Andreas Bertsch

Do foraging bumblebees scent-mark food sources and does it matter?

SummaryThe foraging of worker bees of Bombus terrestris visiting artificial feeders in a climatic test chamber was investigated. The behaviour of worker bees visiting rewarding and unrewarding feeders is completely different. Of all flower visits to rewarding feeders 94% are probing-visits, i.e. the bees land on the flower and probe for nectar. In contrast, only 0.3% of all visits to unrewarding feeders are probing-visits, whereas 47% are approach-visits, i.e., the bees approach the feeders without landing. Exchanging feeder discs proves that the signal used for discrimination must be associated with the plastic disc used as landing platform. Most probably it involves scent-marking of the rewarding feeders with components of high and low volatility. The mean foraging efficiency of bees in a scent-marked foraging arena is 5.7 mg sugar/min and drops to 2.8 mg sugar/min after the scent marked discs are replaced by clean ones. Three components generate this drop in foraging efficiency: (1) the between-flower flight time increases, i.e. the bees search for a longer time before landing on flowers, (2) the bees no longer discriminate between rewarding and unrewarding feeders, and (3) the bees probe empty feeders longer than necessary; obviously they expect to find nectar.

Foraging in male bumblebees (Bombus lucorum L.): maximizing energy or minimizing water load?

SummaryThe O2, CO2, and H2O exchange of single flying male bumblebees (Bombus lucorum and B. terrestris) were measured simultaneously. A respiratory quotient RQ=1 was found for all activities investigated (torpor-flight). The dependence of respiratory CO2 production in flight on body-weight was measured: for a 220-mg male bumblebee it amounts to 24.5 mg CO2/h (=56.4 ml O2/g·h). The corresponding evaporative water loss amounts to 6 mg H2O/h. Males tranferred to a climatic test chamber and conditioned to artificial flower feeders started to fly, after a few days of acclimatization, in typical scent-marked flight-paths. The daily pattern of flight activity was recorded: the mean total time in flight amounts to 244 min, and the corresponding daily flight length is about 17 km. At 20°C and 50% relative humidity (RH) a daily uptake of 180 μl (≙ 220mg) of 50% sugar solution was measured, equal to the mean body weight of the male bumblebees. Since the body weight remains constant on consecutive days a 24-h energy- and water-budget could be calculated. The energy-budget is balanced; the activities observed can be fuelled with the sugar available. About 70% of the energy is used for the 4 h of flight activity. With the daily nectar volume 110 mg of water is ingested; in the oxidation of 110 mg sugar, 66 mg of metabolic water is produced and 40 mg water is dissipated by the evaporative water-loss. Thus, to have a balanced water-budget, 136 mg of water must be voided in 24 h, which equals the total body-water of the bumblebees. Nectar is a nutrient of high water content which not only provides the sugar necessary for activity but also, in most circumstances, an excess of water. The effect of this high water load in limiting daily activity is discussed and compared with the water- and osmoregulation of hummingbirds. The strategy of maximizing energy for a male bumblebee must be one of minimizing water load.

Nectar production of Epilobium angustifolium L. at different air humidities; nectar sugar in individual flowers and the optimal foraging theory.

SummaryThe nectar production of Epilobium angustifolium L. was investigated at 20°C and 50%, 78% and 94% ambient humidity in the climatic test chamber. By means of permanent pipettes, freshly produced nectar was sucked off immediately after secretion, and nectar samples were also taken at 10-h and 48-h intervals to investigate the postsecretory influence of ambient humidity. Volume and sugar concentration of samples from individual flowers were measured and the sugar contained was calculated. The rate of sugar production remains constant for all ambient humidities and extraction intervals investigated; the mean value for all 180 samples is 1.55 mg sucrose equivalents/24h. Sugar concentration of secretion nectar is linearly dependent on ambient humidity over the range investigated, and nectar volume and sugar concentration change according to the theoretically expected curve for solutions with a sugar content of 1.55 mg sucrose. The response of secretion nectar to steplike changes in ambient humidity was investigated and the transient function described. The nectaries respond immediately to changes in ambient humidity. The consequences of the results for nectar production and nectar reward of individual flowers in the field and for the optimal foraging of pollinators are discussed. Discussion concentrates particularly on the following questions: what influence the variability of nectar reward in individual flowers may have on flower-visiting bumble-bees; whether these animals have the sensory capabilities to measure sugar exactly; and whether the water relations of pollinators may also influence foraging behaviour.

