Equivocal Evidence for Colony Level Stress Effects on Bumble Bee Pollination Services

Abstract

Climate change poses a threat to global food security with extreme heat events causing drought and direct damage to crop plants. However, by altering behavioural or physiological responses of insects, extreme heat events may also affect pollination services on which many crops are dependent. Such effects may potentially be exacerbated by other environmental stresses, such as exposure to widely used agro-chemicals. To determine whether environmental stressors interact to affect pollination services, we carried out field cage experiments on the buff-tailed bumble bee (Bombus terrestris). Using a Bayesian approach, we assessed whether heat stress (colonies maintained at an ambient temperature of 25 • C or 31 • C) and insecticide exposure (5 ng g -1 of the neonicotinoid insecticide clothianidin) could induce behavioural changes that affected pollination of faba bean (Vicia faba). Only the bumble bee colonies and not the plants were exposed to the environmental stress treatments. Bean plants exposed to heat-stressed bumble bee colonies (31 • C) had a lower proportional pod set compared to colonies maintained at 25 • C. There was also weak evidence that heat stressed colonies caused lower total bean weight. Bee exposure to clothianidin was found to have no clear effect on plant yields, either individually or as part of an interaction. We identified no effect of either colony stressor on bumble bee foraging behaviours. Our results suggest that extreme heat stress at the colony level may impact on pollination services. However, as the effect for other key yield parameters was weaker (e.g., bean yields), our results are not conclusive. Overall, our study highlights the need for further research on how environmental stress affects behavioural interactions in plant-pollinator systems that could impact on crop yields.

Response mechanisms to heat stress in bees

“…The adverse effects of heat stress on bees include multiple aspects such as their growth, development, task-related physiology, immunocompetence, foraging activity, pollination services, and reproduction (Alqarni 2020;Bordier et al 2017b;Greenop et al 2020;Medina et al 2018). Heat stress can trigger malformations of the proboscis, stinger, wings, and legs in A. mellifera carnica (Groh et al 2004).…”

“…However, hive location in nectar-rich Acacia trees decreases the negative impacts of hot-dry-windy weather to A. mellifera foraging activity (Alqarni 2020), which indicate that providing high-quality diets and sufficient sources of nectar may help to maintain the foraging activity of A. mellifera under heat stress. Bean plants pollinated by heatstressed bumblebees have lower total bean weight and proportional pod set compared with those pollinated by non-heat-stressed bumblebees (Greenop et al 2020). Increasing heat on stingless bee foragers (Melipona subnitida ) results in increased flight distance in a Brazilian tropical dry forest (Souza-Junior et al 2020), which may finally result in an overall decrease in the foraging activity and pollination services of stingless bees.…”

“…These results indicate that bumblebees and A. mellifera may "benefit" from defending against infection under heat stress to some extent. However, the benefit of heat to defense against infection also incurs costs, because heat stress has adverse effects on immunocompetence, pollination services, task-related physiology, growth, and development of bees (Alqarni 2020;Bordier et al 2017b;Greenop et al 2020;Medina et al 2018).…”

“…Heat stress affects the growth, development, and foraging activity of bees and also increases oxidative damage by promoting ROS generation, thereby threatening the survival of bees (Alqarni 2020;Bordier et al 2017b;Finkel and Holbrook 2000;Greenop et al 2020). However, bees also deploy defense systems against excessive ROS content, thereby slowing the oxidative damage caused by heat stress (Figure 2) (Li et al 2020a, b).…”

“…Among these factors, heat stress affects pollination services and foraging activity, as well as the growth and development of bees; the impacts may be not exactly the same in different bee species (Alqarni 2020;Bordier et al 2017b;Greenop et al 2020;Rasmont et al 2015;Souza-Junior et al 2020). These adverse impacts can become more obvious with climate change, with especially evident changes occurring in the tropics, cold regions, and mountains (Alqarni 2020;Deutsch et al 2008;Kerr et al 2015;Medina et al 2018;Rasmont et al 2015;Soroye et al 2020).…”