The combined effect of photoperiod and temperature on egg dormancy in an island and a mainland population of the hemlock looper, Lambdina fiscellaria.

Embryonic dormancy characteristics of the hemlock looper (HL), Lambdina fiscellaria (Guenee) (Lepidoptera: Geometridae), an important coniferous defoliator, were investigated using eggs from an island (Newfoundland) and a mainland (Quebec) population in eastern Canada. We determined (1) the pre-diapause duration or time required for eggs to change color at 15 °C, under L16:D8 and L12:D12, (2) the percent hatch and time to hatch of early-diapausing eggs that were, as of 1 September, either (a) incubated under each combination of three photoperiods (L16:D8, L12:D12, or L8:D16) and three temperatures (15, 20, or 25 °C) or (b) stored outdoors prior to their monthly incubation (October-May) under the same treatments, and (3) the diapause duration at 15 °C. The two HL ecotypes completed their pre-diapause phase in ca. 15 days under long-day photoperiod. In the mainland ecotype, photoperiod did not influence the pre-diapause duration. Regardless of photoperiod and ecotype, only eggs incubated continuously at 15 °C hatched successfully after ca. 120 days. Temperature was the most important factor modulating the dormancy of eggs acclimated outdoors, in both ecotypes. From October to December (diapause phase), percent egg hatch at 20 and 25 °C increased from low (20%) to levels similar to those obtained at 15 °C (70%). These percentages remained stable throughout the post-diapause phase (January-May). Time to hatch, which was shorter at warmer temperatures, decreased from October to December at all temperatures. It remained stable from January to March (quiescence), however, and declined thereafter. Eggs from the island were heavier than those from the mainland and their odds of hatching were 2.3 times higher. At 15 °C, diapause lasted ca. 90 days. Our findings indicate that eggs from the two HL ecotypes (1) undergo an obligatory diapause, (2) complete diapause without pre-exposure to cold, (3) respond similarly to photoperiod and temperature during diapause and post-diapause, and (4) do not hatch successfully after prolonged exposure to high temperatures.