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| Fig. 1: Honey bees in front of an artificial hive. (Source: Wikimedia Commons) |
Annual crop pollination is a crucial step for a huge variety of flowering food crops, however, with the demands of modern high density agriculture and the general collapse of wild insect populations, nature no longer reliably accomplishes the job. With pollination amounting to roughly 10% of the crop's final market value there is significant economic incentive to develop controlled pollination technologies. [1] By far the most robust and economical method has been the deployment of farmed bees living in artificial hives, as show in Fig. 1. These hives can and are transported at continental scale allowing for pollination as a scheduled service, where and when a farmer needs them. The maintenance and movement of bee hives for pollination purposes is now a hundred million dollar per year industry in the US alone. [2]
Pollination is in fact a symbiotic side effect of bee's natural foraging behavior. This is, from the human perspective, what makes bees so great at the job; not only will they automatically seek out flowers in the local area, they feed themselves while they're at it. The human input is only in creation and maintenance of the hive while the bees handle all the heavy lifting at zero additional cost in dollars or joules. Bees are also highly efficient from an optimization perspective at both the individual and collective level. An individual bee can travel up to 10 km from its hive, averaging 6 km a day, allowing it to visit hundreds of flowers. Further, the individuals of a hive work communally, communicating forage locations and assisting each other even if they personally don't need additional food. With a single hive comprising tens of thousands of bees, they can all together roam over thousands of acres. [3] In the agricultural setting of course, hives are placed at much higher density such that they both thoroughly and quickly pollinate an entire farm.
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| Fig. 2: Drone Sprayer. (Source: Wikimedia Commons) |
Various approaches to artificial pollination are under development, seeking to programmatically optimized yield as well as resilience against natural incidents like the weather or human made problems such as pesticides, both of which can plague bee populations. [4] One technology receiving significant attention is flying drones modified to carry agricultural equipment, as in Fig. 2, serving like mini crop dusters. [5] The benefits to such drones are that they can be preprogrammed with flightpaths through a farm or orchard to reach all crops at any time of year. The chief limit is that these drones are wide-dispersal, spraying or drop dispersing their payload rather than reaching individual flowers. This makes them ineffective at reaching the flowers of many crops and the inherent inefficiency of un-targeted dispersal results in additional expense. There do exist proposals for micro drones that can seek out individual flowers, closer to a one-to-one replacement for bees, but these are generally unfeasible. [6] Even the larger more efficient drones have significant technological challenges to contend with, including but not limited to, flight range, energy consumption, and computation requirements. In other words, all the things that bee's handle by virtue of biology need to be re-engineering in-silico.
Taking the bee as a reference point we can calculate the necessary energy consumption and flight performance of a hypothetical pollinator drone to be competitive. As above, we know a honey bee will fly 6 km from its hive in a day while foraging, feeding itself on flower nectar as it goes. The energy use of a modern drone is dominated by propulsion, which under near ideal conditions can reach a rate of roughly 20 m/J. [7] The amount of energy for a medium propellor drone to travel as far as a single bee's day is then:
| E | = | 6 km 20 m/J |
= | 300 J |
Of course we don't want to buy a drone for every bee. A more useful reference value is to compare to a hive, which has tens of thousands of bees:
| Ehive | ≈ | 10000 × 300 J | ≈ | 0.83 kWh |
The good news is this can be readily supplied by solar panels. [8] As a practical engineering concern we also consider the necessary size, in units of weight, for such a drone. A modern ithium- ion battery has an energy density of 250 Watt-hour/kg, yielding a minimum drone weight of: [9]
| m | = | 830 Watt-hour 250 Watt-hour/kg |
= | 3.3 kg |
Of course, one has to account for the rest of the drone, and for medium class drones we can approximate the battery and payload being equal weight, leading to overall estimation of 6.6 kg. This is substantial for a flying platform, but not entirely unreasonable. I note that this 'just possible' case is an ideal, and under non-ideal conditions the energy consumption rate for drones easily rises by an order of magnitude. Furthermore, the ideal case assumes a relatively fast drone flight speed without frequent acceleration maneuvers, imposing further challenges on pollen dispersal methods.
Significant technological challenges remain for commercial scale flying drones to be at all competitive with natural bees as pollinators. Natural bee's amenability to being farmed and shipped themselves makes them easily deployable while their natural behavior makes them near optimal at the job. In contrast, even if flight energetic requirements can be met, flying drones must rely an inefficient and expensive 'carpet bombing' strategy to keep up, and this approach is unsuitable for many crops. Bee's remain the obvious answer.
© Andrew Land. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] L. Hein, "The Economic Value of the Pollination Service, a Review Across Scales," Open Ecol. J. 2, 74 (2009).
[2] J. K. Bond et al., "Honey Bees on the Move: From Pollination to Honey Production and Back," U.S. Deartment of Agriculture Economics Research Service, Report No. 290, June 2021.
[3] J. M. Beekman and F. L. W. Ratnieks, "Long-range Foraging by the Honey-Bee, Apis mellifera L." Funct. Ecol. 14, 490d (2000).
[4] K. Arias-Calluari et al., "Modelling Daily Weight Variation in Honey Bee Hives", PLoS Comput. Biol. 19, e1010880 (2023).
[5] M. A. Broussard, M. Coates, P. Martinsen, "Artificial Pollination Technologies: A Review," Agronomy 13, 1351, (2023).
[6] R. Gleadow, J. Hanan, A. Dorin, "Averting Robo-Bees: Why Free-Flying Robotic Bees are a Bad Idea," Emerg. Top. Life Sci. 3, 723 (2019).
[7] J. K.Stolaroff et al., "Energy Use and Life Cycle Greenhouse Gas Emissions of Drones For Commercial Package Delivery," Nat. Commun. 9, 409 (2018)).
[8] S. Gulkowski, "Specific Yield Analysis of the Rooftop PV Systems Located in South-Eastern Poland," Energies 15, 3666 (2022)).
[9] V. Viswanathan et al., "The Challenges and Opportunities of Battery-Powered Flight", Nature 601, 519 (2022).
