Egg Development in Hexagenia limbata (Ephemeroptera:Ephemeridae) from Southern Indian Lake, Manitoba: Temperature Effects and Diapause

Abstract

Egg development was investigated for Hexagenia limbata (Serville) from Southern Indian Lake (SIL) in northern Manitoba, Canada. The SIL population is located near the northern range limit of H. limbata, and some important differences in hatching dynamics were seen in comparison with southern populations. Hatching success and egg development time were related to temperature, as reported in the south, but the developmental threshold temperature for the SIL population was near 8°C, 2° lower than previously reported. Storage of the eggs at 8°C resulted in slow but measurable development rather than low-temperature quiescence as expected, leading to inconsistent degree-day estimation among temperature treatments. Following storage at 8°C, both egg development rate and hatching success increased with increasing temperature. However, hatching success was improved by increasing the time the eggs spent in storage, ranging from only 50% hatch in unstored eggs to >90% hatch in eggs stored for ∼6 mo. In addition, a bimodal hatching response was seen in the eggs; about half of the eggs deposited by individual females hatched immediately under favourable conditions, and the remainder hatched after exposure to cold (4°C). A lower developmental threshold than previously reported and a previously undescribed egg diapause may enable Hexagenia limbata to persist near the northern extent of its distribution.