S. states, predominately as an urban, suburban and general roadside weed, but sometimes invades forests as well.

One North American is happy to see it
. While most of our human citizens rue the day it came, one native moth, Atteva aurea, thrives on it. The moth, now known as ailanthus webworm, occurred no further north than southern Florida, the northern limit of its host, paradise tree. Then came tree of heaven, a plant in the same family as paradise tree. The moth was attracted to it,  consumed it, and grew well. Following its new host, the moth spread up the eastern states all the way to Canada. 

The ailanthus webworm builds communal nests within host leaves, which are tied together like a tent. As the larvae grow, they defoliate the branch on which they feed. The older caterpillars eat the bark and twigs as well. Seedlings and young sprouts can be left leafless, with young branches withering and dropping off. At the State Arboretum of Virginia in 2012, whole young populations of the tree were left defoliated from mid to late summer (see photo on left). It is a very satisfying sight. If you have ever battled this tree, you think "I will protect this moth from birds. I will hatch armies of them in my basement and help it kill every tree of heaven in the world!"

At this time, the possible impact of ailanthus webworm on tree of heaven has received little study. A review paper in 2006 by Ding et al. considered the moth to show some promise as a potential biological control organism where its populations are high enough and where other natural enemies of tree of heaven also exist. Alas, the older trees do not seem greatly affected. Still, if this moth can slow the spread of seedlings and reduce the success of resprouts, it might help. The moth might get better at it over time as well --it has only been feeding on this tree for a couple hundred years, at most. If anyone finds a population of moth that destroys adult trees of heaven, let me know. My basement is available and I have friends.