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Moth ecology

Introduction

Massive outbreaks of herbivorous insects, especially the two geometrid moth species Epirrita autumnata (autumnal moth) and Operophtera brumata (winter moth) is the most important natural disturbance factor in the sub-arctic birch forest in Fennoscandia. The great imFig1. Click for larger imagepact of these insects is due to recurrent defoliation of the forest in population outbreak years, which occurs cyclically in both species at approx. 10 yrs interval. These outbreaks have become a prime example of cyclic population dynamics in the literature, mainly because of their often severe, large scale impactson the forest ecosystem. The geographical extent (in terms of latitude and altitude) of the region with the most devastating outbreaks is climatically determined.

Larvae of the winter moth (top) and the autumnal moth (bottom) feeding on the same host tree. In Norway the winter moth is mainly found on coastal near locations, while the autumnal moth tend to be more common inland. Still a large sympatric zone, where both species are found, exists in particular in northern Norway.

 

Life history Fig2. Click for larger image

Winter moth and autumnal moth have similar life cycles. The adults fly in the autumn, autumnal moth from mid August to mid September and winter moth approx. 1 month later. The females deposit their eggs on mountain birch stems and branches and the eggs overwinter and hatch at leaf burst the following spring. The eggs are extremely cold-tolerant with experimental lethal temperatures as low as -35°C (winter moth) to -37°C (autumnal moth). The timing between larval hatch and budburst is critical for the survival and development of the larvae. The larvae go through 5 instars in course of 2-4 weeks (autumnal moth) or 4-8 weeks (winter moth) before they migrate to the ground and pupate.

Outbreak dynamics

Both winter and autumnal moth in northern Fennoscandia exhibit strong fluctuations in abundance with population peaks every 9-12 yrs. Historical outbreak records dating back to the 1860s document that this is a recurring and consistent pattern (Tenow 1972). Individual outbreak periods vary tremendously however with regard to both geographical extent, duration and the degree of spatial synchrony. While some outbreak periods are short with simultanous outbreaks over large distances (high synchrony), others seem to spread in a wave-like fashion. Historical records also indicate that outbreaks may linger for years in local areas causing recurring defoliation and forest death. This is currently the situation in certain areas in Finnmark county in northern Norway.

The factors creating these characteristic population dynamical patterns (cyclicity, spatial synchrony and travelling waves) in moth species in Fennoscandia are not well understood. Syntheses of research conducted over the last three decades have lead to the suggestion that parasitoid-host interaction is the most likely mechanism underlying the the cyclic outbreaks (Ruohomäki et al. 2000, Klemola et al. 2002). Further evidence for this mechanism is currently sought by several Fennoscandian research teams, including our own. The presence of large-scale spatial synchrony has mainly based on Tenow (1972) been considered to be the rule in winter and autumnal moth in Fennoscandia and a climatic Moran effect has been suggested as the most probable synchronizing factor (Tenow 1972; Ruohomäki et al. 2000; Tanhuanpää et al. 2002). However, Ims et al. (2004) demonstrated that local populations of winter moth separated by as little as a few km of unsuitable habitat could be maximally out of phase suggesting that biotic mechanisms acting on a local scale is able to decouple spatial synchrony if such is indeed present. Klemola et al. (2006) on the other hand, analyzed time series of autumnal moth densities by means of cluster analyses and concluded that sampling localities tended to cluster according to geography and that the correlation between time series decreased with distance. Both findings support the presence of spatial synchrony.

Further reading

Berryman, A.A. 1996. What causes population cycles of forest lepidopthera? Trends Ecol. & Evol. 11:28-32.

Bjørnstad, O., Ims, R.A. and Lambin, X. 1999. Spatial population dynamics: analyzing patterns and processes of population synchrony. Trends Ecol. & Evol. 14: 427-431.

Ims, R.A., Yoccoz, N.G., and Hagen, S.B. 2004. Do sub-arctic winter moth populations in coastal birch forest exhibit spatially synchronous dynamics?. J. Anim. Ecol. 73: 1129-1136.

Klemola, T., Huitu, O. and Ruohomäki, K. 2006. Geographically partitioned spatial synchrony among cyclic moth populations. Oikos 114: 349-359.

Liebhold, Koenig & Bjørnstad 2004. Spatial synchrony in population dynamics. Annu. Rev. Ecol. Syst. 35: 467-490.

Mjaaseth, R.A., Hagen, S.B., Yoccoz, N.G., Ims, R.A. 2005. Phenology and abundance in relation to climatic variation in a sub-arctic insect-herbivore mountain birch system. Oecologia 145: 53-65.

Ruohomäki, K., Tanhuanpää. M., Ayres, M.P., Kaitaniemi, P., Tammaru, T. and Haukioja, E. 2000. causes of cyclicity of Epirrita autumnata (Lepidopthera, Geometridae): grandiose theory and tedious practice. Popul. Ecol. 42: 211-223.

Tanhuanpää, M., Ruohomäki, K., Turchin, P., Ayres, M.P., Bylund, H., Kaitaniemi, P., Tammaru, T., and Haukioja, E. 2002. Population cycles of the autumnal moth in Fennoscandia. In Population cycles: the case for trophic interactions ( ed A.A. Berryman ), pp. 142-154. Oxford University Press, Oxford .

Tenow, O. 1972. The outbreaks of Oporinia autumnata Bkh. and Operopthera spp. (Lep. geometridae) in the Scandinavian mountain chain and northern Finland 1862-1968. Zool. Bidr. Uppsala, Suppl. 2, 107 pp.