Bumblebee Behavior on Flowers, but Not Initial Attraction, Is Altered by Short-Term Drought Stress

Abstract

Climate change is leading to increasing drought and higher temperatures, both of which reduce soil water levels and consequently water availability for plants. This reduction often induces physiological stress in plants, which in turn can affect floral development and production inducing phenotypic alterations in flowers. Because flower visitors notice and respond to small differences in floral phenotypes, changes in trait expression can alter trait-mediated flower visitor behavior. Temperature is also known to affect floral scent emission and foraging behavior and, therefore, might modulate trait-mediated flower visitor behavior. However, the link between changes in flower visitor behavior and floral traits in the context of increasing drought and temperature is still not fully understood. In a wind-tunnel experiment, we tested the behavior of 66 Bombus terrestris individuals in response to watered and drought-stressed Sinapis arvensis plants and determined whether these responses were modulated by air temperature. Further, we explored whether floral traits and drought treatment were correlated with bumblebee behavior. The initial attractiveness of drought and watered plants did not differ, as the time to first visit was similar. However, bumblebees visited watered plants more often, their visitation rate to flowers was higher on watered plants, and bumblebees stayed for longer, indicating that watered plants were more attractive for foraging. Bumblebee behavior differed between floral trait expressions, mostly independently of treatment, with larger inflorescences and flowers leading to a decrease in the time until the first flower visit and an increase in the number of visits and the flower visitation rate. Temperature modulated bumblebee activity, which was highest at 25°C; the interaction of drought/water treatment and temperature led to higher visitation rate on watered plants at 20°C, possibly as a result of higher nectar production. Thus, bumblebee behavior is influenced by the watered status of plants, and bumblebees can recognize differences in intraspecific phenotypes involving morphological traits and scent emission, despite overall morphological traits and scent emission not being clearly separated between treatments. Our results indicate that plants are able to buffer floral trait expressions against short-term drought events, potentially to maintain pollinator attraction.

A meta‐analysis of responses in floral traits and flower–visitor interactions to water deficit

“…Finally, in addition to visitation rates, changes due to reduced water availability might also affect pollinator behavior on flowers (Höfer et al, 2021). Behavioral changes, such as increasing handling time, can impact pollen transfer to the stigma and pollen removal and can be better proxies for pollination efficiency than visitation rates (Willcox et al, 2017;Willmer et al, 2017).…”

Pollinator-Mediated Selection on Floral Traits of Primula tibetica Differs Between Sites With Different Soil Water Contents and Among Different Levels of Nutrient Availability

“…A meta-analysis from Kuppler and Kotowska (2021) demonstrated consistent decreases in floral size, number of flowers and nectar volume in response to reduced water availability. Although Höfer et al (2021) found that drought did not significantly affect floral volatiles of Sinapis arvensis plants, the mean total scent emission per flower tended to be higher in drought-stressed plants than watered plants. Nutrient stress decreased flower production by Lithophragma bolanderi ( Friberg et al, 2017 ), whereas soil nutrient addition increased floral display size ( Brodbeck et al, 2001 ).…”

A model of wild bee populations accounting for spatial heterogeneity and climate‐induced temporal variability of food resources at the landscape level

“…Plant crops under drought stress have reduced photosynthesis and thus decreased growth and produce lower reward and visual cues for pollinators (Descamps et al, 2020 ; Rering et al, 2020 ), affecting the nutritional quality and availability of floral resources for bees and having an impact on their survival (Wilson Rankin et al, 2020 ). Simultaneously, drought can induce changes in floral traits and morphology important for interactions with bumblebees (e.g., number of flowers, size of the flower, petal length and width, and depth of the nectar tube), and flowers might be seen as less attractive to forage on (or difficult to handle if they have a reduced size), affecting the bumblebee behavior and resulting in a reduction in the flower visits, as experimental setups found (Höfer et al, 2021 ; Kuppler et al, 2021 ).…”

“…Simultaneously, drought can induce changes in floral traits and morphology important for interactions with bumblebees (e.g., number of flowers, size of the flower, petal length and width, and depth of the nectar tube), and flowers might be seen as less attractive to forage on (or difficult to handle if they have a reduced size), affecting the bumblebee behavior and resulting in a reduction in the flower visits, as experimental setups found (Höfer et al, 2021;Kuppler et al, 2021).…”