Reproductive success of the invasive weed Linaria dalmatica and seed loss due to predation by Rhinusa neta in northern California, USA

Abstract

AbstractLinaria dalmatica (L.) Mill. (Dalmatian toadflax, Plantaginaceae) is a serious invasive weed of rangelands and natural areas in the western USA and Canada. The adventive weevil Rhinusa neta (Germar) (Coleoptera: Curculionidae) is a seed predator of L. dalmatica and has recently spread into northern California, USA. Seed loss due to R. neta predation may impact the reproductive success and population abundance of local host plant populations. The aim of this study was to determine the intensity of seed predation and how it may be influenced by the reproductive success of L. dalmatica at six field sites in northern California in 2019 and 2020. Flower production ranged from 40 to 208 flowers per inflorescence, fruit production ranged from 6 to 57 fruit per inflorescence, and seed production ranged from 171 to 3734 seeds per inflorescence. These values are very low (<20%) compared to L. dalmatica plants growing in other regions. Fruit set, an indication of pollination success, ranged from 6.9 to 30.6% in 2019 and from 10.9 to 23.7% in 2020. Average seed set ranged from 36 to 138 seeds per uninfested fruit. The intensity of attack of R. neta ranged from 0 to 70.4%. Fruit attack appeared random at most locations. Average seed set for infested fruit ranged from 0 to 35 seeds per fruit and did not differ significantly between terminal and branch fruit. The population‐level impact of R. neta on seed production was directly related to the intensity of attack at a location. Intensity of attack ranged from <1 to 70% and resulted in corresponding seed losses of 0 to 62.7% among six locations. Predation intensity is expected to increase and stabilize at much higher levels in the future as R. neta continues its immigration into northern California.