Effects ofPuccinia carduorumon Musk Thistle Herbivores

Abstract

Puccinia carduorum Jacky, a rust fungus recently introduced into the United States for biological control of musk thistle ( Carduus thoermeri Weinmann), may interact with three established thistle herbivores, Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), Rhinocyllus conicus Froelich (Coleoptera: Curculionidae), and Cassida rubiginosa Müller (Coleoptera: Chrysomelidae). Tests were conducted to determine the effects of P. carduorum on feeding, oviposition, longevity, egg production, egg hatch, and larval development of the three insects. When offered both rust-infected and healthy musk thistle leaves, adults of C. rubiginosa and T. horridus consumed significantly more healthy than infected foliage. On infected leaves, feeding and oviposition were confined largely to pustule-free areas. The amount of rust-infected and healthy foliage consumed by C. rubiginosa larvae was not significantly different, but feeding on the infected leaves was again confined to rust-free leaf areas. Rust infection did not reduce oviposition by the three insects, and R. conicus oviposited only on healthy portions of the bracts. There were no significant differences in longevity, egg production, percentage of egg hatch, and larval development for any of the three insects fed continuously with both healthy and rust-infected leaves. Thus, effects of P. carduorum on the herbivores were slight, and among the interactions that were found, none is expected to be detrimental to biological control.