André Kretzschmar

Influence of pollen nutrition on honey bee health: do pollen quality and diversity matter?

Honey bee colonies are highly dependent upon the availability of floral resources from which they get the nutrients (notably pollen) necessary to their development and survival. However, foraging areas are currently affected by the intensification of agriculture and landscape alteration. Bees are therefore confronted to disparities in time and space of floral resource abundance, type and diversity, which might provide inadequate nutrition and endanger colonies. The beneficial influence of pollen availability on bee health is well-established but whether quality and diversity of pollen diets can modify bee health remains largely unknown. We therefore tested the influence of pollen diet quality (different monofloral pollens) and diversity (polyfloral pollen diet) on the physiology of young nurse bees, which have a distinct nutritional physiology (e.g. hypopharyngeal gland development and vitellogenin level), and on the tolerance to the microsporidian parasite Nosema ceranae by measuring bee survival and the activity of different enzymes potentially involved in bee health and defense response (glutathione-S-transferase (detoxification), phenoloxidase (immunity) and alkaline phosphatase (metabolism)). We found that both nurse bee physiology and the tolerance to the parasite were affected by pollen quality. Pollen diet diversity had no effect on the nurse bee physiology and the survival of healthy bees. However, when parasitized, bees fed with the polyfloral blend lived longer than bees fed with monofloral pollens, excepted for the protein-richest monofloral pollen. Furthermore, the survival was positively correlated to alkaline phosphatase activity in healthy bees and to phenoloxydase activities in infected bees. Our results support the idea that both the quality and diversity (in a specific context) of pollen can shape bee physiology and might help to better understand the influence of agriculture and land-use intensification on bee nutrition and health.

Topological characterization of pore space in soil — sample preparation and digital image-processing

The connectivity of pore space is an important topological property of soil. Together with the porosity and the pore-size distribution, it governs the transport of water, solutes and gases. This paper describes the Euler-Poincare characteristic (EPC) as an index of connectivity of soil pore space in three dimensions, and shows how the EPC can be estimated by analysing pairs of parallel images, so-called disectors, automatically. We investigated the connectivity of pores > 50 μm within two horizons of a Terra fusca from serial surfaces through undisturbed soil impregnated with resin. A method is proposed for the segmentation of the grey level images into solid and void which is especially suitable for the detection of the pore edges. This results in identification of topologically distinct features. The 3D EPC was estimated from pairs of adjacent surfaces, the disector, using an algorithm to count the topological features by digital image processing. The EPC from 10 adjacent disectors within the two horizons are significantly different, and confirm qualitative observations of more intensely connected pore space in the upper Ah horizon. Additional results from 100 adjacent disectors in the lower AhBv horizon show the continuous evolution of the EPC estimates over a distance of 5 mm where two different scales of variation can be distinguished clearly: a small-scale variation which is attributed to methodological errors and a large scale variation reflecting the variability of the structure.

3D skeleton reconstructions of natural earthworm burrow systems using CAT scan images of soil cores

Abstract Four soil cores (length, 20 cm; diameter, 16 cm) were sampled in a Swiss pre-Alpine meadow with high earthworm abundance (>400 individuals/m2); two cores were taken in October 1993 and the other two cores in April 1994. The cores were described using computer assisted tomography which gives a series of section images every 3 mm. A method for reconstructing the three-dimensional (3D) skeleton of the earthworm burrow system is presented and discussed. This method provides an image of the structural organisation of the burrow system and was found to be adequately sensitive for use in ecological and functional studies. The seasonal variation of these 3D skeletons was investigated using two approaches, i.e. the analysis of: (1) global burrow system characteristics, and (2) individual burrow characteristics. At the scale of the global burrow system no difference was found between seasons (same number of burrows and same total burrow length) except for the vertical segment distribution, which was homogeneous in spring and decreased with depth in the fall. The study of individual burrow characteristics revealed that burrows tended to be more vertical in spring and that their branching intensity was higher in this season.

