Can sunspot activity and ultraviolet–B radiation explain cyclic outbreaks of forest moth pest species?

Abstract

Cyclic outbreaks of forest moth pest species have long remained a puzzle for foresters and ecologists. This paper presents time-series exhibiting a strong negative relationship between sunspot numbers and population indices of autumnal and winter moths, both in a mountain birch forest in central Norway and in a mixed lowland forest in southern Norway. In the latter area, also the population level of a moth species feeding entirely on lichens was negatively related to sunspot numbers. Low sunspot activity leads to a thinner ozone layer and thus higher surface ultraviolet (UV)-B radiation. As winter moth larvae prefer leaves subjected to enhanced UV-B radiation, we suggest that the causal relationship between sunspots and moths is that the metabolic costs of producing UV-B-protective pigments during periods of low sunspot activity reduce trees' and lichens' resistance to herbivores, and thus increase the survival of moth larvae. Higher peak densities of moth cycles in mountain forests could be explained by the general higher UV-B radiation at higher altitudes.

Periodic travelling waves in cyclic populations: field studies and reaction–diffusion models

“…Changes through time in spatial dynamics were also found recently for the cyclic autumnal moth populations in northern Scandinavia Tenow et al 2007). Studies by Selås et al (2001Selås et al ( , 2004 suggest that synchrony in the population cycles may be induced by region-wide synchrony in their food supply, which is possibly connected to sunspot activity. In contrast, Tenow et al (2007) showed that the population cycles of the autumnal moth were sometimes organized into periodic travelling waves, and on other occasions the cycles occurred synchronously over wide areas.…”

Recurrent and synchronous insect pest outbreaks in forests

“…Particular weather conditions in Spring enhance synchronization of egg hatch and leaf development, thus giving rise to an increase in fertility. This means that the parameter c increases -Sunspot troughs (Myers, 1998;Selas et al, 2004;Johnson et al, 2005). In a period of low sunspot activity, surface ultraviolet B radiation is high so that the production of protective phenolics increases at the expense of production of chemicals for herbivore resistance so that the half-saturation constant b decreases -Acidic deposition (Gragnani et al, 1998).…”

“…Some outbreaks are impressive and involve variations of insect density from 1 to 10000 (or more), like in the case of larch budmoth (Zeiraphera diniana) in the Oberengadin Valley (Baltensweiler, 1989). Sometimes the outbreaks are triggered by the occurrence of quite peculiar random exogenous factors (Berryman, 1996;Jones et al, 1998;Myers, 1998;Selas et al, 2004;Johnson et al, 2005) in areas with susceptible forest stands and then eventually propagate to other areas ("epicentre hypotheses" Hardy et al (1983)), giving rise to recurrent mosaics of infestation. By contrast , in other cases, the outbreaks are due to endogenous density-dependent mechanisms and are therefore periodic or almost periodic.…”

“…After the investigations of recurrent insect pest outbreaks in many forests around the world, the attention has shifted toward outbreak synchronization (Barbour, 1990;Myers, 1988;Hawkins and Holyoak, 1998;Liebhold and A. Kamata, 2000;Williams and Liebhold, 2000;Peltonen et al, 2002;Selas et al, 2004;Raimondo et al, 2004;Johnson et al, 2005) and, more recently, toward waves of outbreaks over large areas Bjorstad et al, 2002;Johnson et al, 2004;Tenow et al, 2007). On the basis of the available information it seems that synchronization is ubiquitous, in particular when outbreaks are of remarkable intensity, i.e., when they concern those insects that are called pests.…”

Population fluctuations of moths and small rodents in relation to plant reproduction indices in southern Norway

“…By this approach, we removed positive 1-year autocorrelations in the series and also the impact of long-term fluctuations, such as a 10-year pattern known for some moth species in our study area (Selå s et al 2004). The second variable used in the moth analyses was the mean temperature during the flight months, because some moth species may increase their flight activity in periods with high temperatures.…”