Global declines in insects have sparked wide interest among scientists, politicians, and the general public. Loss of insect diversity and abundance is expected to provoke cascading effects on food webs and to jeopardize ecosystem services. Our understanding of the extent and underlying causes of this decline is based on the abundance of single species or taxonomic groups only, rather than changes in insect biomass which is more relevant for ecological functioning. Here, we used a standardized protocol to measure total insect biomass using Malaise traps, deployed over 27 years in 63 nature protection areas in Germany (96 unique location-year combinations) to infer on the status and trend of local entomofauna. Our analysis estimates a seasonal decline of 76%, and mid-summer decline of 82% in flying insect biomass over the 27 years of study. We show that this decline is apparent regardless of habitat type, while changes in weather, land use, and habitat characteristics cannot explain this overall decline. This yet unrecognized loss of insect biomass must be taken into account in evaluating declines in abundance of species depending on insects as a food source, and ecosystem functioning in the European landscape.

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Bees require large amounts of pollen for their own reproduction. While several morphological flower traits are known to have evolved to protect plants against excessive pollen harvesting by bees, little is known on how selection to minimize pollen loss acts on the chemical composition of pollen. In this study, we traced the larval development of four solitary bee species, each specialized on a different pollen source, when reared on non-host pollen by transferring unhatched eggs of one species onto the pollen provisions of another species. Pollen diets of Asteraceae and Ranunculus (Ranunculaceae) proved to be inadequate for all bee species tested except those specialized on these plants. Further, pollen of Sinapis (Brassicaceae) and Echium (Boraginaceae) failed to support larval development in one bee species specialized on Campanula (Campanulaceae). Our results strongly suggest that pollen of these four taxonomic groups possess protective properties that hamper digestion and thus challenge the general view of pollen as an easy-to-use protein source for flower visitors.

To assess the pollen hosts of 60 western palaearctic bee species of the genus Colletes (Colletidae), we microscopically analysed 1336 pollen loads of collected females. Twenty-six species (43.3%) were found to be specialized at the level of plant family, subfamily or genus. Thirty-four species (56.7%) proved to be pollen generalists to varying degrees, visiting the flowers of up to 15 different plant families. Flowers of the subfamily Asteroideae (Asteraceae) are by far the most important pollen source, contributing 23.6% to the pollen-plant spectrum of the whole bee genus. The high significance of Asteroideae pollen is due to the large number of specialists: 14 Colletes species belonging to four different taxonomic groups harvest pollen exclusively or predominantly on flowers of the Asteroideae. By striking contrast, Asteroideae pollen plays only a marginal role in the diets of the pollen generalists: it was recorded in only 2.7% of the pollen loads and in seven out of the 34 pollen generalists. Among the few generalists exploiting Asteroideae for pollen, three closely related species have ancestors which were possibly specialized on Asteraceae. The pattern of use of Asteroideae pollen by the Colletes bees supports recent findings that this pollen possesses unfavourable or protective properties, which render its digestion difficult, and suggests that bees need physiological adaptations to successfully utilize it.

