Phenotypic and Genetic Characteristics of the European Cranefly Following its Introduction and Spread in Western North America

Abstract

Density, size change and spread of the European cranefly, T. paludosa, were monitored in an introduced population in western North America. In field populations, larval growth rate was positively related to asymptotic larval weight and negatively correlated with larval density. Asymptotic larval weight and winglength of males were positively correlated. Laboratory experiments showed that size of larvae was reduced by crowding only if the larvae had been briefly exposed to field conditions and not if they were reared from eggs. Protozoan parasites are implicated as causing the size-density relationship, and possibly they have contributed to the establishment of the reduced equilibrium population density. The asymptotic weight of larvae reared singly in the laboratory after brief field exposure predicts 64% of the winglength variation of males emerging from that population the next autumn. Adult size is determined early in development and is not modified by abundant food and individual rearing. Craneflies on the edge of the southern spread are larger than those from populations which have passed through the post-introduction eruption. Attempts at monitoring genetic change were frustrated by the lack of electrophoretic variability. Comparisons are made with British studies of the European cranefly and indicate that the introduced population has a stronger size-density relationship, a higher average density, and lacks an association between larval density and rainfall during the oviposition period.