Temperature Effects on Growth and Molt of Nematus calais (Hymenoptera: Tenthredinidae)

Abstract

We studied the effects of temperature on growth, consumption, assimilation, conversion efficiencies, and molt of Nematus calais Kirby (Hymenoptera: Tenthredinidae), a willow sawfly, in interior Alaska. Growth rate of fourth instars increased across the full range of naturally encountered temperatures (biomass doubling time of 6.1 d at 6°C versus 0.85 d at 30°C). Growth rate of fifth instars also increased from 6 to 24°C, but had generally lower Q 10S than fourth instars, and actually decreased from 24 to 30°C. Patterns in growth rate across temperatures were almost entirely a function of patterns in consumption rate; relative consumption rate increased sixfold from 6 to 24°C, whereas conversion efficiencies were constant across temperatures (approximate digestibility = 0.41 to 0.45, efficiency of converting digested matter = 0.24 to 0.31). The duration of the fourth molt decreased from 1.32 d at 6°C to 0.17 d at 24°C and then increased to 0.40 d at 30°C. Compared with more southerly distributed insects, growth rates of N. calais tended to be less temperature sensitive and to peak at lower temperatures. Similarities between N. calais and Dineura virididorsata (Retzius), a sawfly from northern Europe, suggest that the Tenthredinidae may be fundamentally different in their molting physiology from the Lepidoptera and Coleoptera; the ability of tenthredinids to molt rapidly at low temperatures may contribute to their ecological success in high-latitude environments.