Ecologists learn how to increase thieving by bumblebees

Plants need pollinators, and often they offer nectar as a reward for pollinators. But what if a visitor cannot access the nectar? They could avoid the plant and not pollinate it – or they could steal the nectar. Sabrina Gavini and colleagues examined Campsidium valdivianum, a plant usually pollinated by hummingbirds in the forests of Patagonia. The long flowers are inaccessible to bumblebees, who want to access the nectar. While the bees cannot get into the flower from the top, a little chewing means they can enter from the side. Gavini and colleagues have studied what leads to the bees deciding to steal from the flowers. They wondered if the surrounding plants made Campsidium more or less likely to be attacked.

Scientists have studied Campsidium valdivianum and found that the longer the corolla, the more likely that Bombus dahlbomii, a native long-tongued bumblebee, would rob the nectar from the flower. However, in Patagonia, the native bumblebee has been driven out by the invasive B. terrestris. B. terrestris is a short-tongued bumblebee and so even more likely to rob nectar from Campsidium, if it interacts at all.

Gavini and colleagues hypothesized that the floral neighbourhoods of Campsidium would make a difference to nectar robbery. They believed that suitable flowers for short-tongued bumblebees would act like a magnet to draw in bumblebees. With bumblebees in the area, the Campsidium flowers would then be targets for attack.

The ecologists staked out plants in the Puerto Blest, Nahuel Huapi National Park. The site is rain forest, and on the trees is where the hummingbirds and bees find Campsidium as a climbing liana. At the same time as Campsidium flowers, so too does the native Berberis darwinii and the alien Cytisus scoparius. In November 2019, the team sampled sixty-four plants along a trail from Puerto Blest to Puerto Frias and examined them for signs of nectar robbing. This would be a hole somewhere in the corolla.

Along with the Campsidium plants, they also looked at what plants were in the neighbourhood. The neighbourhood, in this case, was a 10 × 5 metre plot, longwise to the trail. This seems a bit lopsided for a plot. Why not a square? Gavini and colleagues explain in their article that the plants they studied were always by the edge of the trail. Taking the plot five metres ahead and behind on the trail, as well as five metres deep, gives the oblong shape of the plots.

In the plots, the team recorded all the bumblebee pollinated plants, their density and the distance from the closest bumblebee plant to the Campsidium plant. They then put the data through R to examine it.

There was a lot of data to crunch. The sixty-four plants produced over seven hundred flowers. On average, the plants had 15% of their flowers robbed, and over half of them had at least one flower robbed. The critical factor in the proportion of robbed flowers was the presence or absence of bumblebee-pollinated plants. If there was a bumblebee-pollinated plant in the area, then on average, over a third of the flowers were robbed, compared to just one in twenty in areas with no bumblebee-pollinated plants.

Gavini and colleagues note that usually, when people study nectar robbery, they explore how it affects reproductive success. This research differs as it examines what drives the robbery in the first place. It’s unlikely that ecologists will want to train bumblebees to rob flowers. But if nectar robbery is a problem for a species, then it will be helpful to understand what causes it.

ORIGINAL RESEARCH

Gavini, S.S., Moreno, E., Zamorano-Menay, F., Morales, C.L. and Aizen, M.A. (2022) “Bumblebee floral neighbors promote nectar robbing in a hummingbird-pollinated plant species in Patagonia,” Arthropod-Plant interactions, https://doi.org/10.1007/s11829-022-09895-z