Dispersal of Early Instars of the Douglas-fir Tussock Moth1,2,3

Abstract

Early instar dispersal of the Douglas-fir tussock moth, Orgyia pseudotsugata (Mc-Dunnough) was investigated in the laboratory and in the field at Enterprise, Oregon, and Kamloops, British Columbia. Field studies showed 96% of dispersing larvae were 1st instars and the rest were mostly 2nd instars. Dispersal lasted 10—20 days, occurred only in daylight, and usually peaked before noon. At Enterprise, the number of larvae spreading into an adjacent, uninfested stand was too small for detectable defoliation in the year of dispersal; 1 yr later the population had disappeared. At Kamloops, the dispersing population was large enough to cause heavy defoliation to closely adjacent stands in the year of dispersal. The buoyancy of 1st instars was positively correlated with the length of their silk thread. Field data and theoretical calculations suggest that even larvae with very long silk threads do not normally drift far from their source trees.