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

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, and the populations increased to an average of 45 weevil pupal chambers per 100 cm of stem length in 2012, and 100% of L. dalmatica stems within 15 m were infested. Mecinus janthiniformis also spread to three nearby check sites (10–72% infestation). A wildfire in May 2013 destroyed the weevil population, but the toadflax recovered by 2014, and M. janthiniformis was released again. By 2017, the weevil populations had again increased at all release sites, with up to 47 weevil pupal chambers per 100 cm of stem length and 100% of stems infested. The weevils had dispersed at least 427 m, and population densities were similar at the release and check sites. Survivorship of immature and adult weevils overwintering inside the stems was high (92% survival), probably due to mild winter temperatures and absence of parasitism. The relatively warm climate probably allowed females to realize their maximum fecundity at this southernmost release site in North America. Dalmatian toadflax cover declined 99% from 2014 to 2019, whereas annual and perennial grasses increased, annual forbs remained abundant, and perennial forbs and shrubs remained rare.