Abstract
At the south western border of its extensive distribution, the multivoltine large white butterfly, Pieris brassicae L., is exceptional in undergoing summer diapause or aestivation. In all other regions investigated, P. brassicae pupae only hibernate. The transitional zone from non-aestivating to aestivating populations is a geographically stable region south of the Pyrenees. The restric- tion of this response to this region cannot be accounted for in terms of genetics as aestivation is intermediately inherited, with the heritability (h 2 ) of aestivation in inbreeding lines between 0.35 and 0.77. Two hypotheses are presented to explain why this species does not aestivate in more northern regions. First, aestivation is a behaviour that serves to synchronize generations in areas where this species produces a high number of generations per year. Second, aestivation reduces the incidence of parasitism suffered by the butterfly by desynchronizing its life cycle from that of its main parasitoid, Cotesia glomerata. The two hypotheses are not mutually exclusive and both seem to be adaptive where the species is multivoltine.
Highlights
Insects have evolved numerous life history strategies, which have enabled them to adapt their ontogeny and reproduction to seasonal changes in their environment (Masaki, 1996; Tauber et al, 1986; Musolin & Saulich, 1999; Danks, 2002, 2006a)
In all of the populations of P. brassicae investigated for a photoperiodic response, only those from the Iberian Peninsula responded by undergoing a summer diapause (Table 1)
This well known species is a classical example of a multivoltine species, which undergoes hibernation diapause induced by day lengths, which is dependent on the geographical location of the population
Summary
Insects have evolved numerous life history strategies, which have enabled them to adapt their ontogeny and reproduction to seasonal changes in their environment (Masaki, 1996; Tauber et al, 1986; Musolin & Saulich, 1999; Danks, 2002, 2006a). Multivoltine species rely on exogenous factors such as environmental signals to trigger a developmental arrest This has the advantage that species may respond flexibly to varying life conditions (Bradshaw & Holzapfel, 2007), producing a continuous series of generations until environmental conditions force a developmental arrest. This process is termed facultative diapause and enables a species to adjust the number of generations to local conditions and it further allows to exploit the whole period for which circumstances remain favourable in different geographical regions (Spieth, 1985; Gomi, 1997; Takeda, 1997)
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