Urban bees – can businesses help through green infrastructure?

In this study we focus on honeybees and other pollinators in an urban landscape. The central point of the study are the Grey Street Gathering planters: two small flowerbeds that are seasonally installed in the very heart of Newcastle upon Tyne. Apart from the social and economical benefits this novel green space brings, we aim to establish whether small sites like this can have an impact on urban pollinators. In order to do so, we monitored 14 urban flower beds of different sizes in Newcastle. In addition, we also monitored two rural sites for comparison. At each site, we identified the plants in flower, counted the numbers of floral units per plant species and observed which insects (and how many) were visiting the flowers. An overview of the sites is given in Table 1. In addition to the field work we also analysed honey samples from a selection of urban and rural hives and identified the pollen in this honey.

 

Table 1. Overview of sites monitored. Size: actual surface area monitored. Average number of Floral Units was calculated by taking the average of FU counts over the season. Average number of insects was calculated by taking the average of total insects on flowers observed over the season. Average number of insects per 1000 FU: the number of insects per 1000 FU was calculated at each visit; an average was taken of these ratios over the season.

 

1) Do honeybees use the Grey Street Gathering?

It has been shown that small urban planters benefit sweat bees (Simao et al., 2018), but it is not known if honeybees will use them. At the end of summer 2018 we did observe honeybee use of the Grey Street Gathering, but this was data collected for a short period only. Does the planter have value to honeybees over the whole season? To answer this question, we performed pollinator surveys on Grey Street Gathering.

The planter was installed in April 2019, and in its first months attracted very few flower visitors (Fig. 1). Honeybees started visiting the planters in Late June, and from that time onwards remained one of the most abundant visitors.

Figure 1. Flower visitors to the Grey Street Planters in 2019. Upper panel, visitors time period over the season. E = early part of month (1-15); L = late part of month (16-30/31). Lower panel, proportion of total visitors calculated over the entire flowering season observed (April-September). Bombus – bumblebees; Apis mel – honeybee, Hym other – all other bees and solitary wasps; Syrphidae – hoverflies; Diptera oth – all other Diptera, Lepidopt – butterflies and moths.

At the Grey Street Gathering, honeybees predominantly visit sage flowers (Salvia sp.) There are three varieties planted here; all three are frequently visited by both honeybees and bumblebees (Fig. 2). Visits to other flowers in the planters are few in number.

Why did honeybees only start visiting the Grey Street Gathering in Late June? The sages they forage on had been flowering in good numbers for several weeks already, and had been frequented by bumblebees (Fig. 1). Yet honeybees only visit from the end of June onwards. Our nearest beehive supports this argument with abundant Rowan pollen and occasional Privet and Box pollen in samples. The city bees also show a preference for Knapweeds, Hairy Willowherb, clovers, Meadowsweet, Lavender and Toona. Interestingly this hive did not yield any pollen of sage flowers. It is possible this hive was not using the Grey Street Gathering. Other hives are located closer to the site, but the beekeepers were unable to join the study.

 

Figure 2. Flower choice of honeybees at the Grey Street planters.

2) Does size matter - do small green sites like the Grey Street Gathering have a significant impact on bee species diversity?

 

Is there an effect of size of green sites on pollinators – is there a “critical size” a site needs to have in order to be “good” for pollinators? In order to answer this we first looked at the total number of flower-visiting insects in each site, and compared this with the size of the site in square meters and the amount of Floral Units available. As could be expected, there is a significant relationship between the number of insects and both size of the observed site, and floral unit present. Bigger sites have more floral units, and attract more insects (Fig. 1a and c). There is also a significant correlation between site size and floral units (Fig. 1b). Does size matter in other ways – are bigger sites more efficient? Our results suggest this is not the case, the relation between size (either in surface area or floral units) and average total insects is linear. Moreover, there is no significant correlation between density of flower-visiting insects and size (Pearson correlation) (Fig. 1d).

 

 

Figure 3. Overall flower visiting insects per site. Upper left: correlation between average number of flower-visiting insects and the average total number of Floral Units per site (p = 5.48 x 10^-8). Upper right: average total number of Floral Units versus site size (p = 9.3 x 10^-6). Lower left: average total number of insects versus size of site (p = 2.06 x 10^-5). Lower right: average number of total insects per 1000 FU versus size of site (p = 0.89).

 

3) Do city centre beehives use the Grey Street Gathering?

 

So far our pollinator monitoring results show this to be “yes”, but the near absence of Sage flower pollen in any of the samples so far analysed is rather perplexing. Especially given that 88% of observations were on Sages. We see Geranium and Vervain pollen in our city centre honey and pellet samples, but these only make up 12% of honeybee observations in the field and are common in many city centre sites. As previously stated, there are a number of beekeepers with hives closer to the Grey Street Gathering who were unable to provide samples, perhaps it was their honeybees we observed.

As part of the project we widened the honey and pellet sampling net to cover the entire city and surrounding region. We distributed over 200 sampling kits and received 94 back. Despite this low return rate, we have samples from almost all city postcodes and a number of more rural areas. The results from this have provided some insight into the diversity of non-native plants that urban bees are using, such as: Spirea (Spiraea spp.), Begonias, Sempervivums and Chinese Cedar (Toona sinensis). Spirea and Chinese Cedar are both bushes/trees and highlights the other key finding from across the North East – trees are important for bees. As well as these exotics, we recorded pollen of Buckthorn, Chestnut, Elder, Hawthorn, Horse Chestnut, Lime, Maple, Pear, Rowan and Willow. We also encountered pollen of typically wind-pollinated trees such as Oak and Pine – contamination or intentional? Even discounting the wind-pollinated trees, the diversity and in some samples abundance of tree pollen shows the importance of these flowering skyscrapers. Therefore, whilst the Grey Street Gathering was important for local honeybees (41% of observed pollinators), honey and pellet analysis shows the value of urban and rural trees.