Over the summer of 2016, with the help of our student intern Giri, we conducted a trial to find out whether cultivated varieties of nasturtium (Tropaeolum majus) differ in their attractiveness to bumblebees. We compared seven different varieties of nasturtium which we grew from seed: Black Velvet, Cherry Rose Jewel, Empress of India, Gleam, Ladybird, Vesuvius, and Wina. These ranged in colour from pale yellow, to cerise pink, to dark red.
We needed enough plants to create 3 replicate plots, each containing 3 plants of each variety, plus a few spares in case of losses. This meant raising over 100 plants, for which we used the same peat-free compost that had worked so well in our Pollinator Pots. Once the nutrients in the compost had started to dwindle, we used an organic liquid feed made from comfrey which is rich in potassium – an important nutrient for plants that are producing flowers.
Once the plants were flowering, we conducted a series of 20 minute observations on each plot, during August and September. We recorded the bumblebee species present, what the bumblebee was doing (for example drinking nectar, collecting pollen, resting etc and for how long) and which variety of nasturtium was visited. This involved one recorder taking notes, whilst the other kept a close eye on the bees. We found that the majority of bees visiting the nasturtiums were Common Carder Bees (possibly because it was rather late in the year), but other species such as the Red-tailed Bumblebee and Garden Bumblebee also visited.
We also collected nectar from the flowers to find out if the volume of nectar and its sugar content varied between varieties. Extracting nectar from the nasturtiums was quite straightforward as they secrete reasonably large amounts which is easily accessible and can be drawn out of flowers without damaging them. We did this by placing a small glass capillary tube into the spur (where the nectar collects) of each flower sampled – this drew up the nectar by capillary action and the nectar was then pipetted onto a refractometer to estimate the degrees Brix sugar content (grams of sugar in 100g solution).
We also collected the anthers from flowers to determine if the amount of pollen produced varied between the varieties. Collecting pollen was also simple, as nasturtiums have quite large anthers which were be snipped off using small scissors and placed in vials ready for analysis. We currently have the help of an undergraduate student, Becky, who is busy in the lab counting pollen grains.
The next step will be to analyse the data, but at present, most of our efforts are going into creating content for the Blooms for Bees app and website which launch in April 2017.