The Diversity of Overwintering Strategies Utilized by Separate Populations of Gall Insects

Abstract

The variability of overwintering strategies of four insect species occupying identical hibernacula are compared over two climatic regions. Stepwise acclimation was accomplished over a 12-week "seasonal" cycle between 5, -40, and 20 C. The ball gall hibernaculum of Solidago sp. provides a minimal thermal buffer to its inhabitants. Separate populations of the gall maker Eurosta solidagensis demonstrate markedly different adaptive strategies during acclimation. Low temperature exposure results in an elevation of supercooling points (SCPs) in northern populations so that early freezing occurs. Southern populations depress the SCP upon similar exposures to avoid tissue-freezing encounters. The SCPs become synchronous between populations when ambient temperatures are reduced below the supercooling limit (-20 C). Warm acclimation is characterized by a major depression in the SCP in both populations. However, the temperature trigger has been shifted to higher temperatures in northern species (>5-< 20 C vs. >0-< 5 C for southern groups). Three other species overwinter in the gall: Mordellistena unicolor, a coinhabitant with E. solidagensis, and Eurytoma obtusiventris and Eurytoma gigantea, two predators. Each of these species have distinct overwintering strategies.