One hypothesis in weed biological control is that multiple herbivores are necessary to exert sufficient control. Species that feed on different plant tissues and/or occur at different times will not directly compete. However, interactions can occur if one herbivore alters the host plant that serves as a common resource for both species. Musk thistle (Carduus nutans species group) is a noxious weed of pasture and rangeland. Two insects were introduced for its control, Trichosirocalus horridus, which attacks the vegetative crown, and Rhinocyllus conicus, which infests flower heads. These weevils presumably are complementary in their impact on musk thistle, but this assumption has never been tested. We examined whether vegetative feeding by T. horridus alters musk thistle as a resource for the later-arriving R. conicus.
Low to moderate levels of infestation of musk thistle by T. horridus resulted in the production of significantly more and (under moderate infestation) shorter flower stems compared to uninfested thistle. However, the first appearance of flower heads and the rate of new head formation by plants were generally not affected by T. horridus infestation. There were no differences in the mean number of R. conicus adults or eggs per plant, or in R. conicus survival, among T. horridus treatments over the course of the summer. Therefore, different growth forms of musk thistle resulting from T. horridus infestation do not appear to have had an influence on R. conicus.
In contrast, it appears that severe infestations of T. horridus can alter resource availability for and oviposition by R. conicus.
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA