Honeybee thermoregulation

Evaluation of herbicide formulations and spreading agents on survival of the bumblebee Bombus impatiens following spray and contact exposure

Surfactants are often included as co-formulants in the application of herbicides to aid in spreading and adherence to plant surfaces, but toxicity to native bees has not been extensively tested. In a set of progressive experiments, we evaluated effects of products containing glyphosate and spreading agents, as well as spreading agents alone, on bumblebees (Bombus impatiens Cresson) using parametric survival analysis. We test spreaders from multiple chemical classes including Silwet L-77© (trisiloxane), Alligare 90© (polyoxyethylene), and Southern Ag SA-50© (C10-16 alcohols). We report low lethality of high-glyphosate herbicide formulations (Rodeo©), but bee mortality increased ~20% with addition of a silicone-based spreading agent (Silwet L-77©). Spreaders alone strongly affected bee survival: effects were concentration-specific and did not differ depending on exposure method (spray application vs. application to surfaces contacted by bees). The widely used trisiloxane-based spreader Silwet L-77© was especially hazardous, and exposure to high concentrations of Silwet L-77© caused rapid and near-total mortality in B. impatiens. Analysis of whole-bee cuticle extracts after exposure revealed clear differences in the cuticular hydrocarbon profiles associated with exposure to spreading agents: the alkane n-hexacosane was present in all extracts but was detected in greater relative abundance from bees exposed to Silwet L-77© and Alligare 90©. To support wild bee conservation efforts, we recommend substituting alcohol-based spreaders for siloxane-based spreaders when possible. In addition, certain cuticular hydrocarbons may be useful as biomarkers of previous exposure to certain surfactants, which can aid investigations evaluating causes of bumblebee decline across landscapes.

A thermal performance curve perspective explains decades of disagreements over how air temperature affects the flight metabolism of honey bees

While multiple studies have shown that honey bees and some other flying insects lower their flight metabolic rates when flying at high air temperatures, critics have suggested such patterns result from poor experimental methods as, theoretically, air temperature should not appreciably affect aerodynamic force requirements. Here, we show that apparently contradictory studies can be reconciled by considering the thermal performance curve of flight muscle. We show that prior studies that found no effects of air temperature on flight metabolism of honey bees achieved flight muscle temperatures that were near or on equal, opposite sides of the thermal performance curve. Honey bees vary their wing kinematics and metabolic heat production to thermoregulate, and how air temperature affects the flight metabolic rate of honey bees is predictable using a non-linear thermal performance perspective of honey bee flight muscle.

The effect of spinning conditions on the mechanics of a spider's dragline silk

We studied the mechanical properties of dragline threads of the edible golden silk spider Nephila edulis that are produced under spinning speeds ranging from 0.1 to 400 mm s(-1) and temperatures ranging from 5 to 40 degrees C. These conditions affected the silk in all of the mechanical traits we tested (strain at breaking, breaking energy, initial Young's modulus and point of yielding). We argue that both trade-offs (between mechanical properties) and constraints (in the manufacturing process) have a large role in defining spider silk fibres.

Mite not make it home: tracheal mites reduce the safety margin for oxygen delivery of flying honeybees

Many physiological systems appear to have safety margins, with excess capacity relative to normal functional needs, but the significance of such excess capacity remains controversial. In this study, we investigate the effects of parasitic tracheal mites (Acarapis woodi) on the safety margin for oxygen delivery and flight performance of honeybees. Tracheal mites did not affect the flight metabolic rate of honeybees in normoxic (21% oxygen) or hyperoxic (40% oxygen) air, but did reduce their metabolic rate relative to uninfected bees when flying in hypoxic air (5 or 10% oxygen), demonstrating that mites reduced the safety margin for tracheal oxygen delivery. The negative effects of mites on flight metabolic rate in hypoxic atmospheres were graded with the number of mites per trachea. For example, in 10% oxygen atmospheres, flight metabolic rate was reduced by 20% by moderate mite infection and by 40% by severe mite infection. Thus, the safety margin for oxygen delivery in honeybees allows them to retain normal flight metabolic rate and behavior despite tracheal mite infection under most conditions. However, the reduction in tracheal gas-exchange capacity may constrain activities requiring the highest metabolic rates, such as flying in cool weather. In support of this hypothesis, bees that were unable to return to the hive during late-winter flights showed significantly higher levels of mite infection than bees that returned safely.

Metabolic rate and thermal stability during honeybee foraging at different reward rates

During honeybee foraging, the stabilization of thoracic temperature (Tth) at elevated values is necessary to meet the power requirements of flight at different air temperatures (T(a)). To understand how the bee achieves thermal stability at different reward rates, the metabolic rates of undisturbed foraging bees were measured at different T(a) values and different sucrose solution flow rates. Metabolic heat production, calculated from the rate of carbon dioxide production, decreased linearly from 49.7 to 23.4 mW as T(a) increased from 19 to 29 degrees C (sucrose flow rate 1.75 microl × min(−1), 50 % w/w). In contrast, crop load and inspection rate remained constant. Metabolic rate displayed a linear relationship with both T(a) and the logarithm of the flow rate of sucrose solution (range analyzed 0.44-13.1 microl × min(−1), 50 % w/w). Metabolic rate decreased by 3.13+/−0.52 mW (mean +/− s.e.m., N=37) for every 1 degrees C increase in T(a) and increased by 4.36+/−1.13 mW for a doubling in flow rate. These changes in metabolic power output might be used to achieve thermal stability during foraging. It is suggested that the foraging bee might increase its Tth in accordance with the reward rate.

Mechanisms of thermal stability during flight in the honeybee apis mellifera

Thermoregulation of the thorax allows honeybees (Apis mellifera) to maintain the flight muscle temperatures necessary to meet the power requirements for flight and to remain active outside the hive across a wide range of air temperatures (Ta). To determine the heat-exchange pathways through which flying honeybees achieve thermal stability, we measured body temperatures and rates of carbon dioxide production and water vapor loss between Ta values of 21 and 45 degrees C for honeybees flying in a respirometry chamber. Body temperatures were not significantly affected by continuous flight duration in the respirometer, indicating that flying bees were at thermal equilibrium. Thorax temperatures (Tth) during flight were relatively stable, with a slope of Tth on Ta of 0.39. Metabolic heat production, calculated from rates of carbon dioxide production, decreased linearly by 43 % as Ta rose from 21 to 45 degrees C. Evaporative heat loss increased nonlinearly by over sevenfold, with evaporation rising rapidly at Ta values above 33 degrees C. At Ta values above 43 degrees C, head temperature dropped below Ta by approximately 1–2 degrees C, indicating that substantial evaporation from the head was occurring at very high Ta values. The water flux of flying honeybees was positive at Ta values below 31 degrees C, but increasingly negative at higher Ta values. At all Ta values, flying honeybees experienced a net radiative heat loss. Since the honeybees were in thermal equilibrium, convective heat loss was calculated as the amount of heat necessary to balance metabolic heat gain against evaporative and radiative heat loss. Convective heat loss decreased strongly as Ta rose because of the decrease in the elevation of body temperature above Ta rather than the variation in the convection coefficient. In conclusion, variation in metabolic heat production is the dominant mechanism of maintaining thermal stability during flight between Ta values of 21 and 33 degrees C, but variations in metabolic heat production and evaporative heat loss are equally important to the prevention of overheating during flight at Ta values between 33 and 45 degrees C.