Release and establishment of the weevil Mecinus janthiniformis for biological control of Dalmatian toadflax in southern California

Authors: Smith, Lincoln; WOODS, DALE - California Department Of Food And Agriculture; Wibawa, Maria - Irene; POPESCU, VIOLA - California Department Of Food And Agriculture; Moran, Patrick; VILLEGAS, BALDOMERO - California Department Of Food And Agriculture; PITCAIRN, MICHAEL - California Department Of Food And Agriculture; HON, CHRIS - California Department Of Parks And Recreation

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2021
Publication Date: 4/21/2021
Citation: Smith, L., Woods, D.M., Wibawa, M.I., Popescu, V., Moran, P.J., Villegas, B., Pitcairn, M.J., Hon, C. 2021. Release and establishment of the weevil Mecinus janthiniformis for biological control of Dalmatian toadflax in southern California. Biological Control. 161. Article 104633. https://doi.org/10.1016/j.biocontrol.2021.104633.
DOI: https://doi.org/10.1016/j.biocontrol.2021.104633

Interpretive Summary: Dalmatian toadflax is a perennial invasive alien weed originating from Europe that has invaded grasslands in North America. It is toxic to cattle, and most wildlife do not eat it. A large infestation of toadflax was found on the Hungry Valley State Vehicular Recreation Area in southern California in 2004. The stem-boring weevil, Mecinus janthiniformis, has been approved as a classical biological control agent in Canada and many US states. However, it was not approved for California because of possible risk of attacking a native plant. The State of California authorized release of the weevil in southern California, which is far from where the native plant occurs. Weevils were released in 2008 and quickly multiplied, attacking up to 100% of toadflax stems at the release sites by 2012. However, a wildfire in 2013 killed all the weevils. More weevils were released in 2014 and they again multiplied and dispersed. Toadflax density began decreasing by 2016, and by 2019, very few plants remained. The climate at the release area, which is much further south than its native range in Europe, was very suitable for the weevils. The weevils are expected to persist and provide self-perpetuating control of the weed.

Technical Abstract: Dalmatian toadflax (Linaria dalmatica) is a perennial forb that has invaded grasslands in North America. The weevil, Mecinus janthiniformis, has been released as a classical biological control agent in the western U.S. and Canada, but not in California because of concern that it might attack some nontarget native snapdragons (Antirrhinum spp.). An infestation of Dalmatian toadflax was discovered in southern California in 2004 at the Hungry Valley State Vehicular Recreation Area. In 2008, we released the weevil at three sites which were paired with nearby check sites. Weevil populations increased at the release sites to an average of 45 weevils per 100 cm of stem length in 2012, and up to 100% of Dalmatian toadflax stems were infested. The weevil also spread to the three check sites (10-72% infestation). A wildfire in May 2013 destroyed the weevil population, but the toadflax recovered by 2014, and new weevil releases were made. By 2017, the weevil populations had again increased at all release sites in the original release area, with up to 47 weevils per 100 cm of stem length and 100% of stems infested. The weevils had dispersed at least 438 meters, and similar densities occurred at the release and check sites. Survivorship of weevils overwintering inside the stems was high (92% survival), and the relatively warm climate probably allowed females to realize their maximum fecundity. Dalmatian toadflax cover declined 99% from 2014 to 2019 at this southernmost release site in North America.