In eastern North America a species of weed has become an aggressive invader. Garlic mustard (Alliaria petiolata) is one of the most rapidly increasing woodland invasive plant species, spreading across northeastern and midwestern USA and southeastern Canada at a rate of nearly 2,500 square miles per year.
The plant was most likely introduced to North America in the 19th century, taken from its native habitat of Eurasia by settlers for medicinal and culinary use. Although the crushed leaves and seeds of garlic mustard smell like cultivated garlic and have been used as flavouring in cooking for centuries, the plant actually belongs to the cabbage family.
As an allelopathic herb, garlic mustard releases compounds which inhibit the growth of other plants around, negatively affecting tree seedlings and mycorrhizal fungal* communities. In addition, it contains secondary compounds which act as a defence mechanism to reduce herbivory.
Native North American species suffering from garlic mustard’s proliferation, include the rare West Virginia white butterfly (Pierisvirginiensis). This species normally lays its eggs on native tooth wort (Dentaria spp.), but female butterflies are also attracted to laying on garlic mustard. However, due to the different chemical composition of garlic mustard, larvae are unable to develop on the plant, making it an ecological trap for this important butterfly species.
Garlic mustard is usually found in the undergrowth of disturbed woodlots and forest edges, recent findings have shown that it has the ability to establish and spread even in pristine areas. Garlic mustard is one of very few non-native plants able to successfully invade forest understories with low light levels.
The threat to native ecosystems and lack of competition or natural enemies has made controlling garlic mustard a priority for land managers in the region.
Traditional methods of control are costly, need to be repeated over several years and may face regulatory restrictions. Examples include, hand removal for small infestations and fire, cutting and herbicide treatments for larger infestations.
Biocontrol using natural enemies from the plant’s native habitat could provide a more sustainable solution to controlling garlic mustard in North America.
In August and October 2018, Dr. Rob Bourchier, from Agriculture and Agri-Food Canada (AAFC) released the first biocontrol agent against garlic mustard in North America – the root mining weevil Ceutorhynchus scrobicollis. Weevil larvae in lab-inoculated plants and adults were released at a garlic mustard site in Ontario, Canada.
A population model predicted that of all agents investigated, C. scrobicollis will have the most significant impact on garlic mustard. If the weevils survive the winter and establish themselves in the wild, they are expected to reduce the density of the weed at existing sites and slow its spread to new sites. The project will continue to closely monitor the weevil’s progress and assess whether they live up to the model’s prediction.
The scientific and regulatory process of releasing C. scrobicollis has been years in the making. It started when CABI’s Switzerland centre was commissioned to look for natural enemies of garlic mustard in its native European range in 1998. After initial literature and field surveys, several potential agents (mostly weevils) were studied.
Thorough safety testing done at CABI in Switzerland and under quarantine conditions at the University of Minnesota in the U.S. demonstrated that C.scrobicollis targeted garlic mustard effectively without damaging other native species. Building on this breakthrough, the first petition for field release of the weevil was submitted to the North American regulatory authorities.
Following extensive consultation, supplemental testing and review by regulators, Canada approved the release of the biocontrol agent in June 2018. The research teams are hopeful that a release permit will soon be granted for the United States, which is being pursued by Drs. Jeanie Katovich, Roger Becker, George Heimpel and Ph.D. candidate Mary Marek-Spartz at the University of Minnesota.
In 2019, the weevil rearing program at the University of Minnesota, European field collections by CABI and field expertise from AAFC will be combined to expand the number of weevil release sites in Ontario. These release experiments will focus on defining the conditions that favour C. scrobicollis survival and population growth.
While conducting pre-release studies, researchers collaborating from the U.S., Canada, and CABI have amassed a wealth of experience rearing this insect. This knowledge will be a huge benefit as the teams develop a prescription for the use of C.scrobicollis in North America.
* Mycorrhizas are fungal associations between plant roots and beneficial fungi.