We tested whether the world’s favourite ornamental plant, Rose (Rosa chinensis Jacq.), can help conserve the world’s most important and propagated solitary bee pollinators, leafcutter bees, in the present investigation. The answer to this is that, yes, but with an epilogue, do not spray pesticides on the plants! This well-replicated study in various parts of India suggests that a range of plant-specific, landscape-related, and management factors predict the bees’ use of Rose plants as a leaf-forage resource. For instance, the bees used Roses well in lowlands and urban places than in very high lands and rural places. The pesticide use negatively affects the bee’s use of Rose leaves. Leaf size or plant variety are not important drivers of plant selection by the bees.
There are reports that pesticides have a negative effect on bees3,29. It is postulated that the solitary wild bees are worst affected than the honey bees30,31. However, little evidence has come from studies for the routes other than the flowers the bees exposed to pesticides32. Our findings report that the Rose plants subjected to periodical pesticide treatment are less preferred by the bees, and this strongly suggest that the leafcutter bees can experience and expose to pesticides through the leaves. Albeit a pilot study, we report the first experimental proof that the bees avoid pesticide-treated foliage for foraging leaf fragments. The bees abandoned our experimental plants after four days of visits and foraging leaves from control plants. We limited our pilot study with a contact pesticide, which degrades faster in the environment. A targeted study should explore the response of the bees to a range of new-generation pesticides, neonicotinoids, biopesticides, and systemic pesticides to make recommendations for safer pesticides for the ornamental plant industry, which perceive leafcutter bee as a major defoliator and “pest”25–27. Moreover, it is shown that Megachile rotundata, the first successfully propagated managed solitary bee, has no mechanism to detoxify pesticides and therefore is highly sensitive to pesticide exposure28,33.
The bees explored the Roses in urban environment more than rural environment. Urbanization is considered as a threat to biodiversity and certain biotic functions34. However, the response of bees to urbanization has been often depicted positive by recent studies35–38, and our findings support it. Leafcutter bees are cavity nesters and use almost all unpredictable sites for constructing nest tubes39,40. Since they are above-ground nesters, they are less affected by the soil, which is often polluted and compacted in urban places and agricultural landscapes11,41. Urban places are likely to provide better nesting and foraging opportunities for the bees35,42,43. Plants belong to Rosaceae, and Rosa in particular predominate in the list of ornamentals in global cities44–47. This suggests that urban places might be a heaven for the leafcutter bees. They use plants in Rosid clade for both foraging and leaf resources13,16,21.
Leafcutter bees hardly used Roses cultivated above 1000 m asl. This suggests that their distribution might be driven by the altitude in India, if not everywhere18. In northern Arizona, a greater diversity of Megachilidae and Megachile spp is reported above 2000 m asl18. Southwestern USA is a heaven for the Megachile spp48. In northeast India (Darjeeling), the senior author of the present study had surveyed Roses for three years in altitudinal bands ranging from 150 m asl to about 2000 m asl in 2018, 2019, and 2021, in which Roses had notches on leaves for about 1000 m asl in all the surveys. In south India, Roses were surveyed in 2018 and 2020 at certain places at 12oN (Kasaragod district). In both sites, the Roses that had leaf cuts had cuts every year of sampling (pers. observ.). As part of another investigation, the senior author of the present study (PAS) was monitoring flower visitors of wild, crop, and ornamental flowering plants in the Darjeeling hills for over three years. However, species belonging to Megachilidae were totally a miss on the flowers above 1100 m sl. In Amomum subulatum (large cardamom) – a cash crop and a major nectar source for the bees and birds in Darjeeling and Sikkim Himalayas – Megachile sp was a visitor to the flowers up to an elevation of about 800 m asl49. It is therefore likely that leafcutter bees are underrepresented in high altitudes of India, rather than to hypothesize that they might be using plants other than Roses for foraging leaves at these sites.
