DNA barcoding to identify leaf preference of leafcutting bees

Research DNA barcoding to identify leaf preference of leafcutting bees

Leafcutting bees (Megachile: Megachilidae) cut leaves from various trees, shrubs, wildflowers and grasses to partition and encase brood cells in hollow plant stems, decaying logs or in the ground. The identification of preferred plant species via morphological characters of the leaf fragments is challenging and direct observation of bees cutting leaves from certain plant species are difficult. As such, data are poor on leaf preference of leafcutting bees. In this study, I use DNA barcoding of the rcbL and ITS2 regions to identify and compare leaf preference of three Megachile bee species widespread in Toronto, Canada. Nests were opened and one leaf piece from one cell per nest of the native M. pugnata Say, N leaf pieces, and the introduced M. rotundata Fabricius, N and M. centuncularis (L.), N were analysed. From one hundred seventy-four individual DNA sequences, fifty-four plant species were identified. Preference by M. rotundata was most diverse (thirty-six leaf species, H prime equals three point zero eight, phylogenetic diversity P D equals two point nine seven), followed by M. centuncularis (twenty-three species, H prime equals two point three eight, P D equals one point five one) then M. pugnata (eighteen species, H prime equals one point eight seven, P D equals one point two two). Cluster analysis revealed significant overlap in leaf choice of M. rotundata and M. centuncularis. There was no significant preference for native leaves, and only M. centuncularis showed preference for leaves of woody plants over perennials. Interestingly, antimicrobial properties were present in all but six plants collected; all these were exotic plants and none were collected by the native bee, M. pugnata. These missing details in interpreting what bees need offers valuable information for conservation by accounting for necessary (and potentially limiting) nesting materials.

One. Background

The Megachilidae is the second largest bee family with over three thousand nine hundred species and a worldwide distribution. These bees are solitary and important pollinators in most terrestrial environments. Many of these bees use foreign materials to construct their nests. For some megachilid species, these nesting materials consist of little more than fine particle mud, sand and smaller pebbles. Others depend on plant materials including leaves, flower petals, resins and plant hairs. The collection of foreign material is one factor that may have promoted the diversification of megachilid bees.

Megachilid bees and especially those in the genus Megachile predominantly collect leaves by cutting pieces with their mandibles to construct their nests. Not all leafcutting bees actually collect leaves, some use flower petals (e.g. M. montivaga Cresson) and others collect plant resins (M. sculpturalis Smith, M. campanulae Robertson), or even tile caulking and plastic bags. Although leafcutting bees are well-studied compared with other solitary bees, surprisingly little information is available on which leaf types are cut and by which bee species.

Nesting material choice may limit the geographical ranges and/or abundances of particular bee species. However, the leaf preferences of megachilid bee species have rarely been investigated. Identifying leaf preference could inform management and conservation, research and the improvement of experimental design, as well as plant selection and horticulture to reduce damage to ornamental plant species. For example, Horne experimentally evaluated the leaf preference of M. rotundata among eleven plant species. In enclosures, bees cut all plant species offered to them but showed a significant preference for leaves of buckwheat (Fagopyrum esculentum Moench), which were also significantly larger than the leaves of the other ten species. Kim observed M. apicalis Spinola visiting Wisteria sp. for leaf pieces in nature and subsequently used the plant in pots in enclosures to study relationships between female body size and fecundity. Finally, Nugent and Wagner compared the level of leaf defoliation by unidentified Megachile bee species on seven different Populus cultivars and found one cultivar was defoliated significantly less than all others. Mechanical properties of leaves are evidently important for leaf preference in leaf-collecting Megachile bees. However, studies of resin- and trichome (leaf hair)-collecting megachilid bees suggest antimicrobial factors are important to protect brood from diseases, moulds and parasites, and so these properties might also be desirable among leaf-collecting Megachile bees.

Studies that report the identities of leaves cut by megachilid bees do so based on the characteristic damage to leaves. For example, one study made observations on Hoplitis producta (Cresson) collecting Fragaria sp. leaf pieces, Michener noted Hoplitis pilosifrons (Cresson) using Oenothera sp. leaves, M. centuncularis was observed cutting leaves from beech (Fagus) trees, and another study found the ground-nesting M. integra Cresson using blackberry (Rubus sp.). Strickler et al. linked M. relativa Cresson to cutting plants in the genera Fragaria and Epilobium, and M. inermis Provancher to (among many others) Acer, Betula, Rubus and Amelanchier. The authors associated the size of the cuts on the leaves to the two different bees studied because the first bee species cut noticeably larger pieces than the second one.

Determining leaf preference from observations of the bee in the act of cutting, or by surveying for the characteristic leaf cuts on plants, gives partial lists that can offer useful details about local megachilid bee nesting requirements. However, methods that more accurately link leaf-bee species interactions could provide details about inter- and intraspecific variation in leaf preference, contributing to gaps in knowledge on these essential pollinators. One technique that could ameliorate our understanding of leafcutting bee nesting preferences is DNA barcoding, which uses the diversity in short gene sequence regions to improve species-level identification. These techniques are especially useful in cases where morphological identification is difficult or not possible, as is the case with leaf fragments in bee nests.

In this study, I use DNA barcoding to identify leaf preference of three leafcutting bee species, one native (M. pugnata Say) and two exotic bees that are naturalized in the study region, M. centuncularis (L.) and M. rotundata. The leaf fragments were removed from nests constructed in artificial nest-boxes that were set up in the city of Toronto in twenty twelve and twenty thirteen. I first compare the species diversity in leaf preference among the three bee species and as previous studies show bee species tend to prefer leaves from only a few species, I hypothesize that bees would not overlap in their leaf preferences. Second, because anthropogenic landscape change can reduce phylogenetic diversity, I hypothesize that the leaf preference of the native bee studied in an urban environment would be more phylogenetically constrained than that of the two exotic bees. Native bees prefer native plants, and so similarly I hypothesize that the native bee (M. pugnata) would prefer native leaves rather than exotic ones for nesting material. Lastly, I determine whether any bees exhibit specialization or affinities for native or exotic species, or particular kinds of vegetation, including trees, shrubs or flowering plants, which would have applications for conservation planning and management.