Waves and synchrony in Epirrita autumnata /Operophtera brumata outbreaks. II. Sunspot activity cannot explain cyclic outbreaks

Authors


A. C. Nilssen, Zoology Department, Tromsø Museum, University of Tromsø, N-9037 Tromsø, Norway. E-mail: arnec.nilssen@tmu.uit.no

Summary

  • 1In recent studies, it has been argued that sunspot activity forces the Epirrita autumnata 9–10-year outbreak periodicity in the mountain birch forest of Fennoscandia. For the following reasons, we challenge this conclusion.
  • 2With a 10-year outbreak cycle of E. autumnata and the 11-year sunspot cycle, it is expected that the cycles will run in-phase, out-of-phase and in-phase within 10 × 11 years. Hence, given such cycle lengths, sunspot activity should not affect outbreak periods. For a test, the E. autumnata series should be at least 110 years in length.
  • 3A well-documented E. autumnata outbreak series of 81 years (1888–1968; outbreak periods IV–XII) exists. This series is here lengthened to 114 years by adding outbreak frequencies for three decades (1969–2001).
  • 4By lengthening the series, three more E. autumnata/Operophtera brumata periods (XIII, XIV, XV) are identified. Period XV, like several earlier periods, was of the moving type, i.e. outbreaks moved in a wavelike manner from northern Fennoscandia to southern Norway.
  • 5As with several earlier outbreak periods in central northern Fennoscandia, the main timing of periods XIII–XV centred at the middle of the decades. In contrast, outbreaks at the extreme north-western coast of Norway centred at the decadal shifts, i.e. about 1979, 1989 and 1999. Supported by historical documents, we explain the 1979 and 1999 outbreaks as the final expressions of east–west outbreak waves that branched off from the main waves which moved southward during periods XIII and XV. These side-waves in the north are new observations. Outbreaks at the decadal shift 1989/1990 may have been of a more complex nature.
  • 6We find that sunspot activity does not explain outbreak waves. Furthermore, a test of our 114-year long E. autumnata series against the contemporaneous sunspot series shows that the two series run in-phase and out-of-phase. The observed interval between the two cycles coming in-phase agrees with the expected interval. This challenges the hypothesis of sunspot synchronization of the E. autumnata (and O. brumata) outbreaks.

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