PHOTOPERIODISM AND LIFE-CYCLE PLASTICITY OF AN APHID, MACROSIPHUM-EUPHORBIA (THOMAS), FROM CENTRAL NORTH-AMERICA

Photoperiodic responses are described for five clones of the potato ap hid, Macrosiphum euphorbiae (Thomas) (Homoptera: Aphididae), from Mani toba, Canada, a region with a strongly seasonal, continental climate. When exposed to long nights, parthenogenetic wingless females produce winged males and parthenogenetic winged females that subsequently prod uce mating females. The young adult males and winged females are restl ess and take flight readily, which is typical of a post-teneral migrat ory phase that-could carry them to 1 winter host. These traits are cha racteristic of host-alternating species. However, wingless females als o produce mating females, and winged females also produce parthenogene tic wingless females, traits which are characteristic of nonhost-alter nating species. Clones differ in the frequencies of phenotypes they pr oduce, some being more host-alternating than others. Critical night le ngths are shortest and frequencies of males highest for clones that ar e closest to the host-alternating type of life cycle. The importance o f phenotypic and genotypic plasticity in the life cycle is discussed.