Weed biological control agents that feed on different plant tissues and/or occur at different times may compete indirectly 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 early in the plant's development, and the later-arriving Rhinocyllus conicus, which infests flower heads. Previously, we demonstrated that severe infestations of T. horridus could reduce the quantity of resource available for ovipositing R. conicus. We now show that T. horridus may also affect host plant quality of musk thistle.
At high T. horridus infestation levels, the quality of the earliest produced flower heads appeared to be reduced for developing R. conicus larvae. The survival rate of R. conicus larvae experiencing minimal intraspecific competition was 76.9% from T. horridus-damaged thistles compared to 96.4% for undamaged thistles. The average size (length, biomass) of surviving adult R. conicus was not affected. In contrast, a reduction in R. conicus survival resulting from T. horridus-infested plants was not observed when R. conicus larvae were present at higher densities that normally occur early in the season.
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