Seasonal variation in exploitative competition between honeybees and bumblebees

Abstract

Honeybees (Apis mellifera) and bumblebees (Bombus spp.) often undergo exploitative competition for shared floral resources, which can alter their foraging behaviour and flower choice, even causing competitive exclusion. This may be strongest in summer, when foraging conditions are most challenging for bees, compared to other times of the year. However, the seasonal dynamics of competition between these major pollinator groups are not well understood. Here, we investigate whether the strength of exploitative competition for nectar between honeybees and bumblebees varies seasonally, and whether competitive pressure is greatest in summer months. We carried out experimental bee exclusion trials from May to late September, using experimental patches of lavender, variety Grosso, in full bloom. In each trial, we compared the numbers of honeybees (HB) foraging on patches from which bumblebees had been manually excluded (bumblebee excluded, BBE) versus control (CON) patches, HB (BBE-CON). This measure of exploitative competition varied significantly with season. As expected, mean HB (BBE-CON) was significantly greater in summer trials than in spring or autumn trials. This was despite high nectar standing crop volumes in BBE patch flowers in spring and autumn trials. Mean HB (BBE-CON) was not different between spring and autumn trials. Our results show that nectar competition between honeybees and bumblebees varies seasonally and is stronger in summer than spring or autumn, adding to current understanding of the seasonality of resource demand and competition between bee species. This information may also help to inform conservation programs aiming to increase floral resources for bees by showing when these resources are most needed.

Plants and pollinators: Will natural selection cause an imbalance between nectar supply and demand?

“…Honey bee colonies gain more weight during spring (Balfour et al, 2017; Garbuzov et al, 2020) and a greater proportion of foragers return with empty crops in summer than spring (Couvillon et al, 2014). In addition, standing crops of nectar in lavender flowers are greater in spring than summer (Wignall et al, 2020). Summer is also a season of intense nectar competition among flower visitors (e.g., Inouye, 1978; Weatherwax, 1986; Balfour et al, 2015; Balfour et al, 2021) during which foragers compete for flowers containing minute quantities of nectar (c. 0.1 μl; Williams, 1998; Balfour et al, 2015) and nectar robbing among honey bee colonies occurs (e.g., Sakofski et al, 1990).…”

Honey robbing: could human changes to the environment transform a rare foraging tactic into a maladaptive behavior?

“…Floral resources decline substantially in late summer [46]. Simultaneously, other factors compound the effects of food shortage, including increased competition from other bee species at their peak population abundance [47], heightened pressure to accumulate sufficient honey for the winter [48,49], and declining temperatures, which make nectar processing more energetically costly [50]. Further studies across a range of climate regions are needed to determine how robbing risk is predicted by any one of these seasonal changes.…”

Honey bee hives decrease wild bee abundance, species richness, and fruit count on farms regardless of wildflower strips

“…Research in native plant systems suggests competition between managed and wild bees over floral resources can suppress wild bee abundance and plant seed set 4 – 7 . This may be especially true under resource limitations 6 – 8 , which can increase niche overlap between honey bees and wild pollinators and heighten negative impacts on wild pollinator populations 9 – 12 . Competition from managed bees can affect wild pollinator populations and crop production in agroecosystems as well 13 – 17 .…”