The effects of low temperatures on metabolic rates in Anopheles quadriannulatus

Abstract

The effects of low temperatures on the metabolic rates of Anopheles quadriannulatus were investigated by measuring the rate of carbon dioxide emission (VCO2) at different temperatures using a closed respirometry system. In general, the metabolic rates of A. quardiannulatus decreased with decreasing ambient temperatures. However, the decreases were not uniform over the temperature range of 30 °C and 10 °C. Mean VCO2 measured at 30 °C and sometimes at 25 °C, were statistically different from those measured at other temperatures. Surprisingly, at 15 °C and 10 °C, the mean VCO2 were statistically similar. It was found that metabolic rates of the mosquitoes were affected differently if a 5 °C reduction in temperature occurred at 30 °C and when it occurred at 15 °C. Thus, a 5 °C drop in temperature at 30 °C and 15 °C did not yield similar percentage decrease in metabolic rates. Age was found not to impact on the metabolism of mosquitoes except at 10 °C and 15 °C in blood fed mosquitoes (both mated and unmated). Lack of a correlation between age and metabolic rates in these mosquitoes was a result of constant body masses as mosquitoes aged. Mean VCO2 of four days old mosquitoes in different mating and feeding states were found to be significantly differently only at 10 °C. Mated and blood fed mosquitoes from this age group retained constant metabolic rates despite changes in temperature. The finding that A. quadriannulatus is not capable of significantly dropping its metabolic rates at low temperatures (15 °C-10 °C) is an indication that the species most probably lacks a physiological overwintering mechanism. Consequently, its survival and longevity is highly compromised during winter. Thus, it can be concluded that if adult A. quadriannulatus can overwinter in cooler parts of sub-Saharan regions, it does so using non-physiological mechanisms. Otherwise, this species and related members of Anopheles gambiae overwinters in developmental stages other than adulthood. In this way, these mosquitoes are able perpetuate their populations post winter season.