Thermal biology of the invasive ladybird Harmonia axyridis: physiological plasticity, thermoregulatory behaviour and fitness
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Physiological and behavioural responses to changes in temperature may enable ectotherms to buffer the effects of climate change or facilitate movement into novel environments. The thermal biology of the harlequin ladybird Harmonia axyridis is of considerable interest since it is a globally invasive species and threatens native aphidophagous coccinellids. Here, laboratory experiments were used to assess the effects of thermal acclimation (rearing temperature) on the thermal performance curve for walking speed, and to test the assumptions of the cost benefit model of thermoregulation, while also examining subsequent fitness consequences. Results showed that exposure to temperature treatments either during larval development or adulthood significantly affected thermal preference, thermal tolerance and locomotion in adult H. axyridis. The responses varied between the traits measured and the life stage during which exposure occurred, in some cases reflecting complex trade-offs. Thermal preference was affected by thermal acclimation during adulthood, and was 15 - 20°C lower than the optimum temperature for walking speed in all treatment groups. Critical thermal maximum ¬ (upper activity limit) was strongly affected by exposures to temperature treatments as adults, but critical thermal minimum did not vary. Using thermal arenas in which thermal quality was artificially manipulated (high quality vs. low quality environment), the movement of individual H. axyridis females was tracked over a five-day period, after which fecundity was scored. H. axyridis spent longer periods travelling and moved further in the low quality arenas compared to the high quality arenas but maintained body temperatures further from the preferred temperature in the low quality arenas; no significant fitness consequences were observed in terms of fecundity in either arena type. Combined, these findings suggest that H. axyridis uses both thermal acclimation and thermoregulation to readily adjust to changing conditions, which may contribute to its invasiveness. These responses should be accounted for when assessing the ecological impacts of climate change on invasive species.