How insects handle thermal variation: constant opposed to fluctuating temperatures

Insects are greatly affected by temperature but temperature-dependent performances can be difficult to understand when temperature fluctuates. A factor that adds difficulty is that fluctuations can fall outside the adequate range for regular insect development (permisive range) and enter extreme temperature ranges. We asked how insects are affected by constant temperatures (CT) opposed to fluctuating temperatures (FT) and why that answer might change. We reared pea aphids (Acyrthosiphon pisum) on fava (Vicia faba) in different temperature treatments: 3 constant and 1 fluctuating temperature. One set was in a permissive range (CT: 16, 20, 24, and FL: 16/24 C). The second ranged from permissive to extreme (CT: 24, 28, 32, and FL: 24/32 C). We compared aphid popoulations after 9 days. In a permissive range, CT and FT with the same average temperature (20 vs 16/24) have similar sized populations. But when one temeperature in the FT becomes high (28 vs 24/32), populations can be as much as ten times lower than populations in CT. Our experiment showed that insect response to FT is not equal to the average CT when part of the range is particularly high. In fact, just a short exposure to that high temperature gave aphid populations as low as when aphids were at that high temperature constantly. Even short periods of time in extreme temperatures can have a strongly negative effect on insect performance. This could also mean that in situations with high variability, the range of variability may be less important than the maximum temperature