Dr. Lisa Tewksbury describes ‘biocontrol’ as “putting the food web back together.” As Director of the University of Rhode Island Biocontrol Laboratory, Tewksbury and her staff are constantly seeking organisms, most often insects, that prey on invasive plants and pests. Why? When species are moved from their native homeland, their natural checks and balances are left behind and they are then free to thrive, often flourishing too well and reproducing so vigorously that they crowd out native organisms.
Take black swallow-wort (Vincetoxicum nigrum), for example. Deliberately introduced to North America in the mid-19th century as an ornamental and medicinal plant, the non-native species soon escaped from cultivation, overrunning hedgerows, brushy areas, woodlands, and riverbanks throughout the northeast and parts of the Midwest and California. It has proven to be a persistent weed in farm fields — it is toxic to grazing animals — and gardens as well. In addition to overwhelming and displacing the indigenous vegetation, it also poses a threat to monarch butterflies. Not only does the black swallow-wort crowd out native milkweeds on which the monarch caterpillars feed, but as a relative of milkweed, it functions as a kind of trap. Black swallow-wort resembles milkweed, duping monarchs into mistakenly laying their eggs on it. The swallow-wort, however, contains toxins that the caterpillars cannot process and the caterpillars then perish.
In an effort to control black swallow-wart in North America, Dr. Tewksbury’s predecessor, Richard Casagrande, enlisted the help of then-Ph.D. student, Aaron Weed, who went to the black swallow-wort’s European homeland to find insects that fed upon the vine. Weed located and brought back to the United States five species of pests that the laboratory then began to evaluate not only for their impact on the invasive weed, but also for their potential impact on related native plants.
A species of moth from the Ukraine, Hypena opulenta, was identified as the most promising biocontrol agent, and a multi-year project of testing whether this insect would also attack native North American plants began. Ultimately, 79 different native North American plants, including 20 milkweeds, were exposed to the moth’s caterpillars, which demonstrated an appetite only for swallow-worts.
This research was shared with various organizations and federal agencies for scrutiny and posted for public review. Following the review, the United States Department of Agriculture approved the release of the moths into the wild in August of 2017 and by this past summer, their presence was demonstrating some success in controlling swallow-wort on test plots. Only time will tell, but the hope is that the moths will establish themselves in wild populations, helping to reduce the competitiveness of swallow-wort and perhaps limit or at least slow its spread.
The swallow-wort project is just one of many being conducted at the URI Biocontrol Laboratory. It has also helped to curb another garden pest brought over from Europe, the lily leaf beetle, through the introduction of parasitic wasps. After watching the beetles devour and kill the lilies in my Connecticut garden, I had given up on those beautiful garden plants. Now I can give them another try.
The advantage of this sort of biocontrol is that it can reduce the invader to manageable levels without the harmful, and expensive, use of toxic pesticides, but Dr. Tewksbury emphasizes that careful and deliberate research is essential to this process. Mistakes can be ecologically costly. For example, insects imported to control invasive, old world thistles, were subsequently found to attack native American species of thistles as well, and their use had to be discontinued. Balancing these hazards, though, are the considerable advantages of a properly conducted project of biocontrol. The URI Laboratory, for instance, is currently experimenting with moths that attack the invasive common reed (Phragmites australis) from Eurasia that now dominates huge areas of American wetlands. Once again, the concern is with the insect’s possible impact on a rarer, non-invasive American subspecies of this plant. Likewise, the URI Laboratory has begun to release wasps that parasitize the emerald ash borer, a non-native beetle that has devastated ash trees throughout most of their North American range.
In an era of a globalized world economy, biocontrol can provide an essential tool for combating the threats posed by invasive organisms.
Be-a-Better-Gardener is a community service of Berkshire Botanical Garden located in Stockbridge, Mass. Its mission to provide knowledge of gardening and the environment through a diverse range of classes and programs both informs and inspires thousands of students and visitors each year. Thomas Christopher is a volunteer at Berkshire Botanical Garden and is the author or co-author of more than a dozen books, including Nature into Art, The Gardens of Wave Hill. His companion broadcast to this column, Growing Greener, streams on WESUFM.org.