Honeybees exert flower power over other species

Honeybee on an ivy plant in Cambridge. Photograph by co-author Dr Tom Ings

Honeybees are outcompeting wild pollinators in the UK and new research has found evidence of their aggressive behaviour, recording for the first time the extent to which they restrict access to food sources and attack and chase other species.

The study, led by researchers at Anglia Ruskin University (ARU) and published in the journal Ecological Entomology, took place across two years at five sites in and around Cambridge, all within 300m of known honeybee colonies.

The researchers focused on the behaviour of insects foraging on bramble (Rubus fruticosus) and ivy (Hedera helix), both important sources of nectar and pollen, and visited in large numbers when in flower.

The flower visitors were placed into five categories: honeybees, bumblebees, solitary bees, hoverflies and ‘other’, which included butterflies, beetles and wasps. During 93 hours of observation, researchers recorded all interactions where one pollinator disrupted or prevented another from accessing a flower.

The outcomes were recorded as a ‘win’, where one gained access to a flower resulting in a ‘loss’ of forage for another insect, or a ‘share’, where both gained access with no conflict.

The results found that higher densities of honeybees were associated with both lower numbers of other pollinators and more competitive interactions, with honeybees having a higher proportion of ‘wins’ during their encounters with other flower visitors.

On bramble, a doubling of honeybee density was associated with a 14% reduction in the abundance of bumblebees, while on ivy, an increase in honeybees was linked to fewer solitary bees and hoverflies.

Not only did honeybees win the majority of their interactions, they did this by instigating aggressive interactions, often winning resources by chasing other insects away. Honeybees were actively avoided by other pollinators in 28% of interactions on bramble and 36% on ivy, and they physically displaced or chased rival pollinators in at least one in five winning outcomes on both plant species.

The researchers recorded very few aggressive interactions that were not initiated by honeybees. The only species that contested interactions with honeybees was the ivy bee – a non-native solitary bee that was first recorded in Britain in 2001 – and these were observed defending their food source by displacing and chasing honeybees.

The study is part of a wider project aimed at understanding and mitigating the impact of competition between pollinator species. At a time when the number of honeybee hives are increasing in the UK, with the number of registered beekeepers rising from 19,000 in 2010 to 47,000 last year, the study also suggests climate change could further intensify competition between wild and managed pollinators.

The researchers found that periods when there were higher temperatures were associated with an increase in competitive interactions. It is known that ‘heat stress’ can cause flowers to produce less nectar, and the more limited forage available could be one reason for the increase in interactions between species.

At the same time, pollinators react in different ways to temperature changes, influencing the dynamics of competition. While low temperatures can improve the foraging capacity of hoverflies and bumblebees, which are sensitive to higher temperatures, the opposite is true for honeybees. A previous study found that a 12°C temperature increase led to a 10-fold increase in honeybees leaving their nests.

The full open access study, published in the journal Ecological Entomology, is here https://resjournals.onlinelibrary.wiley.com/doi/10.1111/een.70139