Spatial and seasonal distribution of adults were investigated in a five-acre clearing and surrounding aspen forest is northern Lower Michigan by means of window traps, sweep-net, and direct observation. Trapping measured flight activity, a parameter directly related to population density, which reached its maximum in the clearing during the period of June 4 to 16. This corresponds to the time during which bracken fronds in the clearing were 3 to 10 days old, the stage in which this sawfly oviposits. The low sawfly population levels in the forest appears to be due to the advanced development and consequent immunity of the host plant there. Bracken fronds emerge earlier and stands mature more rapidly in forest than in clearings.