Interactions between Operophtera brumata and Tortrix viridana on oak: new evidence from time-series analysis

1. Yearly population estimates of the green oak leaf roller moth, Tortrix viridana (Lepidoptera: Tortricidae), and the winter moth, Operophtera brumata (Lepidoptera: Geometridae), were taken from the pedunculate oak, Quercus robur, from 1951 to 1966 in Wytham Woods, Oxfordshire, U.K. Larval mortality from parasitoids was recorded for both species over the same period. 2. Operophtera brumata population density fluctuated around a constant mean, whereas T. viridana population density exhibited a linear decline over the sampling period. Population counts were subjected to time-series analysis after the linear decline was removed from the T. viridana data by detrending. Multiple regression models were built so that variation in the per capita rate of increase of each population could be partitioned among (a) current and previous population density, (b) current and previous population density of the second moth species, and (c) rates of larval parasitism. 3. Multiple regression analysis suggested that variation in the per capita rate of increase of O. brumata could be explained by negative feedback from O. brumata density at T–2 (32%), a negative relationship with T. viridana density at T–1 (18%), and a positive relationship with parasitism suffered by T. viridana at T–1 (24%). 4. The T. viridana population time-series was dominated by a rapid feedback process such that per capita rate of increase at time T was negatively related to population density at time T–1, explaining 53% of variance in population growth rate. Per capita rate of increase was unrelated to larval parasitism or densities of O. brumata. 5. In light of previous life-table and experimental studies on O. brumata and T. viridana, the current analyses suggest that pupal predation (data not presented) and interspecific competition are significant determinants of O. brumata population growth rates in Wytham Woods. In contrast, T. viridana population growth rates appear to be dominated by rapid negative feedback consistent with intraspecific competition.