The author has declared that no competing interests exist.
Conceived and designed the experiments: WSR. Performed the experiments: WSR. Analyzed the data: WSR. Contributed reagents/materials/analysis tools: WSR. Wrote the paper: WSR.
Giant honey bees (
The giant honey bee,
Migration distances have been inferred by the presence of colonies on combs in different places at different times of year. Swarms reportedly cross the 50-km-wide Strait of Malacca between Sumatra and Malaysia
Bivouacking
Although I recently described congregated bivouacs
Finally I stress the possible implications of BCSS’s for conservation of this spectacular honey bee, in light of worldwide pollinator decline, Southeast Asia’s rapid deforestation, dwindling numbers of giant honey bees and other insect species’ vulnerability to ecological bottlenecks.
I began searching for bees on 25 August 2009 at the Mae Hong Son Agricultural Research and Development Center. The Center is 4 km southwest of Mae Hong Son, Thailand, elevation 200 m, 19°16′09 N, 97°56′39 E. Searches were approximately weekly until 7 September, when I encountered 2
In 2010 I began searching for bivouacs at the Agricultural Center 17 August. Observations concluded 19 November, thus bracketing the 2009 study period. Although the study ended 3 weeks later than in the previous year, the dry season was not nearly as advanced.
Note concentration of bivouacs in riverside mango trees. In 2010 one other bivouac was seen, 1 km NE of study site, along transect road. A = airstrip; C = assorted vegetable crops; F = forest; M = mango orchards; Ma = macadamia orchard; OMA = old mango trees; P = pomelo orchard; PS = passion fruit; R = road; U = undergrowth.
The rainy season in northern Thailand is April through September, peaking in August. The dry season is November to February. The summer and fall of 2009 were abnormally dry, while in 2010 a strong monsoon caused widespread flooding. From July through October, more than twice the amount of rain fell in 2010 as in 2009 (
Source: Air Force Datsav3 Weather Station 483000, call sign VTCH.
Dates of arrival and departure of bivouacs are presented in
Bars followed by a question mark represent bivouacs still on site at conclusion of 2010 study.
Bivouacs favored mango trees more in 2010 and were also more densely concentrated than in 2009 (
Mean size of 2009 bivouacs was 1,807± s.d. 683 cm2; in 2010 mean size was 2,562± s.d. 3,301 cm2. This difference was not significant according to an unpaired t-test (2-tailed P = 0.37; df = 50). However, size range and variability were markedly different in the 2 years, as reflected in disparate coefficients of variation. In 2009 bivouacs ranged in size from 226 to 2,743 cm2; in 2010 the range was 130 to 12,993 cm2. The coefficient of variation
Bivouacs generally rested at lower heights in 2010 than in 2009. Resting height varied from 1.8 to 18.0 m (mean = 10.1±5.0 m) in 2009; in 2010 resting height ranged from 1.0 to 15.0 m (mean = 5.6±4.3 m). This 4.5-m difference in mean resting height was statistically significant (2-tailed P = 0.002; df = 50).
None of the 52 bivouacs built comb. Departing swarms left numerous small beads of white wax behind on the bark at their resting places. In 2010, 2 transitory colonies, not included among the bivouacs in this study, established small (≈30 cm-diameter) combs near the study site during the first week, only to abscond within a few days.
All swarm flights were preceded in the 30–60 min before liftoff by a varying number of simultaneously dancing bees, spread over the outer curtain of bees (
I watched 4 swarms depart in 2009 (
(A) observations from 2009, (B, C) observations from 2010. Dances of each worker were numerous; directions and distances indicated were unanimous for all dancers in each bivouac. Each color represents dances and flight of a single bivouac. Each concentric ring represents either 3 s of dancing (zigzag lines)–waggle runs only for dances of >3 s duration, complete circuits for dances of <3 s–or 30 m of flight (straight lines). Most flights exceeded 150 m and disappeared from view, as indicated by broken lines. At periphery are identification numbers of bivouacs. N = maximum number of workers simultaneously dancing within 30 min before flight.
Flight directions of the departing bivouacs quite accurately reflected preceding waggle dance orientations (
It is possible that BCSS’s occur as migration stopover grounds throughout southern Asia, likely in geographical locations with landmarks that are easily recognizable to bees and could be found instinctively, just as drone congregation areas are found by both instinct and environmental cues
Without marking or radio telemetry studies, it is impossible to know either the origin(s) or ultimate destination(s) of the migrating bees. Given the variety of flight directions from this valley surrounded by higher ground, it appears the bees are making altitude shifts of varying distance similar to what occurs in Nepal
An alternative hypothesis to account for the BCSS is that bees were considering, but discarding, the mango orchard as a possible nesting site. This alternative is supported by the presence in 2010 of 2 small transitory colonies that built comb and quickly absconded, but seems unlikely in that 52 swarms built no comb.
