Environment

Impact of the management methods
of herbaceous areas on wild bees

authors Olivier Lambert (CVFSE), Philippe Férard (JB Nantes)
Andrena sp ©Erwan Balança

general decline in the bee population

Populations of wild and domesticated bees have undergone a general decline and many factors have been blamed for this :
  • the mass and systematic use of pesticides
  • extensive landscape modification
  • depletion of food and nesting resources linked with urbanisation
  • industrialisation and agriculture
  • global warming etc.

Nevertheless, pollinising insects are essential to maintaining biodiversity and agricultural production, as they ensure the fertilisation of nearly 80% of flowering plants.

a new study

Numerous studies have revealed the key role of the development and management of green spaces on the presence and maintenance of vital pollinising insect populations. With that in mind, the CVFSE/Oniris, Nantes’ Green Spaces and Environment Service (SEVE) and the City of Nantes’ planning department have worked together on a study to evaluate the dietary preferences of wild bees and to specify which management method (in terms of timing and frequency of mowing) creates the best environment for the presence and maintenance of wild bee populations. This will allow us to factor them into planning policies and management methods of city green spaces more efficiently.

3 sites and different zones

Three sites in Nantes were surveyed in 2016: Bottière-Chénaie, Grand Blottereau Park and the Bois des Anses (Doulon Gohards). On each of these sites, four herbaceous zones were chosen, each subject to a specific management plan: zone 1 was cut regularly (lawn, with multiple uses), zone 2 was mown in mid-May (early mowing), zone 3 was mown in mid-June or at the end of June (mown, then grass waste removed) and zone 4 was mown in September (late mowing).

Each zone was examined once a month between April and September 2016. The surveys were completed by two people alternatively scanning each zone for 10 minutes and capturing all the wild bees they encountered using a net. At the same time phytosociological surveys were completed on each of the different zones.

Bottière Chénaie Grand Blottereau Bois des Anses
Fig. 1 Views of the three sites.

414 wild bees

In total, 414 wild bees were captured and identified. They belonged to 15 different genera and 56 different species. Four genera represented 88% of those captured, of which 40% were Lasioglossum, 22 % Bombus, 16 % Halictus and 10 % Andrena. Four species represented 53% of those captured of which 22 % were Lasioglossum malachurum, 13 % were Halictus scabiosae, 12 % were Bombus pascuorum and 6% were Bombus lapidarius. For 22 species (40% of the species), only one individual was captured (5% of those captured).

Sites

bees captured

genera identifed

species identified

Bottière Chênaie

149

14

41

Grand Blottereau

185

10

34

Bois des Anses

80

5

18

TOTAL

414

15

56

Fig. 2 – Table showing the bees captured at each site

Generally, the zones of each of the sites were unequal in terms of their plant coverage and their populations of wild bees. The 4 zones at the Bois des Anes site presented the largest diversity of flora with 88 different species. However, the lowest number of wild bees were captured there (80). On the other hand, Grand Blottereau had the lowest diversity of flora (with 54 different species) but was where the largest number of wild bees were captured (185). The zones at the Bottière Chénaie site fell between the two with 81 plant species identified and 149 wild bees captured.

The presence of wild bees in the different zones depended on the presence and flowering of certain plant species, including “food resource plants” which are used for the collection of nectar and/or pollen. These plants are present due to the history of the site and its management methods. The majority of the wild bees that were captured were common and generalist species, with a fairly broad dietary spectrum.

27 plant species

Over all the zones on all the sites only 27 plant species were used by the bees, with nearly 90% of the wild bees collected on a third of these species (9 species). From this we can see that Hypochaeris radicata (Catsear/false dandelion), Trifolium pratense (Red clover), Crepis capillaris (Smooth Hawksbeard), Convolvulus arvensis (Field Bindweed), and Bellis perennis (Common daisy) are the principle species used by the bees. Broadly speaking, the bees were captured on perrenial plant species (dicotyledons) prevalent on each of the sites, often flowering over a long period and and most often belonging to the Fabaceae or Asteraceae families.

The 27 species foraged by the bees/the number of wild bees captured on these species/over the 3 sites

Fig. 3 The 27 species foraged by the bees/the number of wild bees captured on these species/over the 3 sites

One concern was that it was not possible to respect all of the different management methods recommended for this experiment (due to weather conditions, issues with equipment). With these results, it’s diffilcult to draw general conclusions. Nevertheless, the results show that mowing herbaceous areas directly impacts the presence of wild bees (there was a clear reduction in the number of captures after mowing) due to the change in the availability of food resources. Method 4 (late mowing) can appear more favourable to wild bees because the plant cover is preserved and method 1 (regular mowing) can appear disadvantageous.

However, the results of this study are more nuanced and show that each of these zones, whichever site they are from, are alternatively more attractive over the course of the season. A combination of all of the methods seems like the ideal compromise for wild bees and probably for numerous other species of fauna.

Inventory of captured wild bees – from left  to right, Olivier Lambert, Marie Le Brasidec and Doriane Blotière of CVFSE/Oniris
Fig 4. - Lasioglossum sp and Hypochareis radicata ©Erwan Balança

Finally, if food resources are an indispensable component for the introduction and maintenance of wild bee populations, the presence and conservation of nesting sites are also equally important. These two types of resources have to be simultaneously present in a confined area. Adapting the methods used to manage these green spaces will ensure the sustainability of communinities of wild bees in Nantes.