• Birch forest;
  • Lepidoptera;
  • outbreak species;
  • phase-dependent;
  • population cycles

Abstract.  1. The aim of this 4-year observational study was to test for the presence of direct and delayed density-dependent larval melanism in the geometrid moth species Operophtera brumata (winter moth) in northern Norway.

2. Data from many populations with a wide range of population densities in time and space facilitated statistical analyses that could separate the effects of current and past density. The data also included different phases of the 10-year population cycle of this species so that eventual non-linear density effects due to population phase could be detected.

3. The results showed that the prevalence of melanism had a strong positive, linear relation to population density within years, whereas there was no evidence for a delayed effect from the year before or dependency on the phase of the population cycle.

4. In combination, these results limit the range of possible explanations of larval melanism in this outbreaking species. The possible reasons why winter moth larvae might benefit from crowding-induced melanism are discussed.