Inheritance and World Variation in Thermal Requirements for Egg Hatch in Lymantria dispar (Lepidoptera: Erebidae)

Mode of inheritance of hatch traits in Lymantria dispar L. was determined by crossing populations nearly fixed for the phenotypic extremes. The nondiapausing phenotype was inherited via a single recessive gene and the phenotype with reduced low temperature exposure requirements before hatch was inherited via a single dominant gene. There was no evidence for sex-linkage or cytoplasmic effects with either gene. Eggs from 43 geographic populations were evaluated for hatch characteristics after being held for 60 d at 5°C followed by incubation at 25°C. There was considerable variation both within and among the populations in the proportion able to hatch, time to first hatch, and average time to hatch. Egg masses with reduced requirement for low temperatures before the eggs were ready to hatch were present in all subspecies of L. dispar and the phenotype was not fixed in most populations. The populations clustered into three distinct groups, and climatic variables were found to be rough predictors of those groups. Variation in hatch phenotypes between populations is likely an adaptation to local climate and within a population provides a bet-hedging strategy to ensure that at least some hatch synchronizes with host leaf-out. Continued vigilance to prevent movement of populations both within and between countries is warranted, because some of the alleles that confer nondiapause or reduced low temperature requirements before egg hatch are not present in all populations and their introduction would increase variation in egg hatch within a population.