Bottom-up and top-down effects in a tritrophic system:: the population dynamics of Plutella xylostella (L.)-Cotesia plutellae (Kurdjumov) on different host plants

1. The effects of host-plant resistance on the population dynamics of the Diamondback moth, Plutella xylostella L., and its solitary parasitoid, Cotesia plutellae (Kurdjumov), were studied in replicated time-series experiments. 2. Host-plant resistance did not affect the equilibrial abundance of the Diamondback moth, but it affected the dynamics of Diamondback moth populations. 3. The mean population size of Diamondback moth showed no significant difference between Brassica rapa (a susceptible host plant) and Brassica napus (a partially resistant host plant) either in the presence or absence of the parasitoid. 4. Time-series analysis suggests that the dynamics of Diamondback moth on B. rapa were underpinned by delayed density-dependent processes. In contrast, the dynamics of the moth on B. napus were influenced by a direct density-dependent process. 5. Although measures of short-term parasitism showed a significantly higher rate of parasitism by C. plutellae on Diamondback moth feeding on B. napus compared with B. rapa, this individual performance does not translate into differences in the population dynamics. Analysis shows no significant difference in the persistence time of the population-level interaction between the host and parasitoid on the two different host plants.