Guest Blog: Invasive species: getting by with a little help from their friends
by Robert Ray, PhD
No matter what your opinion of climate change, there is no doubt that humans have had a profound impact on the natural world – typically a negative one — wherever they have settled. The major impacts of human activity are of two types: the elimination of native species — the extinction of the Dodo and the Passenger Pigeon are celebrated examples — and, somewhat more commonly, the introduction of alien species. Some of the latter are deliberately introduced by humans, such as domesticated animals and food crops, while others are brought along passively as stowaways of one form or another.
Natural ecosystems are complicated networks that exist in a delicate balance, not one based on some utopian harmony between species, but rather, as Darwin pointed out, one of fierce competition between species competing for limited resources. As such, adding or removing components can have far-reaching effects on the system as a whole and dramatically alter the natural landscape. How severe these effects are depends on how the new species fits into the network. Specialist species — those that specific requirements to survive and reproduce — either die out or require constant maintenance when introduced to foreign habitats. Generalist species, on the other hand, are more adaptable and can survive and reproduce without human intervention. In the best (or worst, depending on how you look at it) case these species escape human cultivation and become ‘naturalized’ in their new environment.
Among trees, the most dramatic example of this process is Ailanthus altissima, the Tree of Heaven or simply ‘ailanthus’, which, with the help of humans, has colonized most of the temperate northern hemisphere and parts of the southern hemisphere as well. Ailanthus is native to China, where it is widespread, and it has been introduced at different times to Taiwan, Korea and Japan. The tree was introduced to Europe in the 1740s by the French Jesuit missionary Pierre Nicholas d’Incarville who sent seeds of the plant to his mentor, Bernard de Jussieu at the Jardin Royal des Plantes in Paris. d’Incarville claimed that the seeds came from the Chinese lacquer tree (Toxicodendron vernicifluum), which is a kind of sumac used in the Far East to make red and black lacquerware and this mis-identification led to much confusion over the botanical name in the late 18th century. This problem was resolved in 1785, when Rene Desfontaines, also of the Jardin Royal des Plantes, having seen the winged fruit of the tree planted by de Jussieu, realized that it was not a sumac. Desfontaines placed the tree in the genus Ailanthus, the name of which is derived from the Moluccan work “ailanto” meaning “a tree of heaven.” This is how the tree has come to be known as the “tree of heaven.”
European interest in the tree was driven by a number of factors. Because of its ornamental foliage, rapid growth, and charming exotic appearance, the tree was favored in exclusive gardens, parks, and estates throughout Europe. When carefully tended, the trees have straight tall trunks with a broad, delicate, weeping crown of compound leaves. The small yellow-green flowers appear in mid-summer in dense panicles at the ends of new shoots, with individual trees producing either male flowers or female flowers, with the panicles of the male trees being larger than those of the female. After pollination, the female flowers develop into from 1 to 5 winged fruits, or samaras that are similar to the fruits of ash trees. These samaras mature to either a greenish-yellow or reddish-brown color, depending on the variety, and are a further attractive characteristic.
There was also an economic driver to widespread planting of the ailanthus. The tree is, by coincidence, the host tree for the Ailanthus silkworm (Samia cynthia), which had been bred in China for silk production alongside the more common mulberry silkworm (Bombyx mori). In the 18th and 19th centuries, attempts were made in Southern Europe and the United States to gain independence from the Chinese domination of the silk trade by establishing colonies of the Ailanthus silkworm in places where the tree of heaven could grow. These enterprises largely failed with the only enduring result being the planting of more ailanthus trees and the naturalization of the Ailanthus silkworm in both Europe and North America.
The tree arrived in the United States from England in 1784 when William Hamilton introduced it onto his estate, The Woodlands, on the west bank of the Schuylkill River near Philadelphia. The tree’s lush appearance and its ability to survive in poor soils with little care made it an attractive shade tree for American gardens. By 1820, the tree was praised for its “power to adapt to the dirt and smoke, the dust and drought of cities,” which led to its being extensively planted as a street tree in industrial centers such as New York, Baltimore and Boston. By the 1840s it was common stock in nurseries along the eastern seaboard. Apart from its use as an ornamental, its ability to thrive in poor soils led to its use as a pioneer species in afforestation projects in several US states.
