Abstract
We investigated the variation in body colour and its thermoregulatory function in Meteorus pulchricornis (Wesmael) (Hymenoptera: Braconidae), a parasitoid wasp of Spodoptera and other free-living lepidopteran larvae. We show that the body colour of adult wasps darkens when cocoons are kept at low temperatures. The range in the variation in colour, however, differs for different parts of the body and among uniparental (thelytokous) strains. This melanism enables these wasps to attain a body temperature in sunshine up to 2°C. Moreover, this small gain in body temperature can markedly increase the flight activity of wasps at low ambient temperatures. We conclude that the variation in body colour that resulted from rearing the cocoons at different temperatures enable the wasps to adapt to changing thermal environments. The ecological significance of the difference in the degree of melanism of the different strains is discussed.
Highlights
Melanism is the occurrence of darkly pigmented individuals, either as an intraspecific polymorphism or as a difference between species (True, 2003)
The thermal melanism hypothesis (e.g. Watt, 1968; Kingsolver, 1987) posits that body colour is a significant factor affecting body temperature; i.e., dark coloured individuals can attain higher body temperatures than light coloured individuals when exposed to sunlight and dark coloured individuals may be better adapted to cold regions as they can be active for longer periods there, which is essential for feeding, mating or oviposition
We investigated the variation in body colour and its thermoregulatory function in adults of the wasp Meteorus pulchricornis (Wesmael) (Hymenoptera: Braconidae), whose larval stage is a koinobiont endoparasitoid of Spodoptera and other free-living lepidopteran larvae (Huddleston, 1980; Maeto, 1989; Takashino et al, 1998; Berry & Walker, 2004; Liu & Li, 2006; Yamamoto et al, 2009)
Summary
Melanism is the occurrence of darkly pigmented individuals, either as an intraspecific polymorphism or as a difference between species (True, 2003). The thermal melanism hypothesis (e.g. Watt, 1968; Kingsolver, 1987) posits that body colour is a significant factor affecting body temperature; i.e., dark coloured individuals can attain higher body temperatures than light coloured individuals when exposed to sunlight and dark coloured individuals may be better adapted to cold regions as they can be active for longer periods there, which is essential for feeding, mating or oviposition. There are many studies that investigate the role of body colour in determining body temperature in polymorphically coloured insects. In A. bipunctata, melanics are more active than non-melanic individuals when exposed to the radiation from artificial sources of light (De Jong et al, 1996) and field research indicates that melanic adults tend to emerge, mate, oviposit, and die earlier than non-melanic adults (Brakefield, 1984). There are many studies that support the adaptive significance of thermal melanism in insects (Clusella-Trullas et al, 2007), the role of melanism in thermoregulation in parasitoids has yet to be elucidated. According to the physical model of Stevenson (1985a, b), solar radiation and variation in body colour should have a smaller effect on the body temperatures of small insects weighing less than 10 mg, such as many parasitoid wasps, than of large insects, due to their more rapid loss of heat
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