Although the impact of warming on winter limitation of aphid populations is reasonably well understood, the impacts of hot summers and heat wave events are less clear. In this study, we address this question through a detailed analysis of the thermal ecology of three closely related aphid species: Myzus persicae, a widespread, polyphagous temperate zone pest, Myzus polaris, an arctic aphid potentially threatened by climate warming, and, Myzus ornatus, a glasshouse pest that may benefit from warming. The upper lethal limits (ULT(50)) and heat coma temperatures of the aphid species reared at both 15 and 20 degrees C did not differ significantly, suggesting that heat coma is a reliable indicator of fatal heat stress. Heat coma and CT(max) were also measured after aphids were reared at 10 and 25 degrees C for one and three generations. The extent of the acclimation response was not influenced by the number of generations. Acclimation increased CT(max) with rearing temperature for all species. The acclimation temperature also influenced heat coma; this relationship was linear for M. ornatus and M. polaris but non-linear for M. persicae (increased tolerance at 10 and 25 degrees C). Bacteria known generically as secondary symbionts can promote thermal tolerance of aphids, but they were not detected in the aphids studied here. Assays of optimum development temperature were also performed for each species. All data indicate that M. persicae has the greatest tolerance of high temperatures.
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