Species specificity and complexity of Dufour's gland secretion of bumble bees

1. 1. Dufours gland secretions of nine westpalaearctic bumble bee species, representing Terrestribombus, Pyrobombus and Megabombus, were analysed by gas chromatography and mass spectrometry. 2. 2. The secretions were dominated by uneven numbered straight chain alkanes from heneicosane to hentriacontane, each alkane accompanied by up to 10 positional alkene isomers. 3. 3. Various esters dominate among the 64 oxygen-containing compounds found. 4. 4. This is the first report on Dufours gland secretion of bumble bees. The secretion is species specific. Subgeneric and tribe specific patterns are indicated. 5. 5. Characteristics for species separation of workers of the Terrestribombus group are presented. 6. 6. The possible communicative function of the secretion is discussed.

Non-shivering thermogenesis in asynchronous flight muscles of bumblebees? Comparative studies on males of Bombus terrestris, Xylocopa sulcatipes and Acherontia atropos

Abstract 1. 1. Time course of thorax surface temperature elevation and of muscle action potentials in the dorsoventral flight muscles of Bombus terrestris, Xylocopa sulcatipes and Acherontia atropos were recorded. For continuous long-time measurement the spike activity was integrated (discharge 100-37% was 3.5 sec) before being recorded by a personal computer. 2. 2. In all three insect species “warming-ups” were always coupled with muscle action potentials. While in the moth the wings clearly vibrate during these phases, no movement of thorax or wings could be observed in both Hymenoptera by direct visual observation. 3. 3. In addition, in all three insects individual action potentials and thorax vibrations were measured during such heating periods in parallel. In hawkmoths spike frequencies diverted were between 18 and 25 Hz, while the thorax vibrated synchronously. Similar spike frequencies were also found in carpenter bee night muscles during heating and, unexpectedly, a nearly synchronous mechanical response of the thorax was recorded. The average amplitudes of these movements were 15μm and in maximum they were 25 μm. 4. 4. In contrast, no movements of the thorax were detectable in bumblebees by the vibration monitoring system (disintegration limit: 0.1 μm) during intensive “warming-ups”, although action potentials with up to 20 Hz appeared in the flight muscles. 5. 5. That means, no stretch activation of Bombus asynchronous flight muscles occur and consequently the actomyosin-ATPase of these muscles is not activated during heating, excluding that heat generation by a shivering mechanism taking place.

Fuel uptake, storage and use in male bumble beesBombus terrestris L.

Summary1.During a period of more than ten days the flight activity of male bumble beesBombus terrestris follows a constant daily rhythm and they exhibit a constant pattern of large daily food (sugar) intake. The fate of the sugar consumed was studied with especial regard to the following questions: Is it converted into other storage products and, if so, in which organs are they stored?2.Individual bumble bees were examined at three different times during the daily activity pattern (just before the onset of flight, at the beginning of foraging and at the end of the light period) and the total contents of glucose, fructose, trehalose, sucrose, glycogen and lipid in the thorax, digestive tract and the abdomen (minus digestive tract) measured.3.No storage of glycogen or lipids was found in the male bumble bees. They store the chemical energy in the form of non-polymeric carbohydrates in the digestive tract.4.The rates of fuel utilization during the different phases of the daily activity pattern were calculated. For the first time, the sugar consumption rate in male bumble bees during flight was determined directly (7.6 μmoles hexose units/min·g body mass).5.A 24-h energy budget for male bumble bees was constructed from measurements of daily food uptake and the rates of energy consumption (Table 3).

Analysis of the Labial Gland Secretions of the Male Bumblebee Bombus griseocollis (Hymenoptera: Apidae)

Abstract The labial gland secretions from males of the bumblebee Bombus (Separatobombus) griseocollis De Geer, a bumblebee exhibiting perching behaviour, were analysed by gas chromatography/ mass spectrometry (GC/MS) in the electron impact and positive ion chemical ionization mode. The major compound of the complex mixture of alkenols, acetates, hydrocarbons, wax type esters and steroids is tetradecyl acetate, considerable amounts of hexadecyl, geranyllinaloyl, geranylgeranyl, docosyl, tetracosenyl and hexacosenyl acetate were also found. 1,3-Tetradecanediol diacetate, detected as a minor component, has not yet been identified in male bumblebee labial gland secretions. Besides small amounts of primary alcohols (tetradecanol and hexadecanol) the tertiary alcohol geranyllinalool (3,7,11,15-tetramethyl- hexadeca-1,6,10,14-tetraene-3-ol) was also present. The primary alcohols were also present as esters of butanoic, dodecanoic, tetradecanoic, and hexadecanoic acid. Besides the usual mixture of un- and mono-unsaturated straight chain hydrocarbons, the labial gland contains the isoprenoid hydrocarbons β-springene [(6E,10E)-7,11,15-trimethyl-3-methylenehexadeca-1,6,10,14-tetraene] and two isomers of α-springene [(3Z,6E,10E)- and (3E,6E,10E)-3,7,11,15-tetramethyl-hexadeca-1,3,6,10,14-pentaene]. The close relationship in chemical composition in male bumblebees with perching and flight pass behaviour is discussed.

