Figure 1.
a) Schematic map of the study area indicating the positions of the experimental colony (hive-symbol), the release site (star) and the radar (red arrow) in the agricultural landscape (non-flowering crop/harvested fields: light green; field margins: solid black line; field tracks: brown; hedges: green, dashed black outline; woodland: dark green, solid black outline). Areas with impaired radar tracking are shown in horizontal stripes. b) Graphical representation of a homing flight track (circles) and the two hypothetical flight scenarios in a radar-centred x-y-coordinate system (unit = metres): S1) straight homing flight from the release site (star) to the colony (red) and S2) flight along available field margin connecting the release site and the hive (blue), both represented by the coordinates of 100 points evenly distributed along either path. The distance of the realized homing flight to either scenario was used as a proxy measure of the utilization of landscape features by bees of the different treatments to orientate towards the colony.
Figure 2.
Percentage of non-returning bees during homing flights per treatment group (sample sizes: Nc: 38; CGE: 32; CS: 35). A significantly higher proportion (X2 = 12.3, d.f. = 2, p = 0.002) of bees inoculated with Nosema ceranae failed to return to the colony as compared to the control bees.
Figure 3.
Flight paths of returned bees.
Graphical representation of all recorded flight paths of successfully homed bees: a) sucrose-control bees (CS), b) gut-extract-control bees (CGE) and c) Nosema-inoculated (Nc). The release site is located at the origin of the axes and the hive position indicated by the white arrow. The grey circle illustrates the 60 m perimeter at which track heading direction was assessed and which is illustrated in the inserts (black arrow: overall mean heading direction).
Figure 4.
Flight path of non-returned bees.
Graphical representation of all recorded flight paths of non-retuning bees and the spatial distribution of the final bee positions (CS: double black line, white diamonds, end-position = white cross; CGE: grey line, light grey markers, end-position = grey cross; Nc: black line, black markers, end-position = black cross). The release site is located at the origin of the axes and the hive position indicated by the white arrow. The grey circle illustrates the 60 m perimeter at which track heading direction was assessed and which is illustrated in the inserts (black arrow: overall mean heading direction).
Figure 5.
Box plots (box: median (central line) ± quartiles; whiskers: minimum – maximum values) of heading direction of homed and non-returning bees within each treatment (Nc = Nosema-inoculated, CGE = bee gut-extract-control, CS = sucrose-control) group after crossing the 60 m perimeter around the release site. The direction of the hive is indicated by the central dashed line. As the majority of the bees followed the available landmarks rather than assuming a direct approach to the hive, heading directions after 60 m from the release site generally deviate from the ideal direction. Using a linear mixed model (LMM) fitted by restricted maximum likelihood (REML), there was neither a statistically significant difference between homed and non-returning bees nor among the treatment groups.
Table 1.
Summary of flight parameters.
Figure 6.
Comparison of flight performance.
Biplots of a Canonical Variate Analysis (CVA) of a) normalized and standardized flight parameters (a: max. acceleration, b: mean acceleration, c: max. velocity, d: mean velocity, e: mean gap duration, f: total gap-duration, g: straightness) and b) normalized and standardized track parameters (h: actual total flight-time, i: Approachiness, j: total track duration, k: track length, f: total gap duration, e: mean gap duration; h and f coincident). Treatment mean scores+95% C.I. (solid circles) and circumferential polygons are given as follows: sucrose-control CS: ▴, dotted line; gut-extract control CGE: •, solid line; Nosema-inoculated Nc: ▾, dashed line. Both graphs show a low discriminative power of both types of parameter in separating Nosema-infected and uninfected bees, with only Approachiness (i), highly affected by homing success, allowing some separation of the infected non-returning bees from the rest.