LONG-TERM DYNAMICS OF LEAF MINERS, ERIOCRANIA SPP, ON MOUNTAIN BIRCH - ALTERNATE YEAR FLUCTUATIONS AND INTERACTION WITH EPIRRITA-AUTUMNATA

1. Abundance of leaf-mining larvae of the outbreak species Eriocrania spp. was monitored in northern Sweden in 1955-67 and 1984-92 in a mountain birch forest rejuvenated by an Epirrita autumnata outbreak in 1954-55. 2. Eriocrania mine density fluctuated in a regular biennial pattern, probably due to a semivoltine life cycle. The alternate year fluctuations had shifted phase between the two study periods. 3. The density fluctuated at a significantly higher mean level and with a lower amplitude in the c. 10-year-old forest in the 1960s than in the 30-year-old forest in the 1980s. However, no difference was found in mine density between c. 30- and >60-year-old stands in the 1980s. 4. Significant correlations between the density of mines in high-density years and date of budburst, and between rate of change between high-density years and the time between snow-melt and budburst indicate effects of weather. No correlation was found between yearly average mine density and date of budburst, precipitation or temperatures in May or June. 5. No correlation was found between the mean densities of Eriocrania mines and E. autumnata caterpillars in the same and the two preceding years. However, in the first (1955) and highest of three studied E. autumnata peaks a negative effect on population density of Eriocrania was indicated. 6. A significant, negative correlation between the number of Eriocrania mines and E. autumnata caterpillars on single branches was found in three out of eighteen years. The separation of caterpillars and mines at branch level indicates an effect of avoidance at intermediate population densities.