Females of solitary bees (Megachilids) like these mason bees nest in crevices and tiny holes — hollow stems, bamboo, even pipes. Of the 20,400 species of bees in the world, over 90% do not live in hives and do not make any honey. Photo by Arati Kumar-Rao.

Necessary nesting sites

V.V. Belavadi of the Entomology department at Gandhi Krishi Vignyan Kendra (GKVK) has studied solitary bees for four decades. “The availability of nesting sites is key to the survival of these species,” he says. Solitary bees tend to live separately, with only the female building a nest. Sometimes females will build burrows or boreholes close to one another, but rarely do solitary bees share nests. The males find grasses or other surfaces to hold on to, or little depressions to sleep in at night — they do not build or use nests. Carpenter bees, as their name suggests, carve holes into wood or bamboo with their powerful mandibles. Others, like mason bees, blue-banded bees, and leaf-cutter bees burrow into mud or nest in small cavities.

During the process of urbanisation, mud and bamboo and wood and other permeable surfaces are the first to be paved over or clear-cut, replaced by tarmac, tiles, and concrete. This could mean fewer nesting sites for solitary bees. “Loss of nesting sites is one of the strongest predictors of species biodiversity decline,” says Belavadi.

Somanathan of IISER Trivandrum concurs. “Even more than food sources — for these solitary bees are generalists and could manage fine in urban gardens — it is nesting sites that determine the health of their population.”

Precious pollinators

“We have shown that in pigeon pea (toor dal, in Hindi) plots if you increase nesting sites, you can almost double the pod-set,” says Belavadi, based on some preliminary findings. While pigeon pea can self-pollinate, that is, produce pod sets without the aid of an external pollinator, the yield is only 18-20% in such cases. If left open to pollinate, without any help to the pollinators, the yield rises to 30-35%. But if solitary bees — in the case of the pigeon pea it is leaf-cutter bees — are provided additional nesting sites (like hollow reeds, pipes, bamboos) close to pigeon pea plots, the yield jumps to 50-60%, explains Belavadi, while noting that more research is needed to confirm the findings.

Some important crop plants like chilies, bell peppers, and tomatoes need to be “buzz-pollinated.” The anthers in such flowers require a certain amount of vibration in order for them to release pollen. The exact mechanism of release is still being studied, but that bees play an important role in providing this vibration through their wingbeat frequency is key. “Solitary bees are very efficient at buzz-pollination,” says Somanathan, “Not so much honeybees.” Thus, increasing nesting sites for solitary bees around farms where such crops are grown is known to be beneficial. This holds true for urban and peri-urban farms, kitchen gardens, and terrace gardens as well.

Moreover, studies have shown spillover effects where rural agricultural land that borders urban spaces are helped by a high incidence of pollinators in the urban areas.

Professor Emeritus VV Belavadi of the Department of Entomology at the University of Agricultural Sciences in Bangalore oversees an extensive collection of solitary bees from Karnataka, the Western Ghats, and other parts of India, some dating back over a century. Photo by Arati Kumar-Rao.
Professor Emeritus VV Belavadi of the Department of Entomology at the University of Agricultural Sciences in Bangalore oversees an extensive collection of solitary bees from Karnataka, the Western Ghats, and other parts of India, some dating back over a century. Photo by Arati Kumar-Rao.

While there is little data from India or for Indian cities, studies in other parts of the world have compared the health of pollinator (specifically bee) populations and species biodiversity in natural and semi-natural areas with urban sprawl, manicured urban gardens, dense concrete cities, rural sites of intensive agriculture, and urban and rural commons.

The results are instructive and illuminating for city planners and residents alike, pointing to distinct strategies for increasing pollinator diversity and protecting bee populations.

Studies show that native bee diversity is most adversely affected in areas of intense monoculture agriculture with heavy use of pesticides and chemicals. These landscapes tend to be resource-poor.