The 2010 bivouacs, larger in number, were also more densely crowded than in 2009. Unlike those in Sri Lanka
The range in size of the bivouacs was remarkable, given the typical
Though the sample size is small, the 2 observations of short flights preceded by short-duration waggle runs, with longer flights portended by longer dances, may contradict earlier conclusions for
Pollinators are in decline worldwide
Though attention has been paid to disappearance of giant honey bee nesting sites in large trees
Populations of many migratory animals, and the migrations themselves, have been damaged or destroyed by loss of specific habitat crucial to vulnerable stages of the animals’ life histories
Permission for access to the study site was granted by the director of the Mae Hong Son Agricultural Research and Development Center. No permits were required for very limited insect collecting. Field studies did not involve endangered or protected species.
Commencing Sept 7, at the beginning and end of every day I surveyed the area for newly arrived and departed bivouacs, by walking or bicycling slowly for 2.5 km along the road flanking the west side of the mango orchard. This twice-daily transect included close inspection of the orchard, and of trees on both sides of the road for ≈1 km north and south. I also patrolled the site during lulls in colony activity, and sometimes found new swarms as I made rounds studying bivouacs already present. Field workers occasionally told me of swarms and their arrival times.
At least twice daily I surveyed the macadamia and pomelo orchards to the west. No bivouacs were encountered in the pomelos. I found several bivouacs in the macadamias. Approximately weekly I searched the other mango orchard, west ≈250 m from the river, but never encountered bees there. I also regularly traveled through northern Thailand both near the study area and well outside it, constantly looking for more bivouacs without success. This included, for example, a daily bicycle ride of ∼10 km to and from the village of Mae Hong Son, through human settlements, agricultural and forested land. At the height of the 2009 bivouac season I rafted the Pai River from Pai to Mae Hong Son (∼70 km), looking unsuccessfully along the shores for bivouacs.
Bivouacs were distinguishable by their widely varying sizes and shapes. Upon finding a new bivouac I attached an identifying tag to a nearby branch. With the exception of 2 that took short flights that I was able to track, bivouacs did not move around the study site before leaving it completely, so there was no potential for confusing one with another.
I recorded arrival time as the date I first saw the swarm. Often this was very precise, as I actually saw the swarm land. In rare cases that date could be late by as much as several days, if a newly arrived swarm went unnoticed. I recorded departure time as the date at which I first noted the bivouac’s absence. Again, in many cases I saw swarms depart the study site; in other instances the swarm may have left the previous afternoon but its absence was not noticed until morning. In calculating the mean length of stay, I recorded a stay as 0.5 day if it departed the same day it arrived, 1 day if its absence was noticed the day after arrival, 2 days if it was gone 2 days after its arrival, etc.
I was unsure of the precise arrival time of the first swarms seen in 2009. They were not present during a preliminary search on 1 September, but I did not survey the orchard again until 7 Sept, when I found the first bivouacs. I have thus interpolated 4 Sept as their arrival date. In 2010 I left Thailand on 19 November, when 5 bivouacs were still present, and was unable to record their departure dates. In calculating duration of stay, I used 19 November as the date of departure, so these calculations are overly conservative.
I used a tape measure to measure colony size and height at which swarms rested. For occasional lofty colonies, I estimated height by sighting against a 2-m reference pole, and colony dimensions using the known 17-mm length of an
I employed unpaired t-tests to test the hypotheses that mean swarm size, mean bivouac resting height and mean length of stay varied between the 2 years.
I patrolled the swarms on a 30–60 min circuit, observing any dancing worker bees, and apparent preparations for swarm flight. Bivouacs were often low in the trees and easily viewed from 1–3 m with the unaided eye. I used Leupold™ 10×42 binoculars to view higher swarms. I photographed and video-recorded the bees’ behavior using Canon Power Shot™ S3 IS or SX10 IS digital cameras.
I measured duration of waggle dance circuits
When a swarm flew, I used a compass to determine its flight direction to the nearest 5° by sighting a line from point of takeoff to point of disappearance or observed landing. I used the NOAA Sun Location Calculator
Statistical tests were employed following Mead
(MP4)
Several bees dance 2-s complete circuits as a medium-sized bivouac prepares for a short flight of 150 m.
(MP4)
(MP4)
I thank Maria Katherman for her many valuable insights. Jeffrey Lockwood and Quinn Robinson critiqued the manuscript. Jesseda Weera, Yotin Bootchalaey and Jaroon Kumnuanta provided logistical help in Thailand. Phanida Ponya helped monitor bivouacs. Jane Ifland prepared the figures.