Microclimate variability and the eurythermic nature of goldenrod gall fly (Eurosta solidaginis) larvae (Diptera: Tephritidae)

Abstract

Four insect species inhabit ball galls on goldenrod plants (Solidago sp.) for several months of the year. These sites are highly exposed and conceivably undergo wide fluctuations in temperature on a daily basis. I recorded temperature profiles for galls in western New York between September and November. The capacities of a gall inhabitant (Eurosta solidaginis larvae) to endure high temperature and freezing were also recorded during this period. I noted considerable and concurrent variation in air temperature and gall temperature. Diurnal radiant heating augmented changes in gall temperature so that their daily variations often ranged between 25 and 29 °C. From September to November, Eurosta larvae tolerated 24-h exposures to 35.5 °C and freezing at −6.5 °C. In the latter case, this occurred despite the lack of any appreciable accumulation of cryoprotectants in September and October. Greater freeze tolerance was gained by November (100% survival at −15.5 °C), when hemolymph osmolality increased 60% to 722 mosmol. This study demonstrates the extremely eurythermal nature of ball galls and the highly developed capacities of gall inhabitants to survive extreme temperatures.