Sue W. Nicolson

Do honeybees, Apis mellifera scutellata, regulate humidity in their nest?

Honeybees are highly efficient at regulating the biophysical parameters of their hive according to colony needs. Thermoregulation has been the most extensively studied aspect of nest homeostasis. In contrast, little is known about how humidity is regulated in beehives, if at all. Although high humidity is necessary for brood development, regulation of this parameter by honeybee workers has not yet been demonstrated. In the past, humidity was measured too crudely for a regulation mechanism to be identified. We reassess this issue, using miniaturised data loggers that allow humidity measurements in natural situations and at several places in the nest. We present evidence that workers influence humidity in the hive. However, there are constraints on potential regulation mechanisms because humidity optima may vary in different locations of the nest. Humidity could also depend on variable external factors, such as water availability, which further impair the regulation. Moreover, there are trade-offs with the regulation of temperature and respiratory gas exchanges that can disrupt the establishment of optimal humidity levels. As a result, we argue that workers can only adjust humidity within sub-optimal limits.

Honeybees and nectar nicotine: Deterrence and reduced survival versus potential health benefits

Secondary metabolites produced by plants for herbivore defence are often found in floral nectar, but their effect on the foraging behaviour and physiological performance of pollinators is largely unknown. Nicotine is highly toxic to most herbivores, and nicotine-based insecticides may contribute to current pollinator declines. We examined the effects of nectar nicotine on honeybee foraging choices and worker longevity. Free-flying honeybee (Apis mellifera scutellata) workers from six colonies were given a choice between multiple nicotine concentrations (0-1000 μM) in artificial nectar (0.15-0.63 M sucrose). The dose-dependent deterrent effect of nicotine was stronger in lower sugar concentrations, but even the highest nicotine concentrations did not completely repel honeybees, i.e., bees did not stop feeding on these diets. Nicotine in nectar acts as a partial repellent, which may keep pollinators moving between plants and enhance cross-pollination. In the second part of the study, newly emerged workers from 12 colonies were caged and fed one of four nicotine concentrations (0-300 μM) in 0.63 M sucrose for 21 days. Moderate (≤30 μM) nicotine concentrations had no significant detrimental effect, but high nicotine concentrations reduced the survival of caged workers and their nectar storage in the honey comb. In contrast, worker groups that survived poorly on sugar-only diets demonstrated increased survival on all nicotine diets. In the absence of alternative nectar sources, honeybees tolerate naturally occurring nectar nicotine concentrations; and low concentrations can even be beneficial to honeybees. However, high nicotine concentrations may have a detrimental effect on colony fitness.

Convergence of carbohydrate-biased intake targets in caged worker honeybees fed different protein sources

SUMMARY The nutritional needs of bees are supplied by nectar carbohydrates and by protein and other nutrients in pollen but little is known of how bees achieve nutritional balance. Using newly emerged caged worker honeybees (Apis mellifera scutellata), we investigated whether bees maintain their intake target when confined to pairs of imbalanced complementary diets varying in protein to carbohydrate (P:C) ratio. Diets were formulated using three protein sources [casein, royal jelly or Feed-Bee® (a natural pollen substitute)] and sucrose. Within each protein type, honeybees switched between complementary diets and converged on the same P:C intake target. However, this target differed between protein types: P:C ratios were 1:12, 1:14 and 1:11 on casein, royal jelly and Feed-Bee® diets, respectively. Except for an early peak in protein consumption on royal jelly diets, these strongly convergent ratios remained constant over the 14 day experiment. This is probably due to the absence of brood, reflected in relatively stable values measured for haemolymph protein concentration and hypopharyngeal gland activation in bees on Feed-Bee® diets. Performance of caged workers was also assessed in terms of survival and ovarian activation. Survival was highest on casein diets and lowest on Feed-Bee® diets but ovarian activation was highest on royal jelly diets and lowest on casein diets. This may be due to additional components in Feed-Bee® and royal jelly (e.g. fatty acids), which are needed to activate the ovaries but also reduce survival. Nutrient intake of broodless workers is directly related to their own physiological requirements, and the strong carbohydrate bias may reflect the high metabolic rate of honeybees even under resting conditions.

Response of avian nectarivores to the flowering of Aloe marlothii: a nectar oasis during dry South African winters

In southern Africa, Aloe marlothii flowers during the dry winter season and offers copious dilute nectar to a variety of birds. Avian abundance and community composition were monitored at an A. marlothii forest at Suikerbosrand Nature Reserve, South Africa. Sampling occurred during two summer months (February–March) when no flowers were present, and six months (May–October) that spanned the winter flowering. We hypothesized that an influx of occasional nectarivores to the A. marlothii forest during flowering would lead to significant changes in the avian community. Overall bird abundance increased 2–3 fold at the peak of nectar availability (August). We recorded 38 bird species, of 83 species detected during transects, feeding on A. marlothii nectar; this diverse assemblage of birds belonged to 19 families, including Lybiidae, Coliidae, Pycnonotidae, Sylviidae, Cisticolidae, Muscicapidae, Sturnidae, Ploceidae and Fringillidae. Surprisingly, only two species of sunbird (Nectariniidae) were observed feeding on A. marlothii nectar, and both occurred in low abundance. We predicted that competition for nectar resources would be high, but few aggressive inter- and intra-specific interactions occurred between birds while feeding on inflorescences. During peak flowering, insect feeders (insectivores, omnivores, nectarivores) fed on nectar during the cold morning when insect activity was low, whilst non-insect feeders (frugivores and granivores) fed on nectar in the middle of the day. Our study highlights the importance of A. marlothii nectar as a seasonal food and water source for a diverse assemblage of occasional nectarivores.

Detoxification mechanisms of honey bees (Apis mellifera) resulting in tolerance of dietary nicotine.

Insecticides are thought to be among the major factors contributing to current declines in bee populations. However, detoxification mechanisms in healthy, unstressed honey bees are poorly characterised. Alkaloids are naturally encountered in pollen and nectar, and we used nicotine as a model compound to identify the mechanisms involved in detoxification processes in honey bees. Nicotine and neonicotinoids have similar modes of action in insects. Our metabolomic and proteomic analyses show active detoxification of nicotine in bees, associated with increased energetic investment and also antioxidant and heat shock responses. The increased energetic investment is significant in view of the interactions of pesticides with diseases such as Nosema spp which cause energetic stress and possible malnutrition. Understanding how healthy honey bees process dietary toxins under unstressed conditions will help clarify how pesticides, alone or in synergy with other stress factors, lead to declines in bee vitality.

Nutrition affects survival in African honeybees exposed to interacting stressors

BBSRC,NERC, the Wellcome Trust, Defra, and the Scottish Government nunder the Insect Pollinators Initiative Grant (BB/I000968/1).

Differential dispersal and survival of an African mistletoe: does host size matter?

Mistletoes typically grow on tall old trees. Does this positive size-prevalence relationship result simply from the accumulation of infections as trees age, or do other factors related to tree size lead to differential dispersal, germination, establishment or survival of mistletoes? We examined patterns of infection prevalence and intensity of the mistletoe Phragmantheradschallensis on its main host, Acacia sieberana, in a savannah in Zambia. The probability that an A. sieberana tree was infected increased with tree size, although infection intensity did not. In addition, infected trees were significantly taller than non-infected trees, even after removing the effect of tree age, using trunk diameter as a proxy. To determine whether differential dispersal occurred, we observed the feeding behaviour of the three main avian dispersers of this mistletoe, Pogoniuluschrysoconus, Lybiustorquatus and Cinnyricinclusleucogaster, in relation to tree size. All three preferred perching in taller trees, whether trees were parasitized or not. To test whether differential germination of seeds or establishment of seedlings occurred, we planted mistletoe seeds on small and large A. sieberana trees and recorded germination, establishment and survival of seedlings. All seeds germinated, and seedling establishment at 7xa0months and seedling survival to 14xa0months was no different on small and large trees. Thus, host size did not affect germination and establishment. Finally, we measured the survival of established mistletoes on trees of different sizes over a 3-year period. Mistletoes survived significantly better on taller hosts. Our results support the hypotheses that higher mistletoe infection prevalence in taller trees results from differential dispersal of mistletoe seeds to tall trees as well as differential survival of established mistletoes on tall trees, and is not simply an accumulation of infections as trees age.

