Observations on the range expansion of Mecinus janthiniformis Toševski & Caldara, 2011 and Rhinusa neta (Germar, 1821) (Coleoptera: Curculionidae) into California

Abstract. Dalmatian toadflax, Linaria dalmatica (L.) Mill., and yellow toadflax, Linaria vulgaris Mill. (Plantaginaceae), are exotic forbs that have become widespread in North America, including California. Both species have accumulated several exotic insects from accidental and intentional introductions. Until recently, Mecinus janthiniformis Toševski & Caldara, 2011 was known to occur at only one location in southern California. Recent surveys of L. dalmatica in northern California recovered M. janthiniformis and a second weevil, Rhinusa neta (Germar, 1821), the latter a new California record. From 2010–2015, M. janthiniformis was recovered at 24 of 27 sites in Lassen, Modoc, Plumas, Siskiyou, and Trinity Counties. After 2014, four locations in Trinity County and one in Sierra County were monitored annually through 2020, but no M. janthiniformis have been recovered to date suggesting this species has not extended its range since 2014. In 2018, R. neta was discovered in L. dalmatica seed capsules in Trinity and Sierra Counties. Later surveys found R. neta infesting L. dalmatica at five locations in four counties and Sairocarpus virga (A. Gray) D. A. Sutton (= Antirrhinum virga A. Gray) (Plantaginaceae), a native species, at one location in Colusa County. The intensity of attack of R. neta on L. dalmatica ranged from 9–70% of seed capsules infested. Being found 300 km south of the Oregon border suggests that R. neta spreads more readily than M. janthiniformis, which has only spread 190 km south of the Oregon border but no further since 2014. Repeated annual sampling at uninfested sites found R. neta to have spread 42 km in one year, an unusually high dispersal rate for exotic Coleoptera.