A MODEL OF HYPERPARASITE REGULATION OF THE GALL RUST - LODGEPOLE PINEPATHOSYSTEM

Endocronartium harknessii (J.P. Moore) Hirat, populations never approa ch the level where all susceptible Pinus contorta Dougl. ex Loud var. latifolia Engelm. tissue is infected, even in years when environmental conditions are favourable for infection. A simple simulation model wa s developed to determine whether a hyperparasite could regulate the ru st population and maintain it at an equilibrium level far below host s aturation. The model assumed that galls begin sporulating and first be come susceptible to parasitism 2 years after formation and requires sp ecification of infection rates for the rust and the hyperparasite, mor tality rates for normal and parasitized galls, and the degree to which aeciospore production is reduced following hyperparasite infection. A nalysis of tile model showed that the hyperparasite could regulate the rust population, producing a stable equilibrium, but only if the vari ous rates were such that healthy galls, in their lifetime, produced en ough aeciospores to give rise to more than one new gall, while galls t hat were parasitized during the first year of aeciospore production pr oduced,in their lifetime, fewer than one new gall. The model of hyperp arasite regulation provides one possible explanation of the dynamics o f the gall rust pathosystem. It shows that control measures such as sa nitation and breeding for resistance may reduce the effectiveness of t he hyperparasite in regulating the system and, thus, may fail to achie ve the anticipated goals.