Chemistry of the Cephalic Labial Gland Secretions of Male Bombus morrisoni and B. rufocinctus , Two North American Bumblebee Males with Perching Behavior

The labial gland secretions from males of the North American bumblebees Bombus morrisoni Cresson and B. rufocinctus Cresson were analyzed by gas chromatography/mass spectrometry. In both species, 3,7,11,15-tetramethyl-2,6,10,14-hexadecatetraenyl acetate was found as the major compound of a complex mixture of alkenols, acetates, hydrocarbons, and wax-type esters. In addition to a mixture of saturated and mono-unsaturated straight chain hydrocarbons, the labial gland of both species contained the isoprenoid hydrocarbons (6E, 10E)-3,7,15-trimethyl-3-methylene-hexadeca-1,6,10,14-tetraene [β-springene], three isomers of 3,7,11,15-tetramethyl-hexadeca-1,3,6,10,14-pentaene [α-springene], and two further unidentified cyclic diterpenes. In B. morrisoni, 3,7,11-trimethyl-2,6,10-dodecatrien-1-ol and 3,7,11,15-tetramethyl-6,10,14-hexadecatrien-1-ol were detected as characteristic alcohols, as well as small amounts of 9-hexadecenol and hexadecanol. Furthermore, a large peak of hexadecyl dodecanoate and minor amounts of 9-hexadecenyl, 9-octadecenyl, and eicosenyl 9-tetradecenoate were found as typical esters in this species. In B. rufocinctus, 9-hexadecenol, hexadecanol, and 9-octadecenol were present in considerable amounts, with their acetates and 9-tetradecenoic, tetradecanoic, hexadecanoic, and 11-octadecenoic acids. The chemical composition of cephalic labial glands in male bumblebees with perching behavior is discussed.

Warm-up and substrate cycling in flight muscles of male bumblebees, Bombus terrestris

Abstract 1. 1. Rates of substrate cycling between fructose 6-phosphate and fructose bis-phosphate were measured in the flight muscles of male bumblebees ( Bombus terrestris ) in vivo during four denned activity phases. 2. 2. Shortly after the injection of double-labelled glucose thermocouples were fixed to the thorax to record surface temperature continuously and to determine the phase of activity every individual bee was in, when finally the cycling rate was measured. 3. 3. Based upon the 3 H/ 14 C-ratios, measured in the sugars and hexose-phosphates involved, the following relative cycling rates were determined: torpor: 0.24 ± 0.11; warm-up: 9.14 ± 3.51; “activated” rest: 1.94 ± 0.59, and flight: 0.22 ± 0.11. Using the data of the glycolytic fluxes, determined in the flight muscles of these bumblebees in previous studies, absolute cycling rates were evaluated as follows: torpor: 0.11 ± 0.04; warm-up: 249.91 ± 96.05; “activated” rest: 17.42 ± 5.29, and flight: 5.94 ± 2.95 μmol/min × g muscle. 4. 4. Conversion of these rates of substrate cycling (= rates of ATP-hydrolysis) into rates of heat production demonstrates that in the flight muscles of Bombus terrestris during phases of warming-up without any detectable movements of wings or thorax substrate cycling between fructose 6-phosphate and fructose-bis-phosphate very probably is the predominant mechanism of non-shivering heat production. 5. 5. Possible mechanisms for the regulation of the intensity of this fructose 6-phosphate/fructose bis-phosphate substrate cycle are discussed.

Analysis of the Labial Gland Secretions of the Male Bumble Bee Bombus perplexus Cresson (Hymenoptera: Apidae) from North America

The labial gland secretions from males of the bumble bee Bombus (Pyrobombus) perplexus Cresson were analysed by gas chromatography/mass spectrometry (GC/MS) in the electron impact and positive ion chemical ionization mode. The major compound of the complex mixture of alkenols, alkenals, fatty acids, hydrocarbons, wax type esters and steroids is 3,7,11,15-tetramethyl-2,6,10-hexadecatrien-1-ol (geranylcitronellol), considerable amounts of hexadecan-1-ol and Z-9-hexadecen-1-ol were also found. All alcohols were present as esters of the detected acids. In older samples both the acids and the alcohols sometimes could not be detected in the GC; therefore, the possibility to check the detected acid-alcohol pattern by interpreting the wax type ester peaks is very instructive. Moreover, the labial gland contains a rich mixture of mono- and di-unsaturated straight chain hydrocarbons. The similarity in composition of the labial glands of the North American B. (Pyrobombus) perplexus with the Eurasian species B. (Pyrobombus) hypnorum corroborates the assumption that the two species are conspecific. The likely supposition that the hydrocarbons could play an essential role in the chemical communication in bumble bees is discussed.