Insects and especially wild pollinators are on the decline in north-western Europe and North America. And since pollination is important to nature's ecosystems, the decline in the number of pollinating insects is worrying. Therefore, new policies are being introduced in relation to the management of natural areas that house wild pollinating insects. A recurring concern has been the potential competition for flower resources between wild bees and honey bees. Researchers from Aarhus University, among others, have reviewed the literature to improve their understanding of the potential competition between wild bees and honey bees.

“Wild bees are not only dependent on flower resources, but also on being able to find suitable habitats to build nests and overwinter. There are therefore several reasons why competition between species may arise. However, there may be significant challenges associated with assessing the effect of honey bees on wild bee population dynamics. To create a better understanding, we have focused on the diversity of the food sources that are utilised by the different species,” explains Claus Rasmussen from the Department of Agroecology at Aarhus University.

Almost 300 different wild bees in Denmark

In Germany, researchers have for a century or more documented the food sources for the German species of wild bees, which gives this group of researchers the possibility to gather information about all of the 292 different species of wild bees that are known from Denmark (19 are regionally extinct).

“We have investigated the interaction between honey bees and wild bees in Denmark. Our focus has been on the flower resources shared by honey bees and wild bees. We looked at both threatened bee species and food-specialised species, as we expected that small populations of threatened bee species and pollen specialists are the most sensitive to competition from among others honey bees,” explains Claus Rasmussen.

The researchers have mapped the 292 different species of wild bees that are found in Denmark based on their position on the Danish red list of threatened species, as well as based on their ability to find food.

Specialists and generalists

When it comes to bee food, the different species do not necessarily live of the same flowers. There are bees that can only raise their larvae on pollen from e.g. field scabious, while others use only pollen from willow. These specialists have their entire active life cycle, when these plants are in bloom. Other bees can use pollen from a wide variety of flower species to raise the larvae. 

“In research, we use the term oligolectic, about the bees that are food specialists. That is, they can only use pollen from quite a few plant species, while polylectic species can use pollen from a wide variety of flowering plant species. For honeybees, for example, we know 294 different plant genera in Denmark, from which they can collect nectar and pollen,” says Claus Rasmussen.

And competition occurs between species that utilise the same limited resource. 

"It is far from all species of wild bees that are threatened by or compete with the honey bee, but we can see from our study that there are some species with a high overlap of food and at that at the same time is under pressure. It is those species, we need to be worried about,” says Claus Rasmussen.

Species of concern

According to the researchers, it is important to identify so-called attention-demanding species to ensure the conservation of the wild bee species. 

“We have set up parameters that make it possible to identify species and areas on the basis of concern. This is based on a high food overlap of more than 70% as well as a position as vulnerable, endangered, or critically endangered on the Danish Red List,” explains Claus Rasmussen.

The parameters can be used when making decisions regarding the management of honey bees in areas with known populations of threatened species. For example, the study shows that there is a total of six different species of oligolectic bees, which are threatened and have a food overlap of more than 70% with the honey bee.

The six species are:

• Andrena lathyri
• Andrena marginata
• Dasypoda suripes
• Dufourea halictula
• Dufourea inermis
• Hoplitis anthocopoides 

The table below summarises the results, while the researchers have made a complete count for each species in the report "Evaluating competition for forage plants between honey bees and wild bees in Denmark".

Table 1. Respectively number of living Danish species of bees, non-threatened species, and threatenedspecies (according to the Danish red list categories) that share 50%, 70%, or 90% of their food sources with honey bees. Categories are for all bees, but also divided into polylectic, oligolectic, and kleptoparasitic species. The latter does not collect pollen but takes over the nest and uses pollen from its host. All 19 species that have disappeared in Denmark are not included in the table, and the same applies to the honey bee. In addition to species that have been assessed as' endangered 'and' not endangered ', there are a number of species where there is insufficient information to make an assessment of the risk of extinction based on the species' distribution or population status.

Precautionary principle

“It must be emphasised that our analysis does not determine whether competition between honey bees and other bees can be registered in Denmark, or what density of honey bees may be sustainable in a given area in relation to the occurrence of threatened species of bees. On the other hand, we have identified a set of parameters for all species of bees, which based on a high degree of food overlap with honey bees and red list status can lead to better management in nature,” says Claus Rasmussen.

More specifically, researchers have identified the individual species of wild bees that require attention in relation to beekeeping. In other words, it is species that have a food overlap with honeybees of over 70%, and which at the same time have a Danish red list status, as being threatened - either as vulnerable, endangered, or critically endangered.

"Until we fully understand the complex natural interactions between honey bees and threatened bee species, it seems advisable to apply a precautionary principle and avoid placing hives in areas close to the threatened bees during their active flight period, where food overlap is highest," explains Claus Rasmussen.