Comparative analysis of upper thermal tolerance and CO 2 production rate during heat shock in two different European strains of Sitophilus zeamais (Coleoptera: Curculionidae)

Abstract

The maize weevil is a major pest of maize crops and post-harvest grain stocks in tropical countries that became established in western and southern European countries a few decades ago. The hypothesis that this species has adapted to the environmental conditions of temperate climates would be supported by significant adaptative changes in the responses to key ecological factors. Response to heat tolerance is one area likely to show such adaptive changes. In order to test the adaptation hypothesis, a comparative analysis of critical points of the kinetics of CO 2 production rate during a heat treatment, using two Sitophilus zeamais strains originating in France (Sze_05) and in Portugal (Sze_19), was carried out by the “thermolimit respiration test” that allowed three critical temperature levels to be determined: 1) the spiracle closing point (SCP), 2) the heat stupor point (HSP) and 3) the death point (DP). The SCP was observed at 45.2 °C and 44.3 °C for Sze_05 and Sze_19 respectively. The HSP was observed at 46.9 °C and 46.3 °C and the DP was observed at 50.0 °C and 48.6 °C respectively for the two strains. The difference between the two strains was significant only for the DP temperature ( P = 0.045). Comparison of adult and larval stages revealed a very different physiological response to a “fast” heat increase, with a spiracle closure phase much more marked for larvae than for adults. The understanding of the influence of environmental adaptation on physiological response to heat stress by the thermolimit respirometry test requires further study using fresh strains of S. zeamais originating from temperate countries. However, the methodology used in this trial can be useful for the rapid assessment of the “heat tolerance profile” for insect populations infesting cereal grain or food processing factories so that schedules for heat disinfestation of cereal processing plants can be optimised.