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Mechanistic model for predicting winter survival of the pine processionary moth based on hourly temperature records

Range shifts have been reported recently for a number of species, but few studies have attempted to explain range shifts in mechanistic terms. In this study we sought a mechanistic understanding of the range expansion in T. pityocampa. We studied larval performance during the winter at two expansion areas: one occurring at a latitudinal (central France) and the other at altitudinal scale (northern Italy). In each area we explored natural temperature gradients as spatial analogues for climate change by rearing cohorts of larvae in three zones along each gradient: the core zone (where the moth has been present for over 30 years), the expansion zone (where recent colonization has occurred), and the external zone (outside its 2003 distribution). This approach allowed us to test a mechanistic model for winter-feeding that was developed from laboratory data, and to assess the importance of feeding in winter survival. The model is based on the combined effect of daytime nest temperature, which induces feeding, and minimum temperature for night feeding. The observed patterns in feeding and survival confirm the model's ability to explain the trends in temperature-linked range expansion, and highlight the potential of similar approaches in improving our understanding of species range shifts.

In the coldest months, our model was consistent with the observed patterns of feeding activity: feeding was progressively reduced with increasing latitude or elevation, as predicted by the lower number of hours when the feeding threshold was reached, and negatively affected final survival. Insolation raised nest temperature and increased feeding activity on the south but not the north aspect. Prolonged temperature drops below the feeding thresholds occurred at all sites, leading to starvation and partial mortality. Nonetheless, even the most extreme sites still allowed some feeding, and, consequently, up to 20% colony survival and successful pupation. Given that the present distribution of the oligophagous T. pityocampa is not constrained by the distribution of its actual or potential hosts, and that warmer winters will cause the number of hours of feeding to increase and the probability of the lower lethal temperature to decrease, we expect the trend of improved survival in previously prohibitive environments to continue, causing further latitudinal and altitudinal expansion.

This work highlights the need to develop temperature-based predictive models for future range shifts of winter-limited species, with potential applications in management. It addresses scientists and managers with a simple tool, based on the measurement of temperature, to assess the potential survival of the pest in the expansion area.

A follow-up paper relating the survival to generally available climatic indexes (mean temperature, rainfall) will allow to extend the prediction over a wide geographic scale.

Reported by

University of Padova
Via Università 16a
35020 Legnaro
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