Solitary Bees and Bumblebees in a Danish Agricultural Landscape

Abstract

__________________________________________________________________________ In a survey April through November 1997, a total of 72 solitary bee species and 19 bumblebee species were recorded in the semi-natural habitats of a Danish conventional agricultural landscape. The majority of the solitary non-inquiline bee species (59) were polylectic, but four oligoleges of Salix and six oligoleges of other plant families were recorded. The plant community of the studied area is typical of nutrient-rich soils of a conventional farmland, with annuals and vigorous species that benefit from fertilisers dominating the flora. Abundant and widespread mellitophilous plant species were all ones that may sustain a species rich but polylecticly dominated bee fauna. Abundance of solitary bees and bumblebees were correlated with mellitophilous plant coverage in south-facing areas, whereas no correlation was found for honeybees. Furthermore, abundance of honeybees was not correlated with abundance of other bees. Bee species richness could not be explained by plant species richness or coverage in a multiple regression. Habitat parameters in a generalised linear model were able to predict abundance of males and inquilines, a measure of nest abundances in the habitats. ___________________________________________________________________________ Introduction Bee-pollinated crops account for about 30% of the food produced for humans, and the value of this pollination service exceeds the value of produced honey by at least a factor 50 (Corbet et al. 1991). Furthermore, thousands of wild plants depend on pollination for seed set and thus persistence in their habitat. The agricultural landscape claims the largest proportion of the total area of Denmark (65%) (Ed. by: Prip et al. 1996), and it is also the landscape type being altered the most by man. Pollinators constitute an important part of the fauna in this landscape but apart from the honeybees, nothing is known in Denmark about the current status of an ecologically important group of the pollinators namely the so-called wild bees. Apart from nesting places, the most important feature of a landscape that can sustain a diverse bee fauna is a continuous succession of flowering plants, representing a wide range of plant families. Especially Apiaceae, Asteraceae, Fabaceae, Lamiaceae, Ranunculaceae, and Rosaceae are preferred by many Danish polylectic and oligolectic bee species. The continuous succession of plants is important to the social species, which are mainly active April through September in Denmark. Social bees, and solitary bees with long flight-seasons (e.g. Lasioglossum calceatum and L. leucopus), are most often polylectic and therefore not dependent on the flowering period of only a limited range of plants. MANUSCRIPT I I. Calabuig: Assessment of the foragingand nesting conditions for solitary bees and bumblebees, and their distribution in a Danish agricultural landscape 17 Oligolectic species such as Salix oligoleges (e.g. Andrena clarcella, A. praecox, and A. vaga) have to be closely synchronized in emergence with the flowering of their host plants, and are more vulnerable than polyleges to changes in the flora community of the landscape. The breeding habitats for bees in the Danish agricultural landscape are primarily hedgerows, forest edges, dry-stone walls, and field edges. Generally, soil-nesting species’ nests are placed in south-faced, warm, and sun-exposed sites, with loosely packed, dry, sandy soil and patchy or no vegetation (Cane 1991; Muller et al. 1997; Potts & Willmer 1997; Westrich 1996). Other species are more conservative in their choice of nest substrate, e.g. Colletes cunicularius, which is a pioneer species that prefer areas with bare, loose sand, often near streams or in sand dunes. Breeding sites available to cavity-nesting species are primarily insect borings in dead wood, and in old Rubus stems and Phragmites stands along streams. Generalist species, such as Anthidium manicatum and Osmia rufa, nest in all kinds of holes and crevices, between stones and in old half-timber and mortar walls. Other cavity-nesting species are extreme specialists that nest exclusively in places such as abandoned snail-shells (Osmia aurulenta) or insect-galls (Hylaeus pectoralis). Unless only a limited area is intensively studied, nests of solitary bees are difficult to locate in the field (own observations; Pers. com. C. O’Toole). Therefore, in this local study the nesting conditions are evaluated on the basis of distribution patterns of males and nest parasites. Broadly stated, in species of solitarily nesting females, the males flock around attractive foraging plants that the females visit, or the males patrol territories or leks. In gregariously nesting females, the males’ mating behaviour involves staying close to the nests for female encounters (Cameron et al. 1996; O'Toole & Raw 1999). Nest parasites (inquilines) keep close to the nests for quick and undetected entrance when a host female leaves a nest (Munster-Swendsen & Calabuig 2000). Large trap yields of inquilines and males of a gregariously nesting species may therefore indicate presence of a nesting site in vicinity of a given trap. The recording of species richness and distribution of inquiline bees may also be used to indicate other properties of nest building species populations: 1) If no parasites are present, the bee population may not be stable and / or is newly founded. Conversely, presence of parasites may indicate that host populations have been persistent through a sufficient period for parasite detection and invasion to occur. 2) Presence of parasites may indicate host species yet to be recorded in a survey (O'Toole & Ismay 1995). 3) Parasite / host ratio, termed Cleptoparasitic Load (Archer 1995), may be used as a measure of habitat quality and of species diversity (I. Calabuig, Manuscript III, In prep.) This study aims to assess whether solitary bees and bumblebees occur as persistent populations in the small semi-natural habitats of a Danish agricultural landscape. Though only one year of intensive data collection was carried out, and populations of bees are known to fluctuate markedly from year to year (Frankie et al. 1998), the confirmation of expected seasonal activity patterns of genera, and of presence and activity of inquiline species, are both strong indicators of persistent populations. The purpose of this study was furthermore to monitor on a local scale, the potential foragingand nesting conditions for solitary bees and bumblebees, and possible competition from honeybees. Since arable land claims so large a fraction of the area of Denmark (65%), a survey of bees in an agricultural landscape also gives an indication of the general status of solitary bees in Denmark. MANUSCRIPT I I. Calabuig: Assessment of the foragingand nesting conditions for solitary bees and bumblebees, and their distribution in a Danish agricultural landscape 18 The main questions I address in order to review the foragingand nesting conditions are the following: 1) Do solitary bees and bumblebees occur as abundant and persistent populations in a Danish conventional farmland? 2) What characterises the flora community of the semi-natural habitats, and what foraging potential for bees does it provide? 3) Can local coverage and species richness of mellitophilous plants explain species richness and abundance of solitary bees and bumblebees? 4) Which abiotic habitat parameters are significant in predictions of male and parasite abundances of solitary bees, using a generalised linear model? Materials and methods