Effects ofPuccinia carduorumon Musk Thistle Herbivores

Puccinia carduorumJacky, a rust fungus recently introduced into the United States for biological control of musk thistle (Carduus thoermeriWeinmann), may interact with three established thistle herbivores,Trichosirocalus horridus(Panzer) (Coleoptera: Curculionidae),Rhinocyllus conicusFroelich (Coleoptera: Curculionidae), andCassida rubiginosaMüller (Coleoptera: Chrysomelidae). Tests were conducted to determine the effects ofP. carduorumon 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 ofC. rubiginosaandT. horridusconsumed 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 byC. rubiginosalarvae 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, andR. conicusoviposited 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 ofP. carduorumon the herbivores were slight, and among the interactions that were found, none is expected to be detrimental to biological control.