Despite all its attractive characteristics, there’s a downside to Ailanthus: it was a generalist species, par excellence. Female trees produce an abundance of winged seeds, allowing them to disperse widely from the parent tree and thus spread if left unchecked. To make matters worse, the male trees produce an unpleasant odor when flowering which led to a preference for planting the female trees, which produced even more seeds. Furthermore, the ability for the seeds to germinate and grow in practically any soil allowed ailanthus to colonize and establish itself in disturbed and wasteland sites where native trees could not easily make a foothold. With these characteristics driving it forward, it is not surprise that by 1888, ailanthus had escaped cultivation in Virginia and the surrounding states, spreading along rail lines and road beds where few other trees could survive. By the mid-20th century, the tree could be found throughout the continental United States and southern Canada, leading some botanists to quip that it is by far the most successful Chinese import of all time. To give a sense of the tree’s local success, a 1995 survey of interstate highways in southwest Virginia found that 30% of the mileage along the highways was infested with ailanthus.
The realization of the tree’s invasive tendencies led to the discovery of another even less attractive characteristic: its robust ability to sprout adventitiously from its shallow root system. In the wild, ailanthus is a relatively short-lived tree (30 to 50 years), but when the parent tree dies, saplings sprout from the rootstock to produce a dense colony that can continue to grow and spread indefinitely. Unfortunately, this same effect is achieved by trying to remove unwanted trees by cutting them down. Once ailanthus was deemed to be invasive, early attempts at controlling its spread focused on cutting the trees back, which only made the problem worse since the tree would sprout from the rootstock to produce an even denser colony. Like the broom in Göthe’s The Sorcerer’s Apprentice, hacking back one ailanthus tree only made it return with a vengeance. Studies have shown that the only effective way to eliminate the tree is to cut it down and then apply herbicide treatments to the stump that will completely kill the root system.
The latest chapter in the ailanthus story in the United States is related to the recent arrival of the Spotted Lanternfly (Lycorma delicatula). The Spotted Lanternfly is native to northern China, where records of the species date back to the 1100s. In recent years the species has appeared in Korea (in 2006), where is has become an invasive agricultural pest, Japan, and the United States (in 2014). Despite of the common name, the insect is not a fly, but rather a planthopper related to leaf hoppers and cicadas. As such, it develops through a series of juvenile nymph stages prior to emerging as a fertile adult (as opposed to true flies which pass through larval and pupal stages to become adults). Spotted lanternfly nymphs feed on a wide variety of host plants including fruit trees, ornamental trees, woody trees and vines, and thus threaten primarily the grape and fruit tree industries. Studies in Korea and the US have shown that the adults have a strong preference for ailanthus trees, which they use for feeding, mating, chemical sequestration and depositing egg masses. This preference makes the tree of heaven a target that should be monitored in order to control potential Lanternfly infestations.
The Penn State Extension and the USDA have provided guidelines for monitoring and controlling the spread of the Lanternfly in the US. Since ailanthus is a preferred host, the recommendation is for removal of most – but importantly not all – of the ailanthus trees. In particular, authorities recommend removing female trees and killing the rootstock with herbicide treatments, then leaving the male trees as ‘sentinel plants’ for monitoring the movement of the species. Adhesive paper bands can be placed on high-risk host trees to capture the Lanternflies as they ascend and congregate on the trunk. As of 2017, the species has only been detected in a few counties surrounding Berks County in eastern Pennsylvania, reflecting the behavior of the insect to move by walking or hopping rather than long-distance flight. Given this, it is believed that the colonization of new areas is achieved by people moving objects such as rocks, wood or furniture, which can also be used for egg laying. It is very important to keep an eye out for this species, and if you do see it to photograph, or, if possible to collect the insect and report the sighting to the authorities for verification.