Flower-Visiting Insect Assemblages on Fall-Blooming Native California Sage Scrub Shrubs

Pollinator studies in the endangered California sage scrub ecosystem have focused on spring insect assemblages, when most plant species bloom. Consequently, the insect assemblages using common fall-blooming sage scrub shrubs Lepidospartum squamatum, Ericameria pinifolia, and Baccharis pilularis remain undescribed. Our study aimed to: (1) document flower-visiting insect assemblages on fall-blooming shrubs, (2) assess the efficacy of three sampling techniques in inventorying insect assemblages, and (3) explore, using DNA metabarcoding, which plants are utilized and the extent to which surrounding suburban habitats’ plants are also used. While elevated sampling is required to inventory flower-visiting insects, we describe a diverse assemblage consisting of 123 species. Insect assemblages differed between L. squamatum and B. pilularis, as well as, E. pinifolia and B. pilularis, but not between L. squamatum and E. pinifolia. Direct sampling approaches (netting and photo documentation) collected 115 species not collected by passive malaise traps, highlighting that active observations are required to describe flower-visiting insect assemblages. Sequencing the ITS2 region of pollen from abundant visitors revealed that a majority of pollen is from the sage scrub ecosystem, highlighting its value. Our results indicate that the presence of fall-blooming shrubs may be critical for maintaining diverse sage scrub insect and pollinator assemblages.