Simultaneous stressors: Interactive effects of an immune challenge and dietary toxin can be detrimental to honeybees

Recent large-scale mortality of honeybee colonies is believed to be caused by multiple interactions between diseases, parasites, pesticide exposure, and other stress factors. To test whether a dual challenge has an additive effect in reducing survival, we experimentally stimulated the immune system of caged Apis mellifera scutellata workers from six colonies by injecting saline or Escherichia coli lipopolysaccharides (LPS), and additionally fed them the alkaloid nicotine (0 μM, 3 μM and 300 μM in 0.63 M sucrose). Workers did not increase their sucrose intake to compensate for the immune system activation, and those injected with E. coli LPS decreased their intake on the highest nicotine concentration. In the single challenges, injection and high nicotine doses negatively affected survival. All injected worker groups showed reduced survival. Without nicotine, survival of the saline and E. coli LPS worker groups was similar, but survival of E. coli LPS-challenged workers dropped below that of the saline groups when additionally challenged by nicotine, with bees dying earlier at higher nicotine concentrations. In the dual challenge of saline injection and dietary nicotine, a reduced effect on survival was observed, with lower mortality than expected from the summed mortalities due to the single challenges. However, additive and synergistic effects on survival were observed in workers simultaneously challenged by E. coli LPS and nicotine, indicating that interactive effects of simultaneous pathogen exposure and dietary toxin are detrimental to honeybee fitness.

Honeybees prefer warmer nectar and less viscous nectar, regardless of sugar concentration

The internal temperature of flowers may be higher than air temperature, and warmer nectar could offer energetic advantages for honeybee thermoregulation, as well as being easier to drink owing to its lower viscosity. We investigated the responses of Apis mellifera scutellata (10 colonies) to warmed 10% w/w sucrose solutions, maintained at 20–35°C, independent of low air temperatures, and to 20% w/w sucrose solutions with the viscosity increased by the addition of the inert polysaccharide Tylose (up to the equivalent of 34.5% sucrose). Honeybee crop loads increased with nectar temperature, as did the total consumption of sucrose solutions over 2 h by all bees visiting the feeders. In addition, the preference of marked honeybees shifted towards higher nectar temperatures with successive feeder visits. Crop loads were inversely proportional to the viscosity of the artificial nectar, as was the total consumption of sucrose solutions over 2 h. Marked honeybees avoided higher nectar viscosities with successive feeder visits. Bees thus showed strong preferences for both warmer and less viscous nectar, independent of changes in its sugar concentration. Bees may benefit from foraging on nectars that are warmer than air temperature for two reasons that are not mutually exclusive: reduced thermoregulatory costs and faster ingestion times due to the lower viscosity.

Effects of a neonicotinoid pesticide on thermoregulation of African honey bees (Apis mellifera scutellata)

Thiamethoxam is a widely used neonicotinoid pesticide that, as agonist of the nicotinic acetylcholine receptors, has been shown to elicit a variety of sublethal effects in honey bees. However, information concerning neonicotinoid effects on honey bee thermoregulation is lacking. Thermoregulation is an essential ability for the honey bee that guarantees the success of foraging and many in-hive tasks, especially brood rearing. We tested the effects of acute exposure to thiamethoxam (0.2, 1, 2ng/bee) on the thorax temperatures of foragers exposed to low (22°C) and high (33°C) temperature environments. Thiamethoxam significantly altered honey bee thorax temperature at all doses tested; the effects elicited varied depending on the environmental temperature and pesticide dose to which individuals were exposed. When bees were exposed to the high temperature environment, the high dose of thiamethoxam increased their thorax temperature 1-2h after exposure. When bees were exposed to the low temperature, the higher doses of the neonicotinoid reduced bee thorax temperatures 60-90min after treatment. In both experiments, the neonicotinoid decreased the temperature of bees the day following the exposure. After a cold shock (5min at 4°C), the two higher doses elicited a decrease of the thorax temperature, while the lower dose caused an increase, compared to the control. These alterations in thermoregulation caused by thiamethoxam may affect bee foraging activity and a variety of in-hive tasks, likely leading to negative consequences at the colony level. Our results shed light on sublethal effect of pesticides which our bees have to deal with.