Effects of aposematic coloration on predation risk in bumblebees? A comparison between differently coloured populations, with consideration of the ultraviolet

It has been proposed that sympatric bumblebee species form mimicry rings to profit from learnt avoidance behaviour by predators. This hypothesis can be tested by comparing the predation rates of local bumblebees with those of imported non-native bumblebees, whose coat colour is different from that of local bees, so that their coloration is unfamiliar to local predators. To test whether populations of non-native bumblebees suffer higher worker loss rates during foraging, we conducted transplant experiments in the UK, Germany and Sardinia. The loss rates of foraging workers of four Bombus terrestris populations (Bombus terrestris canariensis, Bombus terrestris terrestris, Bombus terrestris sassaricus and Bombus terrestris dalmatinus) were compared, evaluating data from 989 foragers, whose flight times were monitored precisely (over 8258 h of foraging). While all of these workers display a bright UV-reflecting abdominal tip, the colours in other body parts differ strongly to the eyes of avian predators. The hypothesis that foragers from the non-native bumblebee populations, which differ in coloration from the local native population, would suffer higher predation risk was not upheld. In contrast, in one location (Sardinia) the native population had the highest loss rate. The consistent population rank order we found in terms of forager losses indicates that such losses are more prominently affected by factors other than the familiarity of local predators with aposematic colour patterns.