The Bacterium Pantoea ananatis Modifies Behavioral Responses to Sugar Solutions in Honeybees.
ABSTRACT: 1. Honeybees, which are among the most important pollinators globally, do not only collect pollen and nectar during foraging but may also disperse diverse microbes. Some of these can be deleterious to agricultural crops and forest trees, such as the bacterium Pantoea ananatis, an emerging pathogen in some systems. P. ananatis infections can lead to leaf blotches, die-back, bulb rot, and fruit rot. 2. We isolated P. ananatis bacteria from flowers with the aim of determining whether honeybees can sense these bacteria and if the bacteria affect behavioral responses of the bees to sugar solutions. 3. Honeybees decreased their responsiveness to different sugar solutions when these contained high concentrations of P. ananatis but were not deterred by solutions from which bacteria had been removed. This suggests that their reduced responsiveness was due to the taste of bacteria and not to the depletion of sugar in the solution or bacteria metabolites. Intriguingly, the bees appeared not to taste ecologically relevant low concentrations of bacteria. 4. Synthesis and applications. Our data suggest that honeybees may introduce P. ananatis bacteria into nectar in field-realistic densities during foraging trips and may thus affect nectar quality and plant fitness.
Project description:Due to intensive agriculture honeybees are threatened by various pesticides. The use of one group of them, the neonicotinoids, was recently restricted by the European Union. These chemicals bind to the nicotinic acetylcholine receptor (nAchR) in the honeybee brain. Recently, Bayer AG released a new pesticide by the name of "Sivanto" against sucking insects. It is assumed to be harmless for honeybees, although its active ingredient, flupyradifurone, binds nAchR similar to the neonicotinoids. We investigated if this pesticide affects the taste for sugar and cognitive performance in honeybee foragers. These bees are directly exposed to the pesticide while foraging for pollen or nectar. Our results demonstrate that flupyradifurone can reduce taste and appetitive learning performance in honeybees foraging for pollen and nectar, although only the highest concentration had significant effects. Most likely, honeybee foragers will not be exposed to these high concentrations. Therefore, the appropriate use of this pesticide is considered safe for honeybees, at least with respect to the behaviors studied here.
Project description:In addition to sugars, nectar contains multiple nutrient compounds in varying concentrations, yet little is known of their effect on the reward properties of nectar and the resulting implications for insect behaviour. We examined the pre-ingestive responses of honeybees to sucrose solutions containing a mix of pollen compounds, the amino acids proline or phenylalanine, or known distasteful substances, quinine and salt. We predicted that in taste and learning assays, bees would respond positively to the presence of nutrient compounds in a sucrose solution. However, bees' proboscis extension responses decreased when their antennae were stimulated with pollen- or amino acid-supplemented sucrose solutions. Compared to pure sucrose, bees exhibited worse acquisition when conditioned to an odour with pollen-supplemented sucrose as the unconditioned stimulus. Such learning impairment was also observed with quinine-containing sucrose solutions. Our results suggest that bees can use their antennae to detect pollen compounds in floral nectars. Depending on the type and concentrations of compounds present, this may result in nectar being perceived as distasteful by bees, making it less effective in reinforcing the learning of floral cues. Such reward devaluation might be adaptive in cases where plants benefit from regulating the frequency of bee visitation.
Project description:Gustatory receptors (Grs) expressed in insect taste neurons signal the presence of carbohydrates, sugar alcohols, CO2, bitter compounds and oviposition stimulants. The honeybee (Apis mellifera) has one of the smallest Gr gene sets (12 Gr genes) of any insect whose genome has been sequenced. Honeybees live in eusocial colonies with a division of labour and perform age-dependent behavioural tasks, primarily food collection. Here, we used RT-qPCR to quantify Gr mRNA in honeybees at two ages (newly-emerged and foraging-age adults) to examine the relationship between age-related physiology and expression of Gr genes. We measured the Gr mRNAs in the taste organs and also the brain and gut. The mRNA of all Gr genes was detected in all tissues analysed but showed plasticity in relative expression across tissues and in relation to age. Overall, Gr gene expression was higher in the taste organs than in the internal tissues but did not show an overall age-dependent difference. In contrast Gr gene expression in brain was generally higher in foragers, which may indicate greater reliance on internal nutrient sensing. Expression of the candidate sugar receptors AmGr1, AmGr2 and AmGr3 in forager brain was affected by the types of sugars bees fed on. The levels of expression in the brain were greater for AmGr1 but lower for AmGr2 and AmGr3 when bees were fed with glucose and fructose compared with sucrose. Additionally, AmGr3 mRNA was increased in starved bees compared to bees provided ad libitum sucrose. Thus, expression of these Grs in forager brain reflects both the satiety state of the bee (AmGr3) and the type of sugar on which the bee has fed.
