Tephritid Fly Abundance and Inflorescence Herbivory on a High Alpine Thistle Vary with Environment

Little is known about insect–plant interactions in high alpine environments. We studied the insect herbivore feeding guild in inflorescences of Cirsium funkiae (Asteraceae), an alpine thistle endemic to the southern Rocky Mountains, USA. We asked whether site (open talus above treeline, mid-elevation wet meadow, or lower-elevation forest edge just below treeline) and plant characteristics affect insect abundance, guild composition, and intensity of herbivory. We dissected flower heads collected at season’s end to quantify fly occurrence and measure herbivory damage. Three tephritid flies—Chaetostomella undosa, Paracantha gentilis, and Terellia occidentalis—strongly dominated the herbivore guild. All three species differed in abundance and composition across habitat sites. Chaetostomella dominated in the high, open talus habitat; Terellia peaked in the mid-elevation meadow site; and Paracantha dominated in the lower forest edge site. While Terellia abundance was strongly associated with flower head diameter, abundances of Paracantha and Chaetostomella were only weakly associated with flower head size, suggesting species-specific flower-head selection. Paracantha and Terellia showed elevated co-occurrence at the mid-elevation alpine meadow site, indicating shared distributional preferences likely related to their occurrence on thistles at lower elevations in this region. Unexpectedly, seed damage was extremely high (91.5–98.5%) across all three sites and highest in the characteristic habitat, open high talus slopes. Despite uniformly high seed damage, the low Forest Edge plants had significantly more viable seeds than did the High site plants (8.2% vs. 1.6%), and the difference was associated with differences in guild composition. These findings demonstrate that tephritid flies dominate floral herbivory on the high alpine Cirsium funkiae, that site differences can shape fly feeding guild structure, and that they have a measurable negative impact on plant seed reproductive success in high alpine environments.