Warm Temperatures Reduce Flower Attractiveness and Bumblebee Foraging

Abstract

(1) Background: Plants attract pollinators using several visual signals, mainly involving the display, size, shape, and color of flowers. Each signal is relevant for pollinators foraging for floral rewards, pollen, and nectar. Changes in floral signals and rewards can be induced by an increase in temperature, drought, or other abiotic stresses and are expected to increase as global temperatures rise. In this study, we explored how pollinators respond to modified floral signals and rewards following an increase in temperature; (2) Methods: We tested the effects of warmer temperatures on bee-pollinated starflower (Borago officinalis, Boraginaceae) and determined the behavior of one of its main pollinators, the buff-tailed bumblebee (Bombus terrestris). We measured visual floral traits (display and size) and rewards (nectar and pollen) for plants cultivated at 21 °C or 26 °C. We investigated bumblebee behavior by tracking insect visits in a binary choice experiment in an indoor flight arena; (3) Results: Plants cultivated at 26 °C exhibited a smaller floral area (i.e., corolla sizes summed for all flowers per plant, 34.4 ± 2.3 cm2 versus 71.2 ± 2.7 cm2) and a greater flower height (i.e., height of the last inflorescence on the stem, 87 ± 1 cm versus 75 ± 1 cm) compared to plants grown at 21 °C. Nectar production per flower was lower in plants grown at 26 °C than in plants grown at 21 °C (2.67 ± 0.37 µL versus 4.15 ± 0.22 µL), and bumblebees visited flowers from plants grown at 26 °C four times less frequently than they visited those from plants grown at 21 °C; (4) Conclusions: These results show that warmer temperatures affect floral signals and reduce overall floral resources accessible to pollinators. Thus, the global increases in temperature caused by climate change could reduce plant pollination rates and reproductive success by reducing flower visitation.

Honey bees and climate explain viral prevalence in wild bee communities on a continental scale

“…Here one might argue that these extremes of both precipitation in the warmest and driest quarter as well as temperature in the driest quarter are likely suboptimal for most pollinator species as well as their floral resources. This is either due to a lack of rain-free days for foraging or drought or a combination of drought and heat, which may have a negative impact on the availability and attractiveness of floral resources 44 , 47 . These suboptimal conditions can induce physiological and nutritional stress, which can either result in reduced host populations 73 , 74 and hence lower viral transmission and prevalence or result in an increased virulence of viral infections due to stressed hosts 63 .…”

“…Since most bee pathogens are transmitted via the fecal–oral route, enabling inter- and intra-species transmission via shared flowers 16 , 39 – 42 , climatic variables, such as UV-exposure, temperature and precipitation could potentially influence pathogen survival on flowers and thereby their transmission 40 . Furthermore, climatic variables affect vegetation phenology 43 , flower attractiveness 44 and diversity 45 , which may alter the transmission network via flowers 23 , as well as quality, quantity of floral resources 45 – 47 , which may impact host immunity 48 , 49 and consequently pathogen susceptibility and transmission.…”

Pollination by the hoverflyEristalinus aeneus(Diptera: Syrphidae) in two hybrid seed crops: celery and fennel (Apiaceae)

“…Furthermore, standard beehives are not suitable for pollination in small isolations (Ochiuzzi, 1999; Dag, 2008; Evans et al ., 2019). For spring–summer flowering crops, such as carrot, celery and fennel, the ability of certain hoverfly species like E. aeneus to continue being active at high temperatures can also be an advantage over bumblebees (Nayak et al ., 2020; Descamps et al ., 2021; Sánchez et al ., 2022). Indeed, E. aeneus can cope well with high temperatures (>30°C) and, unlike bumblebees, they do not need to spend time and energy ventilating a hive in order to cool it down and preserve the colony.…”

Pollinaria Reconfiguration Mechanism of Widespread Euro-Mediterranean Orchids: The Effects of Increasing Air Temperature

“…In this study, however, our priority was not to focus on the effects of temperature on insect behavior, but instead, the main goal of the project was to understand how temperature affects the bending mechanism of orchids. In any case, the data in the literature show that warmer temperatures did not directly modify insect behavior in terms of visiting time but decreased floral nectar production, and bumblebees visited flowers suffering from nectar reduction four times less frequently than they visited those plants with a natural concentration of nectar [ 26 ]. In our case, the examined orchids were no nectariferous and so they could not have any modifications of floral reward due to higher temperatures.…”