IMPACTS OF DEFOLIATION BY TUSSOCK MOTHS (ORGYIA-VETUSTA) ON THE GROWTH AND REPRODUCTION OF BUSH LUPINE (LUPINUS-ARBOREUS)

1. Outbreaks of tussock moths (Orygia vetusta Bdv.) on bush lupines (Lupinus arboreus Sims) may be very intense, although spatially localized, and may last >10 years. To understand better how such outbreaks may persist, we defoliated lupine bushes using variable realistic numbers of tussock moth larvae, and measured the immediate and delayed impacts of defoliation on the growth and seed production of lupine bushes. 2. Immediate effects of defoliation included the production of new leaves that were significantly smaller in diameter and biomass, and lower in water content, than the new leaves of undamaged bushes. Also, seed output was reduced by up to 80% on heavily damaged bushes. 3. In the longer term, surviving bushes recovered remarkably well, despite having received the full range of possible (survivable) insect densities. After 1 or 2 years' rapid growth, surviving bushes were not affected in terms of height, basal stem diameter or volume by their previous defoliation. However, there was a significant tendency for juvenile bushes to produce more seeds the more heavily they had been attacked the previous year. 4. The ability of the host plant to recover, in terms of its biomass, helps to explain the sustained nature of the insect outbreak.