A story of good and weevils
Tatlayoko wilderness, British Columbia (Photo by Tim Ennis/NCC)
Weevils were introduced to the Tatlayoko Valley in 2008 to control an invasive plant. Against expectations, the insect population is thriving, helping to protect the region's biodiversity.
Dalmatian toadflax was first introduced to BC as an ornamental garden plant, hailing from southeastern Europe and western Asia. Topped with attractive yellow flowers that look like snapdragons, toadflax spreads both by seed and via its deep root system: roots that can grow to more than three metres in length.
But dalmation toadflax proved to be altogether too comfortable in its new environment, crowding out native grasses and plants, reducing forage available to wildlife and threatening already vulnerable ecosystems.
By the 2000s, the aggressive, perennial weed had spread through the West Chilcotin region of BC and was affecting two Nature Conservancy of Canada conservation areas in the Tatlayoko Valley.
Management of invasive species
Dalmatian toadflax (Photo by Bob Nowierski via Wikimedia Commons)
Invasive species are a significant challenge for habitat conservation groups like the Nature Conservancy of Canada.
“Once you protect a property and secure it from residential and commercial development,” says Hillary Page, the Nature Conservancy of Canada's director of conservation planning in BC. “Invasive species are the next threat we're concerned about, as they negatively affect native plants and animals."
Typical toadflax management strategies include spraying herbicide and hand pulling. Hand pulling wasn't practical in Tatlayoko because of the size of the infestation, and spraying herbicide on such a widespread infestation would put waterways and other plants at risk.
A different solution: biocontrol
Biocontrol is a method of reducing an invasive plant population through use of the species' natural predators (usually insects, parasites or pathogens). The biocontrol agent is introduced into the ecosystem and attacks the invasive plant, hopefully eradicating it.
Stem-boring weevil at Tatlayoko Lake Ranch (Photo by NCC)
“The beauty and elegance of using biocontrols is that they are virtually free, don't pay attention to property lines and don't damage non-target species,” says Peter Shaughnessy, the Nature Conservancy of Canada's project manager for the Tatlayoko area.
Once established, biocontrols work continuously and don't require maintenance.
In Tatlayoko, the proposed agents were weevils: small herbivorous beetles generally seen as pests. In 2008, 2009 and 2010, the BC Ministry of Forests and Range, now the Ministry of Forests, Lands and Natural Resource Operations, collected three species of weevils from the Kamloops area, and several hundred of the insects were released onto NCC's Lincoln Creek Ranch and Tatlayoko Lake Ranch.
One species in particular, Mecinus janthinus, began to flourish. Black in colour and measuring 3.5 to 4 millimetres long, M. janthinus reproduces exclusively on dalmatian toadflax. They are stem-boring, chewing through the stems of the plant to lay eggs. The larvae mine the stems, which limits the plant's growth and productivity. Adult weevils feed on the plant's leaves.
A success story
Lincoln Creek Ranch (Photo by NCC)
In southern BC, weevils had previously been introduced as biocontrol agents by the provincial government. The insect had subsequently been found established in areas across southern BC, but nowhere as far north as Tatlayoko.
Some of the experts involved in the Tatlayoko Valley project doubted the insects would survive the region's harsh winters. Native to the Rhine Valley in France, M. janthinus prefer hot dry conditions in grassland and open forest habitat, and have high winter mortality rates.
“It was kind of like a last-ditch effort,” says Page.
Since their introduction to the area, however, the stem-boring weevils have exceeded expectations. Shaughnessy estimates that the weevil population is now in the millions, spread over a stretch of 30 square kilometres. While the toadflax isn't eliminated, is has been suppressed by up to 90 percent. Wherever dalmatian toadflax is still found, so are the weevils.
“If the weevils are left to do their job here the dalmatian toadflax will no longer be a management issue,” says Shaughnessy. The plant can never be eradicated at this scale – it has been tried in the U.S. – and so it will likely become part of the local flora and fauna, with its own control: the weevils.