Ailanthus altissima, a Problem Land Managers Can Eliminate
Ailanthus altissima is one of many non-native invasive plants that plague land managers in Pennsylvania. In our area, it is the most common non-native invasive tree. Its strengths are It is clonal, a mature female produces a huge number of samaras each year which can spread across landscape, is allelopathic, outcompetes other plants for sunlight by its very rapid growth rate and can “drown” out other plants by forming dense clonal stands from a few parent trees.
With all its strengths Ailanthus has several important weaknesses which can be exploited for its control. The first and foremost is that it is dioecious, having separate male and female trees. The second and possibly greatest weakness is that it is non-native with several relatives in North America. The third weakness is that it tends to form dense clonal stands with no barriers to stop the spread of disease or pests. The fourth weak point is that if has a limited vascular system. Together, these weaknesses present opportunities for land managers.
Dioeciousness allows the focus to be on half of the mature trees in an area. While male trees are important, when doing landscape triage, female trees should be the first target. If a female tree can be killed or sterilized before the seeds mature, the spread of this plant can be contained. Once the female trees are controlled, then the male trees and the immature seedlings and saplings can be eliminated.
Ailanthus altissima is not native to the Americas. However, many family members are. This allows biocontrol biologists to survey native Simbouracae family members for insects and pathogens that can be used as safe native biocontrols. I have seen many dying and dead stands throughout the mid-Atlantic area. There are at least three pathogens and two insects which are actively killing or injuring Ailanthus. My own research has focused on the insects Atteva punctella (native) and Aculops ailanthi with the phytopathogen Fusarium oxysporum in a complex web of interrelationships. Penn State is focusing on the phytopathogen Verticillium albo-atrum and is trying to understand the mechanism of its spread.
Dense interconnected stands of a plant with limited genetic variability opens that stand to its extinction. The limited genetic variability or heterogeneity makes Ailanthus an easy target with limited ability to fight off phytophagous insects or diseases. Once a phytophagous insect or a disease is found in a stand or an area with many stands, the insect will be able to spread from stand to stand and the disease has a strong possibility to do the same. If a phytophagous insect of Ailanthus occurs together a pathogen of Ailanthus, then the pair can act synergistically to control Ailanthus. The herbivorous insect can transport the disease between stands. The disease can make the plant easier food by weakening its defenses. If more than one pathogen and phytophagous insect occur together, the possibilities are even greater.
The limited vascular system of Ailanthus is due to its swift growth. My field research has shown that apparently there is a limited amount of horizontal structure in the vascular cambium with few interconnections between columns of xylem or phloem. This means that herbicides or pathogens will spread swiftly to the roots or stems and leaves from the site of introduction. It also means that once a vascular column is killed, the tree will have a hard time repairing the damaged tissues. Remembering that a tree is basically a water pipe between the roots and leaves, damaging the vascular cambium will block the movement of water and other nutrients throughout the tree in the same way a damaged water main prevents the flow of water in a city. The downside is that it is harder for a pathogen or herbicide to reach all parts of the tree.
The practical application of the weaknesses of Ailanthus is that it is controllable in defined landscapes. I have been developing a method I call “Drill and Fill” so that Ailanthus trees can be efficiently killed by volunteers in parks, arboretums and environmentally sensitive areas with limited human risk and environmental damage. I drill 3/8” holes through the vascular cambium, 1½” to 2” deep, about every 2” around the girth of trees with a circumference of 8” or more. Glyphosate concentration needs to be as high as possible and always at least 40%. I have been using a concentration of 50.2%. Several mL of dish soap can be added to a liter to act as a surfactant to help the glyphosate spread throughout the tree. For branches that are not part of the crown holes need to be drilled directly under them to kill them. Hollow trees can be a special problem of their own as the drill bit will often pass through the vascular cambium into the hollow space allowing the glyphosate to do the same.
One precaution here is that this application needs to be done with the purpose of killing in one application. Apparently, if a tree is not killed in the first application, resistance develops and new holes need to be drilled as the older holes are blocked off by the tree. If this happens, make sure the new holes are offset from the original holes and a much stronger concentration of glyphosate is used.
To this point, I have not focused on the immature clones, seedlings or saplings. My guess is that if the Ailanthus web worm Atteva punctella or the eriophyoid mite Aculops ailanthi are present in the stand or immediate area, these smaller trees will be taken care of. To enhance the population size of Atteva punctella, planting patches of native nectar species nearby such as various species of goldenrod will give the adults a reliable food source and should enhance their fecundity. The goal should be a reliable food source from early May through frost. Near Hawk Mountain in SGL 110 there is a patch of Canadian goldenrod, Solidago canadensis, with a dying Ailanthus stand surrounding it. Adult Atteva punctella have been seen feeding on the goldenrod. Their larvae have been on the trees from seedling to adult size. In close association is the eriophyoid mite Aculops ailanthi. At the same time, the adult trees appear to be dying of disease. This is most probably a Verticillium or Fusarium wilt. My guess is that the mites are being spread by wind, Atteva punctella and possibly birds. The pathogen is most probably being spread in one of two ways. The wind may be spreading the pathogen and it is infecting through the (feeding) wounds caused by Atteva punctella. Or the pathogen is in the gut and/or on the bodies of the web worm and the mite entering the tree though feeding wounds or injected while feeding.
The one action that no one should ever do is cut down or heavily prune Ailanthus trees. This causes a defense mechanism to start producing clones. Once started there will be an almost infinite number of clones from the stumps at distances of up to 30 meters from the parent tree. I have seen the resulting dense stands of several hundred clones in many places, including along our highways.
In summary, Ailanthus altissima is not an insurmountable problem. Chemical and partial biocontrol when used together should be able to control this weed tree in defined areas with some, but not an excessive amount of volunteer time. My experience working alone with a drill, 2 battery packs and a spray bottle of concentrated glyphosate I was able to inject up to 30 adult trees from 8” girth to over 50” girth with the charge of both battery packs, about 2 hours.