Unlike most other insects, the larval development of the pine processionary occurs during fall and winter and is highly sensitive to slight temperature variations. Winter warming increases the survival of the caterpillars and allows them to complete their development in areas where climatic conditions were previously hostile to them. The climatic constraints modulating this development have been precisely established since the mid-2000s:
The lethal temperature, below which not all processionary colonies can survive, has been estimated at -16°C, a value that is now very rarely reached in most parts of Western Europe.
But beyond their survival, caterpillars must also feed to complete their development until pupation. They leave the nest at night to devour pine needles, this night outing being subject to two conditions [1]:
– The nest must have had a minimum temperature of 9°C during the day (activation temperature);
– The air temperature must be above 0°C during this night (supply temperature).
This allows us to calculate the mortality and the number of nights that the caterpillars can feed during winter, and conversely the number of consecutive days of famine, which must remain limited to allow for full development [2].
2. From the late 1970s, a gradual lifting of thermal constraints limiting the insect’s range was achieved
[1] Battisti A., Stastny M., Netherer S., Robinet C., Schopf A., Roques A., et al. (2005) Expansion of geographic range in pine processionary moth caused by increasing winter temperatures. Ecological Applications15, 2084-2096.
[2] Buffo E., Battisti A., Stastny M., Larsson S. (2007). Temperature as a predictor of survival of the pine processionary moth in the Italian Alps. Agricultural and Forest Entomology9, 65–72.
[3] Robinet C., Baier P., Pennerstorfer J., Schopf A., Roques A. (2007). Modelling the effects of climate change on the potential feeding activity of Thaumetopoea pityocampa (Den. & Schiff.) (Lep., Notodontidae) in France. Global Ecology and Biogeography16, 46-471.
[4] Roques A., Rousselet J., Avci M., Avtzis D. N., Basso A. et al. (2015) Climate warming and past and present distribution of the processionary moths (Thaumetopoea spp.) in Europe, Asia Minor and North Africa. pp 81-162 In Roques A. (Ed.) Processionary Moths and Climate Change: An Update. Springer/ Quae.
[5] Battisti A., Avci M., Avtzis D.N., Ben Jamaa M.L., Beradi L. (2015). Natural History of the Processionary Moths (Thaumetopoea spp.): New Insights in Relation to Climate Change. pp. 15-79 In Roques A. (Ed.) Processionary Moths and Climate Change: An Update. Springer/ Quae.
[6] Robinet C, Imbert C.E., Rousselet J., Sauvard D., Garcia J., et al. (2012). Warming up combined with the trade of large trees allowed long-distance jumps of pine processionary moth in Europe. Biological Invasions14, 1557–1569.
[7] Moneo I., Battisti A., Dufour B., Garcia-Ortiz J.C., González-Muñoz M. et al. (2015). Medical and veterinary impact of the urticating processionary larvae. pp. 359- 410 In Roques A. (Ed.) Processionary Moths and Climate Change: An Update. Springer/ Quae.