Project description:Honeybees (Apis mellifera) and bumblebees (Bombus spp.) often undergo exploitative competition for shared floral resources, which can alter their foraging behaviour and flower choice, even causing competitive exclusion. This may be strongest in summer, when foraging conditions are most challenging for bees, compared to other times of the year. However, the seasonal dynamics of competition between these major pollinator groups are not well understood. Here, we investigate whether the strength of exploitative competition for nectar between honeybees and bumblebees varies seasonally, and whether competitive pressure is greatest in summer months. We carried out experimental bee exclusion trials from May to late September, using experimental patches of lavender, variety Grosso, in full bloom. In each trial, we compared the numbers of honeybees (HB) foraging on patches from which bumblebees had been manually excluded (bumblebee excluded, BBE) versus control (CON) patches, HB(BBE-CON). This measure of exploitative competition varied significantly with season. As expected, mean HB(BBE-CON) was significantly greater in summer trials than in spring or autumn trials. This was despite high nectar standing crop volumes in BBE patch flowers in spring and autumn trials. Mean HB(BBE-CON) was not different between spring and autumn trials. Our results show that nectar competition between honeybees and bumblebees varies seasonally and is stronger in summer than spring or autumn, adding to current understanding of the seasonality of resource demand and competition between bee species. This information may also help to inform conservation programs aiming to increase floral resources for bees by showing when these resources are most needed.
Project description:In recent years, conservation biologists have raised awareness about the risk of ecological interference between massively introduced managed honeybees and the native wild bee fauna in protected natural areas. In this study, we surveyed wild bees and quantified their nectar and pollen foraging success in a rosemary Mediterranean scrubland in southern France, under different conditions of apiary size and proximity. We found that high-density beekeeping triggers foraging competition which depresses not only the occurrence (-55%) and nectar foraging success (-50%) of local wild bees but also nectar (-44%) and pollen (-36%) harvesting by the honeybees themselves. Overall, those competition effects spanned distances of 600-1.100?m around apiaries, i.e. covering 1.1-3.8km2 areas. Regardless the considered competition criterion, setting distance thresholds among apiaries appeared more tractable than setting colony density thresholds for beekeeping regulation. Moreover, the intraspecific competition among the honeybees has practical implications for beekeepers. It shows that the local carrying capacity has been exceeded and raises concerns for honey yields and colony sustainability. It also offers an effective ecological criterion for pragmatic decision-making whenever conservation practitioners envision progressively reducing beekeeping in protected areas. Although specific to the studied area, the recommendations provided here may help raise consciousness about the threat high-density beekeeping may pose to local nature conservation initiatives, especially in areas with sensitive or endangered plant or bee species such as small oceanic islands with high levels of endemism.
Project description:Plant defense compounds occur in floral nectar, but their ecological role is not well understood. We provide evidence that plant compounds pharmacologically alter pollinator behavior by enhancing their memory of reward. Honeybees rewarded with caffeine, which occurs naturally in nectar of Coffea and Citrus species, were three times as likely to remember a learned floral scent as were honeybees rewarded with sucrose alone. Caffeine potentiated responses of mushroom body neurons involved in olfactory learning and memory by acting as an adenosine receptor antagonist. Caffeine concentrations in nectar did not exceed the bees' bitter taste threshold, implying that pollinators impose selection for nectar that is pharmacologically active but not repellent. By using a drug to enhance memories of reward, plants secure pollinator fidelity and improve reproductive success.
