Emergence Patterns of Hexagenia bilineata: Integration of Laboratory and Field Data

Abstract

Laboratory findings regarding water and air temperature effects on the growth rate and emergence success of Hexagenia bilineata and field observations of emergence patterns of H. bilineata under known thermal regimes are compared. Several hypotheses based on laboratory results are evaluated. The hypothesis that degree-day accumulation requirements could be used to predict the intervals between onset of emergence each year was rejected. The laboratory derived regression showing that approximately 2337 degree-Celsius days (days having temperature above 10° C) were required for development from oviposition to emergence correlated very well with the degree-days actually occurring (2322 and 2192 degree-Celsius days) in two succeeding years in a cove representing a normal thermal condition. However, in a thermally enriched cove, the predicted time required for development was exceeded by 1500 to 1600 degree-Celsius days in two years of field observations. The hypothesis that a critical thermal threshold was required for complete development into the adult stage was neither strongly rejected nor supported by our field observations. Laboratory tests suggested a critical temperature threshold (between 15 and 20° C) might be involved in determining the onset of emergence. These results lead to a prediction of a four-week to six-week difference in the onset of emergence between the two field sites; however, only a two-week difference was observed. Air temperature is shown to affect survival after emergence but is irrelevant to the onset of emergence. A series of laboratory tests may be required to arrive at a good predictive model for thermal effects on any target species.