What kinds of flora urban spaces afford bees is important too. Most urban areas have been shown to be dominated by generalist species of solitary bees, happy to forage on exotic plants. This crowds out species that have a narrower foraging strategy which would likely have been satisfied in natural or semi-natural spaces with high biodiversity of native flora. This also works the other way around, when pollinators that prefer certain host plants or native species are crowded out, exotic or invasive species take over the urban scapes.

Bees are predated upon by a variety of creatures, including the one named for its bee-loving habit, the bee-eater. Here, green bee-eaters trash the sting and venom out of the bees before feeding on them. Photo by Arati Kumar-Rao.
Bees are predated upon by a variety of creatures, including the one named for its bee-loving habit, the bee-eater. Here, green bee-eaters trash the sting and venom out of the bees before feeding on them. Photo by Arati Kumar-Rao.

Impacts of urbanisation

With urbanisation has come the heat-island effect. Research shows that when trees that line our roads are chopped down, the ambient air temperature increases by 3.5 degrees Celsius. The surface temperature of the tarmac shoots up by 25-30 degrees Celsius. By the end of the century, our cities could be 7 degrees hotter and that oven has been turned on. Such temperature changes favour species that prefer warmth while foraging. They take to the warmer micro-habitats like city centers which may be food-resource rich by way of ornamental flowers, while bee species that prefer the cooler temperatures will likely be edged out to the outskirts (which may not be as rich in food sources) or completely. The effect of light, noise, pollution, and climate change on solitary bee biodiversity needs far more research, but broadly, generalists tend to do better when the going gets tough, while specialists or species that rely on a narrow range of food resources, or certain native species of flora, tend to suffer.


Read more: Urbanisation and climate change sting bees. What can cities do for them?


But there is hope for our cities, and for solitary bees. Urbanisation per se does not mean lowered solitary bee biodiversity. In fact, urban sprawl, with a green corridor that leads in from the rural or natural landscape till the city center, could foster a high degree of biodiversity if urban planners and landscapers built cities with adequate food resources and ground- and cavity-nesting sites for pollinators in mind.

Even ordinary city residents can make a difference. Somanathan urges urbanites to build “bee-hotels” and host solitary bees. “All it takes is some hollow wooden sticks, or bamboo tied together and hung for the cavity-nesting species, and some soil left free in gardens (not paved or tiled-over)  for the ground-nesting species,” she says.

“If we don’t spray harmful chemicals, if we plant cucurbits like pumpkins and tomato plants, or flowering plants like roses and Calotropis — or any plant from the family Fabacae — solitary bees will come,” she added.

They will get their nectar and pollen, we’ll get fruits and flowers.

A Xylocopa carves a hole in a bamboo. Carpenter bees, as their name suggests, carve holes into wood or bamboo with their powerful mandibles. Lack of nesting sites is a threat to the survivial of solitary bees. Photo by Arati Kumar-Rao.

CITATION:

Amala, U., & Shivalingaswamy, T. (n.d.). Role of native buzz pollinator bees in enhancing fruit and seed set in tomatoes under open field conditions. Journal of Entomology and Zoology Studies 2017, 5(3), 1742-1744.

Arne, W., et.al., How urbanization is driving pollinator diversity and pollination – A systematic review. Biological Conservation.

Batra, Suzanne W. T. “India’s Buzzy Biodiversity of Bees.” Current Science, vol. 65, no. 3, 1993, pp. 277–280. JSTOR, www.jstor.org/stable/24095131.

Somanathan, H., Saryan, P. & Balamurali, G.S. Foraging strategies and physiological adaptations in large carpenter bees. J Comp Physiol A 205, 387–398 (2019). https://doi.org/10.1007/s00359-019-01323-7


This article is a part of a series on Bengaluru’s Ecosystems and Biodiversity, a joint project between Mongabay-India and Citizen Matters, supported by the Bengaluru Sustainability Forum (BSF).


 

Banner image: A Xylocopa nasalis Westwood female collecting nectar and pollen for her nest from a Crotolaria juncea (hemp) flower. These flowers were a few feet away from her nest. She would be out foraging until twilight every day, starting at around 10 AM, once the sun was high in the sky. Photo by Arati Kumar-Rao.

Article published by Aditi
, , ,