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To determine the extent of host—plant specialization, the pollen sources of the 72 anthidiine species (family Megachilidae, subfamily Megachilinae, tribe Anthidiini) of Europe, North Africa, and Asia Minor were investigated by microscopic analysis of ≈ 1800 pollen loads of females. By this measure, 31 species (43%) were oligolectic (relatively specialized to pollen source) at the level of plant tribe, plant subfamily, or plant family. Exclusive pollen sources of these bees throughout their geographic ranges are flowers of the Cardueae (Compositae), the Asteroideae (Compositae), the Papilionoideae (Leguminosae), the Lamioideae (Labiatae), the Nepetoideae (Labiatae), the Dipsacaceae, or the Campanulaceae. Thirteen species (18%) were found to exhibit a strong, but not exclusive, preference for the Papilionoideae (Leguminosae), the Labiatae, the Cardueae (Compositae), and Zygophyllum (Zygophyllaceae), respectively, while 25 species (35%) proved to be more markedly polylectic, visiting the flowers of up to 17 different plant families for pollen. The plants exploited by three species (4%) are insufficiently known. By far the most important pollen sources of the anthidiine bees as a whole are the Compositae (41.7%) followed by the Leguminosae (23.1%) and the Labiatae (13.0%). The phylogenetic relationships of the anthidiine bees were estimated by a cladistic analysis based on 115 characters to trace possible evolutionary patterns of diet composition. Based on the estimated phylogeny, at least eight shifts of oligoleges between different plant taxa and six transitions between oligolecty and polylecty appear to have occurred. Four transitions were from oligolecty to polylecty whereas two transitions are of unknown direction, both directions being equally parsimonious. Assuming that the ancestral state in the anthidiine bees was oligolectic, the present distribution of oligolectic and polylectic species can be explained solely by switches from the oligolectic to the polylectic habit and by shifts of oligoleges between different plant taxa. Three of four transitions from oligolecty to polylecty are accompanied by a reduction in bee body size. The significance of this size reduction with respect to the polylectic habit is discussed. The oligolectic anthidiine species visit significantly fewer flower species for pollen during a single foraging bout than the polylectic species. On average, 1.4 plant species were recorded in the loads of specialists compared to 2.2 for generalists. Two monophyletic groups of bees belonging to the genus Anthidium are equipped with a pollen—collecting apparatus consisting of specialized hairs localized either on the face or on the underside of the thorax. It is used to remove pollen from the raised anthers of flowers of the Labiatae and the Scrophulariaceae and to brush pollen from the flat inflorescences of some Compositae, respectively. The observation of flower—visiting females of several anthidiine species revealed that pollen uptake is far from an accidental process. Basic pollen—harvesting ...

Summary 1.Given the enormous quantitative pollen requirements of bees and their high efficiency in pollen removal, flowers should balance the need to attract bees for pollination on the one hand and to restrict pollen loss to bees on the other hand. Although various morphological flower traits have been identified that reduce excessive pollen losses to bees, the question of whether pollen might also be chemically protected remains largely unexplored. 2. In this study we compared the larval performance of the two very closely related and highly pollen generalist solitary bee species Osmia bicornis and Osmia cornuta on four different pollen diets. 3. Despite their very large pollen diet breadth, the two bee species showed striking differences in their ability to develop on pollen of the same plant species. Osmia bicornis developed well on Ranunculus pollen but failed to do so on Echium pollen, whereas the reverse held true for O. cornuta with the exception of two larvae grown on Ranunculus pollen that developed into dwarfish adults. Both bee species performed well on Sinapis pollen, while neither of the two species managed to develop on Tanacetum pollen. 4. The observed differences in larval survival of these two Osmia species when reared on the same pollen diet as well as their failure to develop on Tanacetum pollen clearly demonstrate that bees require physiological adaptations to cope with the unfavourable chemical properties of certain pollen. 5. Our results show a remarkable analogy of bee-flower relationships with herbivore-plant interactions and possibly indicate that the pollen of certain plant taxa might be chemically protected.

To trace the evolution of host-plant choice in bees of the genus Chelostoma (Megachilidae), we assessed the host plants of 35 Palearctic, North American and Indomalayan species by microscopically analyzing the pollen loads of 634 females and reconstructed their phylogenetic history based on four genes and a morphological dataset, applying both parsimony and Bayesian methods. All species except two were found to be strict pollen specialists at the level of plant family or genus. These oligolectic species together exploit the flowers of eight different plant orders that are distributed among all major angiosperm lineages. Based on ancestral state reconstruction, we found that oligolecty is the ancestral state in Chelostoma and that the two pollen generalists evolved from oligolectic ancestors. The distinct pattern of host broadening in these two polylectic species, the highly conserved floral specializations within the different clades, the exploitation of unrelated hosts with a striking floral similarity as well as a recent report on larval performance on nonhost pollen in two Chelostoma species clearly suggest that floral host choice is physiologically or neurologically constrained in bees of the genus Chelostoma. Based on this finding, we propose a new hypothesis on the evolution of host range in bees.K E Y W O R D S : Ancestral state reconstruction, evolutionary constraint, oligolecty, phylogeny, pollination, supermatrix.