On the Front Line: Quantitative Virus Dynamics in Honeybee (Apis mellifera L.) Colonies along a New Expansion Front of the Parasite Varroa destructor

Over the past fifty years, annual honeybee (Apis mellifera) colony losses have been steadily increasing worldwide. These losses have occurred in parallel with the global spread of the honeybee parasite Varroa destructor. Indeed, Varroa mite infestations are considered to be a key explanatory factor for the widespread increase in annual honeybee colony mortality. The host-parasite relationship between honeybees and Varroa is complicated by the mites close association with a range of honeybee viral pathogens. The 10-year history of the expanding front of Varroa infestation in New Zealand offered a rare opportunity to assess the dynamic quantitative and qualitative changes in honeybee viral landscapes in response to the arrival, spread and level of Varroa infestation. We studied the impact of de novo infestation of bee colonies by Varroa on the prevalence and titres of seven well-characterised honeybee viruses in both bees and mites, using a large-scale molecular ecology approach. We also examined the effect of the number of years since Varroa arrival on honeybee and mite viral titres. The dynamic shifts in the viral titres of black queen cell virus and Kashmir bee virus mirrored the patterns of change in Varroa infestation rates along the Varroa expansion front. The deformed wing virus (DWV) titres in bees continued to increase with Varroa infestation history, despite dropping infestation rates, which could be linked to increasing DWV titres in the mites. This suggests that the DWV titres in mites, perhaps boosted by virus replication, may be a major factor in maintaining the DWV epidemic after initial establishment. Both positive and negative associations were identified for several pairs of viruses, in response to the arrival of Varroa. These findings provide important new insights into the role of the parasitic mite Varroa destructor in influencing the viral landscape that affects honeybee colonies.

X-ray computed tomography to quantify tree rooting spatial distributions

Poor root development due to constraining soil conditions could be an important factor influencing health of urban trees. Therefore, there is a need for efficient techniques to analyze the spatial distribution of tree roots. An analytical procedure for describing tree rooting patterns from X-ray computed tomography (CT) data is described and illustrated. Large irregularly shaped specimens of undisturbed sandy soil were sampled from Various positions around the base of trees using field impregnation with epoxy resin, to stabilize the cohesionless soil. Cores approximately 200 mm in diameter by 500 mm in height were extracted from these specimens. These large core samples were scanned with a medical X-ray CT device, and contiguous images of soil slices (2 mm thick) were thus produced. X-ray CT images are regarded as regularly-spaced sections through the soil although they are not actual 2D sections but matrices of voxels similar to 0.5 mm x 0.5 mm x 2 mm. The images were used to generate the equivalent of horizontal root contact maps from which three-dimensional objects, assumed to be roots, were reconstructed. The resulting connected objects were used to derive indices of the spatial organization of roots, namely: root length distribution, root length density, root growth angle distribution, root spatial distribution, and branching intensity. The successive steps of the method, from sampling to generation of indices of tree root organization, are illustrated through a case study examining rooting patterns of valuable urban trees

A population facing climate change: joint influences of Allee effects and environmental boundary geometry

As a result of climate change, many populations have to modify their range to follow the suitable areas—their “climate envelope”—often risking extinction. During this migration process, they may face absolute boundaries to dispersal because of external environmental factors. Consequently, not only the position, but also the shape of the climate envelope can be modified. We use a reaction-diffusion model to analyse the effects on population persistence of simultaneous changes in the position and shape of the climate envelope. When the growth term is of logistic type, we show that extinction and persistence are principally conditioned by the species mobility and the speed of climate change, but not by the shape of the climate envelope. However, with a growth term taking an Allee effect into account, we find a high sensitivity to variations in the shape of the climate envelope. In this case, the species which have a high mobility, although they could more easily follow the migration of the climate envelope, would be at risk of extinction when encountering a local narrowing of the boundary geometry. This effect can be attenuated by a progressive opening at the exit of the narrowing into the available space, even though this leads temporarily to a diminished area of the climate envelope.

Decomposition of 14C-labelled plant material in soil: The influence of substrate location, soil compaction and earthworm numbers

