The processionary moths of the genus Thaumetopoea live on various host plant species in Africa, Asia and Europe, but only a few species among them may colonise conifer trees. These species can be further subdivided according to a main difference in the feeding season, i.e. those defoliating trees across summer ('summer species' such as Th. pinivora on Scots pine in Europe, Th. bonjeani on Cedrus in N Africa and Middle East) or across winter ('winter species' such as Th. pityocampa and Th. wilkinsoni) (Démolin and Frérot, 1993). The winter species are commonly defined as 'pine processionary moth', which in Asia and Middle East are represented by Th. wilkinsoni and in Europe and Africa by Th. pityocampa. The two species were considered as synonyms for a long time but their separation became recently evident thanks to a genetic study (Salvato et al., 2002).
The processionary moth is well distributed in countries in white colour whereas circles indicate countries where the species has been seldom found or its presence needs confirmation.
The life cycle of T. pityocampa in France in relation to variations in altitude and latitude has been rewritten by Speight and Wainhouse (1989) from the work of Démolin (1969).
Using the date of 50% adult emergence (in the example shown, 15 August), the horizontal line predicts the occurrence of subsequent life stages for a particular altitude and latitude (Démolin, 1969).
8. Adult development inside pupae.
6. Post-hibernation processionary larvae moving to pupation sites .
7. Pupal diapause.
5. Post-hibernation development of larvae.
4. Hibernation in nests.
3. Pre-ibernation processionary larvae moving to overwintering sites.
2. Pre-hibernation eggs and young larvae.
The life cycle depends mostly on winter temperature and on the possibility to enter extended diapause at pupal stage. As a general rule, at upper elevation/latitude the feeding ends in late spring, diapause is obligatory, moths emerge in early summer.
At lower elevation/latitude the feeding ends in winter or early spring, diapause is facultative, moths emerge in late summer. The larvae can tolerate temperatures as low as -15°C for several days provided that the colony is numerous and the nest is well constructed.