Project description:Neonicotinoids are often applied as systemic seed treatments to crops and have reported negative impact on pollinators when they appear in floral nectar and pollen. Recently, we found that bees in a two-choice assay prefer to consume solutions containing field-relevant doses of the neonicotinoid pesticides, imidacloprid (IMD) and thiamethoxam (TMX), to sucrose alone. This suggests that neonicotinoids enhance the rewarding properties of sucrose and that low, acute doses could improve learning and memory in bees. To test this, we trained foraging-age honeybees to learn to associate floral scent with a reward containing nectar-relevant concentrations of IMD and TMX and tested their short (STM) and long-term (LTM) olfactory memories. Contrary to our predictions, we found that none of the solutions enhanced the rate of olfactory learning and some of them impaired it. In particular, the effect of 10 nM IMD was observed by the second conditioning trial and persisted 24 h later. In most other groups, exposure to IMD and TMX affected STM but not LTM. Our data show that negative impacts of low doses of IMD and TMX do not require long-term exposure and suggest that impacts of neonicotinoids on olfaction are greater than their effects on rewarding memories.
Project description:Since climate change is expected to bring more severe and frequent extreme weather events such as heat waves, assessing the physiological and behavioural sensitivity of organisms to temperature becomes a priority. We therefore investigated the responses of honeybees, an important insect pollinator, to simulated heat waves (SHW). Honeybees are known to maintain strict brood thermoregulation, but the consequences at the colony and individual levels remain poorly understood. For the first time, we quantified and modelled colony real-time activity and found a 70% increase in foraging activity with SHW, which was likely due to the recruitment of previously inactive bees. Pollen and nectar foraging was not impacted, but an increase in water foragers was observed at the expense of empty bees. Contrary to individual energetic resources, vitellogenin levels increased with SHW, probably to protect bees against oxidative stress. Finally, though immune functions were not altered, we observed a significant decrease in deformed wing virus loads with SHW. In conclusion, we demonstrated that honeybees could remarkably adapt to heat waves without a cost at the individual level and on resource flow. However, the recruitment of backup foraging forces might be costly by lowering the colony buffering capacity against additional environmental pressures.
Project description:Foraging specialization allows social insects to more efficiently exploit resources in their environment. Recent research on honeybees suggests that specialization on pollen or nectar among foragers is linked to reproductive physiology and sensory tuning (the Reproductive Ground-Plan Hypothesis; RGPH). However, our understanding of the underlying physiological relationships in non-Apis bees is still limited. Here we show that the bumblebee Bombus terrestris has specialist pollen and nectar foragers, and test whether foraging specialization in B. terrestris is linked to reproductive physiology, measured as ovarian activation. We show that neither ovary size, sensory sensitivity, measured through proboscis extension response (PER), or whole-body lipid stores differed between pollen foragers, nectar foragers, or generalist foragers. Body size also did not differ between any of these three forager groups. Non-foragers had significantly larger ovaries than foragers. This suggests that potentially reproductive individuals avoid foraging.
Project description:Neonicotinoid pesticides can have a multitude of negative sublethal effects on bees. Understanding their impact on wild populations requires accurately estimating the dosages bees encounter under natural conditions. This is complicated by the possibility that bees might influence their own exposure: two recent studies found that bumblebees (Bombus terrestris) preferentially consumed neonicotinoid-contaminated nectar, even though these chemicals are thought to be tasteless and odourless. Here, we used Bombus impatiens to explore two elements of these reported preferences, with the aim of understanding their ecological implication and underlying mechanism. First, we asked whether preferences persisted across a range of realistic nectar sugar concentrations, when measured at a series of time points up until 24 h. Second, we tested whether bees' neonicotinoid preferences were driven by an ability to associate their post-ingestive consequences with floral stimuli such as colour, location or scent. We found no evidence that foragers preferred to consume neonicotinoid-containing solutions, despite finding effects on feeding motivation and locomotor activity in line with previous work. Bees also did not preferentially visit floral stimuli previously paired with a neonicotinoid-containing solution. These results highlight the need for further research into the mechanisms underlying bees' responses to these pesticides, critical for determining how neonicotinoid-driven foraging preferences might operate in the real world for different bee species.