Environmentally cued size variation in the light-brown apple moth,Epiphyas postvittana (Walk.) (Tortricidae), and its adaptive value in dispersal.

Adult males and females of the light brown apple moth,Epiphyas postivittana (Walk.) (Tortricidae) showed seasonal, cyclic variations in size at two study areas. These variations are caused by the quality of the food available and temperature. Large individuals occur during the cool wet months of the year (autumn-winter generation moths that fly during September-November) and small individuals occur during the warm dry months of the year (from January to May when late-spring and summer generation moths fly). The ratios of body length to wing-length showed that the wing-loadings of the smaller individuals are significantly lower than those of large individuals, even though the latter have longer wings. Those with low wing-loading would be expected to be more adapted for dispersal. It is suggested that the type of flight "dimorphism" ofE. postvittana which is associated with the size of the insect has survival value in enabling the moth to disperse when habitats become adverse for breeding.