Indirect effect of early-season infestations of Trichosirocalus horridus on Rhinocyllus conicus (Coleoptera: Curculionidae)

Field experiments were conducted to determine whether early vegetative feeding by Trichosirocalus horridus alters musk thistle (Carduus nutans) as a resource for the later-arriving Rhinocyllus conicus. Results showed that thistles with high larval densities of T. horridus (66 per plant) produced more and shorter flower stems, significantly fewer flower heads, and delayed flowering by 1 week when compared to uninfested thistles. Also, colonization and oviposition on flower heads by adult R. conicus were as much as three and five times lower, respectively, on thistles heavily infested with T. horridus than on uninfested thistles. As a result, 63% fewer R. conicus adults developed from T. horridus-infested thistles. Musk thistles that were infested with lower densities of T. horridus larvae (<20 per plant) also produced multiple stems that were usually shorter than uninfested thistles. However, no differences were observed in flower head production or in the behavioral responses of R. conicus to T. horridus-infested and uninfested plants. In a greenhouse experiment, the mortality rate of R. conicus was higher on musk thistles that were heavily infested by T. horridus, implying a change in musk thistle quality. However, this effect was apparent only at low R. conicus larval densities. T. horridus can indirectly and negatively affect R. conicus through a variety of mechanisms, but only when densities of T. horridus are high. This finding may have important implications for the continued redistribution of both weevils, which were imported for the classical biological control of musk thistle, a noxious weed in pastures and rangeland.