Impact of climate change on pea aphid development: Is temperature increase the only driver?

There has been a long debate about aphid capacity to adapt development rates to local conditions. Insect development rates vary as a function of temperature and individual variations follow a normal distribution, which provide suitable conditions for natural selection. Global average temperature has been increasing at a rate of 0.18 °C per decade, and most models predict shorter insect development times. In fact, milder winters have been associated with aphid outbreaks, but few studies have examined the impact of other climatic variables such as the number freezing events. Here, we examine the date of the first pea aphid capture in pan traps in a twelve-year dataset to test climate change model predictions of earlier aphid arrivals over time. We found that average temperature in the study sites has been increasing at a rate of 0.10 °C, and that the number of days with sub-zero temperatures has been constant. However, we did not find any evidence supporting an earlier arrival of aphids over time with average temperature increase or milder winters. We found that the number of degree days (DD) required to trigger the first pea aphid capture in the spring is increasing at a rate of 23 DD per year, and that the first capture is delayed by 0.18 days per sub-zero day increase during the first semester. We show that, even though pea aphids might be developing faster with temperature increases, a selection pressure against early developers caused by freezing temperatures is preventing them to arrive earlier to crops.