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THE GENETIC COMPOSITION OF PHILAENUS SPUMARIUS POPULATIONS IN ISLAND HABITATS VARIABLY AFFECTED BY VOLES

A rare opportunity to work on coexisting mammal and insect populations in a restrictive environment was afforded to the present authors in connection with studies on island populations of the meadow spittlebug, Phdaenus spumarius (L.). Some of the populations investigated were found to live in meadows simultaneously inhabited by the field vole, Microtus agrestis L. The populations analyzed live on very small islets which are sufficiently isolated to resemble the fenced plots used in certain modern experiments on the ecological genetics of microtine rodents (Krebs et al., 1973). Fencing a vole population destroys the regulatory machinery which normally prevents overgrazing. At peak densities of the M. agrestis populations, the island meadows investigated by us suffer heavily from overgrazing. The present work covers a period of six years, 1969-74, which is enough to allow inclusion of both "overgrazing growth seasons" and recovery seasons. Philaenus spumarius is considered an object in which changes are produced through disturbance of the environment by Microtus agrestis . The phenotypic variable studied in Philaenus is a multiple color polymorphism, the genetic basis of which has recently been unraveled by Halkka et aI. (1973). At most eleven color morphs determined by seven allelic genes plus a small number of modifiers are found in the island populations investigated. In the present work, the phenotypes caused by modifiers are not considered separately but occur under