ASSESSING NATURAL-SELECTION IN WHITE-PINE WEEVILS (PISSODES STROBI PECK) (COLEOPTERA, CURCULIONIDAE) FOR OVERCOMING RESISTANCE IN TREES - AN EVOLUTIONARY MODEL

An evolutionary model was constructed for the white pine weevil (Pisso des strobi Peck). This weevil attacks Sitka spruce [Picea sitchensis ( Bongard) Carriere], and Sitka spruce trees have two forms, one being s usceptible to the insect attacks and the other being resistant to atta ck. There is a fear that insects may develop tolerance to the resistan t trees. The strategy of interplanting susceptible and resistant trees to minimize the likelihood of the insects developing tolerance mechan isms to circumvent the resistance is examined. It is found that if onl y one gene locus is involved, the development of tolerance occurs more quickly than if resistance is governed by two independent loci. The r ate of evolution of tolerance to tree resistance is retarded by increa sed adult survivorship, the degree of recessiveness of the gene, prefe rential attack of susceptible trees, redistribution of intolerant inse cts from resistant to susceptible trees, and the immigration of wild-t ype insects.