Abstract
Short photoperiods often accelerate preimaginal development in insects that overwinter as adults and thus ensure the timely development of the diapausing stage. However, fast development usually results in small adults with few reserves whereas successful overwintering is dependent on sufficient reserves of nutrients. Thus, an insect faces two opposite challenges: increase adult weight or speed up pre-adult development. One solution to this time-size trade-off is fast development resulting in relatively light-weight but not too small adults, which are able to survive if there is an early onset to winter and have the possibility of further increasing their weight if conditions in autumn remain favourable for longer than usual. To test this hypothesis, we determined the effect of different photoperiods (day lengths of 12 and 18 h at 20°C) on the duration of preimaginal development, size of emerging adults and dynamics of adult weight in Harmonia axyridis. The larvae reared under conditions tended to develop faster and invest resources in structural growth rather than in increasing weight: the daily rate of increase in size under a 12 h long photoperiod was greater than that under a 18 h long photoperiod, whereas the rate of gain in weight showed the opposite tendency. This resulted in a significant decrease in the weight /size ratio of emerging adults. However, their low initial nutritional status was partly compensated for by adult feeding: the relative gain in adult weight of short-day individuals was greater than that of the beetles that developed under long day conditions.
Highlights
It is known that short day length often accelerates preimaginal development in insects that overwinter as adults, which ensures the timely development of the diapausing stage prior to the beginning of winter (Tauber et al, 1986; Nylin & Gotthard, 1998; Saunders et al, 2002; Musolin et al, 2004; Lopatina et al, 2007)
Successful overwintering depends on sufficient fat and glycogen reserves, which are often positively correlated with body weight (Hahn & Denlinger, 2011; Hodek, 2012; Clemmensen & Hahn, 2015)
1st instar larvae that have just hatched from eggs laid over a period of 24 h by more than 50 females from the laboratory culture were evenly distributed between two different photoperiodic treatments
Summary
It is known that short day length often accelerates preimaginal development in insects that overwinter as adults, which ensures the timely development of the diapausing stage prior to the beginning of winter (Tauber et al, 1986; Nylin & Gotthard, 1998; Saunders et al, 2002; Musolin et al, 2004; Lopatina et al, 2007). The speciesspecific growth rate is naturally limited and fast development often results in small and light adults (Abrams et al, 1996; Kingsolver & Huey, 2008; Nijhout et al, 2010; Dmitriew, 2011; Pöykkö & Hyvärinen, 2012; Salminen et al, 2012; Chen et al, 2014). In some species both goals are achieved (Leimar, 1996; Kutcherov et al, 2011), in most cases a compromise is adopted
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