Energy use, diapause behaviour and northern range expansion potential in the invasive Colorado potato beetle

1. As organisms expand their range towards northern latitudes they will encounter selective factors like harsh winter conditions. The ability to cope with and adapt to harsh winters may depend on the variability and evolutionary potential of relevant traits. 2. One adaptation in insects is winter diapause. It is characterized by changes in physiology, behaviour or in both. Physiological changes include lowered metabolic rate that enhances survival by saving limited energy reserves during overwintering. Active behavioural changes like burrowing into the soil allow individuals to escape harsh conditions. 3. We examined variation in overwintering body mass, resting metabolic rate (CO₂ production) and diapause behaviour (burrowing into the soil), and their effects on overwintering success in the adult Colorado potato beetles (Leptinotarsa decemlineata). We conducted a full-sib/half-sib rearing experiment to estimate the evolutionary potential (heritability) of these traits. 4. High overwintering body mass and low metabolic rate were phenotypically associated with high diapause propensity (i.e. burrowing), which was linked to high overwintering survival. We found that once beetles had entered the soil, only large body mass of males was associated with high overwintering survival. However, the heritability estimates in all traits examined were low. 5. Our results show that winter conditions impose selection on diapause behaviour, which was linked to lower metabolic rate and larger body mass. If range expansion to higher latitudes requires adaptive genetic changes in diapause behaviour, metabolism, or body mass, the insufficient genetic variation in these traits suggest that the Colorado potato beetle's future potential to respond to selection due to harsher winters could be limited and thus, its range expansion could be hindered. Both physiological and behavioural adaptations are important to consider when assessing range expansion potential.