Pod fecundity of Ulex europaeus and seed loss due to predation by Exapion ulicis in northern California, USA

Abstract

Common gorse, Ulex europaeus L. (Fabaceae) is a serious invasive shrub of pastures, forest gaps, and natural areas in western North America. The seed weevil, Exapion ulicis (Forster) (Coleoptera: Brentidae) was introduced as a biological control organism at three locations in California and one location in Oregon from 1953 through 1956. To document its performance as a biological control organism, a survey of 11 gorse populations along a north–south transect was performed in 2021 and 2022. The seed weevil was recovered at all locations, but the intensity of attack (percentage of pods infested) varied from 2 to 88%. Various components of pod fecundity (pod length, number of ovules, seed set, number of mature and intact seeds) varied spatially. Seed set (mature seed-to-ovule ratio) ranged from 24 to 60% and production of mature seeds (prior to predation) ranged from 1.4 to 4.6 seeds per pod. When present, E. ulicis larvae destroyed 91% of seed in a pod. For infested pods, the number of weevils per mature seed was 1.72, with an average of 6.3 weevils and 3.3 mature seeds per pod. The number of intact seeds after predation averaged 0.3 seeds per pod. Mean seed loss due to E. ulicis ranged from 1 to 81% among locations and was closely related to the local intensity of attack. Oviposition by female E. ulicis targeted more fecund pods. Three parasitoids were recovered, with local parasitization rates of E. ulicis larvae ranging from 0 to 1.6%, suggesting that parasitoids do not limit population abundance of the seed weevil.