Use of Roses – at least the proportion of plants received cuts – varied considerably among sites. The number of cuts on leaves – another measure used to study bee’s level of dependence on the plants – did not vary much on the cut plants. We took latitude as a measure of the site at a broader scale as the study was well replicated at a higher spatial scale. At each latitude, we had a good number of Roses in several sites. Overall, Roses at 8oN and 12oN are highly used by the bees in south India. Sites within each of the latitudinal points have different proportions of cut Roses (Fig. 2). There could be at least three important drivers that can explain this variation. They are availability of foraging material (pollen and nectar), number of female breeding bees, and nesting places in sites. In the present investigation, we did not quantify them. Natural nesting sites of leafcutter bees can be any unpredictable cavities or crevices in any artificial or natural materials39,40. Two of the authors’ (PAS and AK) observations (N=21) in parts of southwestern USA (PAS) and south India (PAS & AK) are that the nesting places of leafcutter bees are between 0.8 m and 17 m from the leaf sources (PAS & AK, Unpublished). The location of Roses in the gardens, therefore, might play a crucial role in plant use. However, it may be important to note here that out of the 114 Rose plants belonging to Rosa chinensis and its breeds, and Rosa banksiae (N=18) surveyed in the arboretum of the University of Arizona, 101 had the notches of leafcutter bees (PAS, Unpublished). Southwestern USA is a paradise for the bees and Megachile spp in particular. The system offers plenty of pollen resources, such as Mesquites and cacti (PAS: pers. observ.). Leafcutter bees normally return to the preferred plant and even to the selected leaf for more cuttings and until the leaf exhausts13,16 (Supplementary Video S1). Therefore, we are not surprised by the finding that the number of cuts on leaves does not vary much among sites.
All quadrants and houses that we surveyed in east, west, north, and south Guwahati city were located at an altitude of about 50 m asl. White Rose was also not a minority in the sample or not clumped to any particular quadrant or house to account for this variation to the local abundance of the plants or to any other local features of the quadrant, including the altitude. Unfortunately, we did not collect information on the pesticide application for this site. Based on our findings that the bees avoid pesticide-treated plants, it is likely that these plants might have been treated by pesticides. One of our (PAS) studies in the arboretum of the University of Arizona in two springs, however, found that the bees hardly discriminated Rose plants by the variety or species.
Conservation of pollinators is an international and intercontinental agenda since the IPBES report1 flagged the threats the pollinator guild and pollination service are experiencing in the crop systems. Several studies have been concerned about habitat restoration of pollinators and made recommendations for improving the pollinator flowerscape as reports suggest that loss of flowers due to land-use change is a key reason for pollinator decline in the world2,3,30. Improving flower resources becomes a practice in agri-environment schemes and urban landscapes of the developed countries, in particular North America, Europe, and Great Britain. Though late, studies have concerned about the poor quality of nesting grounds of wild pollinators due to tillage and use of agrochemicals in soils5,11.
The leafcutter bees that we are concerned in the present investigation are cavity-nesters. Countries and farmers are relying upon them for crop production, such as Alfalfa and pulses19. Serious efforts have been made to propagate these bees in crop systems33. Because they nest in cavities, installing bee hotels became a fashion for managing leafcutter bees in crop systems, public places, and even private premises of the institutions and houses in several parts of the world20. Yet, recent meticulous field and controlled experiments have documented suboptimal propagation of the broods in bee hotels14,20. Studies have related this to an increasing rate of parasitism in bee hotels and poor choice of nesting materials14. Our findings suggest that Rose leaves may not hamper the development of brood due to leaf chemicals.
Leaves and petals of a range of plant species have been unearthed in the nest tubes of leafcutter bees14,21,50. Plant surveys have reported even a greater number of leaf-foraging plant species in parts of southwest USA13 and Asia16,51. However, no recommendations have, so far, been made for the leaf plants for leafcutter bee habitat management and conservation. The recommended plants must meet at least two basic requirements. First, the plant can be manageable in agricultural landscapes and urban centres easily. Second, the nesting materials must allow the brood development.
We found 100% success for broods sheathed by the leaves of Roses. In fact, two of the authors (PAS and AK) have collected nest cells constructed exclusively by the leaves of Cassia fistula (Golden-Shower) and Swietenia macrophylla (Mahagony tree) in natural nesting places of Megachile lanata and M. disjuncta, and found 100% brood success for all such cases too (Unpublished). Although studies have found suboptimal brood success for cells retrieved from bee hotels14,20, our findings suggest that the success rate could be higher in natural breeding places. However, unlike those trees, Rose plants are easily manageable in any situation, including the towering apartment complexes, and are already propagated to almost all parts of the world as part of beautification projects44–47. Although Roses attract only very few bees to the flowers, so found no place in any lists of bee-friendly plants (reference9 and references therein), are heavily used by leafcutter bees for their reproduction. We, therefore, recommend Roses, for the first time, to the list of bee-friendly plants. Although the leafcutter bees can defoliate the plants considerably, there are no evidences that this unusual “herbivory” hampers the flower output or plant growth. It is an exciting topic for a future investigation. Even though some studies retrieved polyurethane materials in the leafcutter bee nests, which the bee might have foraged under the stress of limited leaf resources, did not show the brood success in such cases52,53. The study underscores that incorporating appropriate nesting material must be an important strategy in solitary bee-, in particular the leafcutter bee-conservation and management.