Abstract The influences of substrate location and of earthworm number on the dynamics of decomposition of 14 C-labelled plant material, freshly-added to soil, were measured in laboratory studies using different conditions of soil compaction. The labelled plant material (mature leaves of Trifolium subterraneum ), was added to columns of soil compacted at three levels (200, 400 and 600 kPa), and was located either as litter on the soil surface, or was mixed with the top layer of soil, or mixed with soil at depth. Earthworms were either absent, or soils contained either 4 or 7 earthworms ( Aporrectodea trapezoides ) per column. The release of CO 2 - 14 C from these soil columns was recorded over a period of 104 days. Total recoveries of 14 C in soil plus evolved CO 2 exceeded 90% in all treatments measured, except when the soil was highly compacted and the plant material was buried at depth. Under these latter conditions, earthworm addition increased the total 14 C recoveries, which ranged from 73 to 88% of input 14 C. Lag periods before maximal rates of 14 CO 2 evolution were evident in treatments with surface-applied litter, and to a lesser extent, with plant material incorporated at depth. In this latter treatment, lag time was highest in soil of high compaction, but was decreased with increasing earthworm numbers. In all other treatments lag time appeared to be unaffected by earthworm numbers and by level of soil compaction. Maximal rates of 14 CO 2 release generally increased with soil compaction but exhibited no consistent trend with earthworm numbers. A clear exception to the general pattern occurred when plant material was incorporated at depth in highly compacted soil. In this treatment maximal rates were relatively very low, but increased with earthworm numbers. The influence of the imposed treatments on the total extent of 14 C evolution reflected closely their effects on maximal rates of 14 CO 2 release. Effects of either soil compaction or substrate location were most demonstrable with treatments which likely limited the aerobic processes of decomposition. In such cases, the marked effects of the presence of earthworms and their number probably depended on the favourable effects of their casts and their burrows in removing physical constraints, both on aerobic decomposition per se of added plant material and also on the efficiency of measurement of evolved gaseous metabolic products. Thus, earthworm influence appeared to be indirect, through the interaction of their activities, soil compaction and substrate location.

Combined neonicotinoid pesticide and parasite stress alter honeybee queens' physiology and survival.

Honeybee colony survival strongly relies on the queen to overcome worker losses exposed to combined stressors like pesticides and parasites. Queen’s capacity to withstand these stressors is however very little known. The effects of the common neonicotinoid pesticide imidacloprid in a chronic and sublethal exposure together with the wide distributed parasite Nosema ceranae have therefore been investigated on queen’s physiology and survivorship in laboratory and field conditions. Early physiological changes were observed on queens, particularly the increase of enzyme activities (catalase [CAT] and glutathione-S-transferase [GST] in the heads) related to protective responses to xenobiotics and oxidative stress against pesticide and parasite alone or combined. Stressors also alter the activity of two other enzymes (carboxylesterase alpha [CaE α] and carboxylesterase para [CaE p] in the midguts) involved in metabolic and detoxification functions. Furthermore, single and combined effects of pesticide and parasite decrease survivorship of queens introduced into mating hives for three months. Because colony demographic regulation relies on queen’s fertility, the compromise of its physiology and life can seriously menace colony survival under pressure of combined stressors.

Size changes in honey bee larvae oenocytes induced by exposure to Paraquat at very low concentrations.

The effects of the herbicide Paraquat were investigated in honey bee larvae with attention focused on oenocytes. Honey bee larvae were exposed to Paraquat at different concentrations in the food: 0, 0.001, 0.01, 0.1 and 1 µg/kg. In controls, between 24 h and 48 h, oenocytes grew from 630.1 to 1643.8 µm2 while nuclei changed in size from 124.9 to 245.6 µm2. At 24 h, Paraquat induced a slight decrease in the size of oenocytes and nuclei. N-acetylcysteine (NAC), an antioxidant substance, slightly lowered the effects of Paraquat. At 48 h, Paraquat elicited a strong concentration-dependent decrease in the size of oenocytes, even at the lowest concentration. NAC reversed the effect of Paraquat at a concentration of ≥0.01 µg/kg. This reversion suggested different modes of action of Paraquat, with an oxidant action prevalent at concentrations ≥0.01 µg/kg. This study is the first which reports an effect of a pesticide at the very low concentration of 1 ng/kg, a concentration below the detection limits of the most efficient analytic methods. It shows that chemicals, including pesticides, are likely to have a potential impact at such exposure levels. We also suggest that Paraquat could be used as a suitable tool for investigating the functions of oenocytes.

Estimation of the relationship between structural parameters of simulated burrow systems and their partitioning effect

Abstract From our previous studies on burrow systems in soils it is apparent that, depending on the structural characteristics of these systems (pore mean length distribution, density, presence of very long burrows), the heterogeneity of the environment can be described using the functional parameters of the poral system. The main structural characteristics of the biological pore systems discussed here is the “shortest pathway” through the system, joining any point of the soil space to the surface. The present paper deals with some ways to measure the pattern of spatial heterogeneity, using simulated burrow systems that are generated by the combination of four parameters: mean burrow length, angular orientation, central coordinates and density. We look first at the patterns of the heterogeneity related to depth when the pore system structure is changing qualitatively (introduction of long burrows) or quantitatively (mean length or density variations); we then focus on the horizontal pattern complexity with depth-dependant variation in pore density. Finally, we characterize the spatial 3D pattern of the partitioning effect of the burrow system by the estimation of tesselation cell density and distribution of the shortest pathway length.