Thursday, December 1, 2011

Local Evolution in the Large White Butterfly

Large White Butterfly Larva, Pieris brassicae, feeding
The bane of any gardener attempting to grow a few tasty brassicas, the larvae of the Large White Butterfly (Pieris brassicae) is a rapacious feeder. Reports in the past of 100% crop loss (1) may have been reduced but globally the loss to its main 18 food plants runs to 1% of total yield (2). Such heavy damage by this widespread, Palaerartic species is due to the practice of females laying batches of eggs, as opposed to its close relatives who tend to lay eggs singly (3). The results can be devastating to behold, with plant leaves reduced to skeletal forms.
Large White Butterfly Adult, Pieris brassicae
Like many other insect species, P. brassicae undergoes diapause in response to short photoperiods which serves to outlast periods with unsuitable conditions such as low temperature and low food supplies (4). However, one population of P. brassicae in Andalusia in southern Spain has evolved an additional response to long photoperiods (3). This summer diapause (or aestivation), which is only found in this Spanish population, last three months and represents something of  a major evolutionary novelty, because it cannot have evolved via a gradual transformation of the ancestral photoperiodic response in which only short day-lengths induced diapause. Its role is unclear as the butterfly loses three generations by undergoing aestivation. It has been shown that the attack rate of its main parasitoid, Cotesia glomerata is reduced as a result of aestivation, thus increasing survival rate of the larvae (5).
Large White Butterfly Larvae, Pieris brassicae
References:
  1. Cansdale, 1876. Entomologist 8-9 p. 257
  2. Feltwell, 1978. Journal of Research on the Lepidoptera 17 pp. 218-225
  3. Held and Speith, 1999. Journal of Insect Physiology 45 pp. 587-598
  4. Beck, 1980. Insect Photoperiodism
  5. P√∂rschmann and Spieth, 2011. Entomological Science 14 pp. 31–36

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