An investigation into the bee's gustatory mechanisms for the detection of toxins in nectar

Many animals, plants, and fungi defend themselves from being eaten by the production or sequestration of toxic chemicals. Thousands of these compounds have been identified from plants. For hungry animals, avoiding death or intoxication by defence compounds can be as important as identifying nutrients in food. Taste is the gatekeeper regulating food ingestion: it permits chemical identification and instructs the appropriate behavioural response. For example, before staring a meal, insects like bees sample food using gustatory receptors (Grs) expressed in neurons in hair-like sensilla on the mouthparts, antennae, and tarsi. If a toxic compound is detected in food, the insect will reject the food item and move on.

Insects have often led the way to greater insight into neuronal mechanisms when they are used as model organisms for understanding how sensory systems function. Previous research has shown that the gustatory systems of vertebrates and insects are organized such that a subset of gustatory cells or neurons is excited by sugars and other nutrients, whereas others are excited by toxic (bitter) compounds. New research in insects, however, has revealed that some gustatory receptor neurons (GRNs) can be activated by both nutrients and potential toxins. A simple model, where the activity in sugar-sensing neurons evokes feeding and the activity in bitter-sensing neurons causes an insect to reject food may be incorrect. Instead, whole populations of GRNs are likely to be involved in encoding information. This has rarely been studied.

Wright's laboratory is developing the bee as a model organism for understanding how taste functions. Bees have few gustatory receptor types, making it tractable to identify what they can detect and how their taste system functions. Research from her laboratory has shown that bees have poor acuity for the detection of potential toxins like pesticides in sugary solutions like nectar. It is unclear whether the sugars in these solutions mask the taste of the toxins, or whether the bees cannot detect the substances at all. The research proposed here will comprehensively study the responses of GRNs on the mouthparts of buff-tailed bumblebees to mixtures of sugars and non-nutrient compounds. We will measure how populations of these neurons respond to stimulation with several types of potential toxins including compounds naturally found in floral nectar. As a result of modern agriculture, bees are increasingly exposed to pesticides and other agrochemicals in floral pollen and nectar which they collect as food. For this reason, we will measure the detection thresholds of 3 additional bee species towards non-nutrient compounds including pesticides, fungicides, and herbicides. We will also perform behavioural assays to identify how the input from the GRNs instructs feeding behaviour.

This research will be the first to characterize how an insect's gustatory system encodes the value of food when it is a mixture of nutrients and potential toxins. These experiments will also identify whether some bee species are better than others at detecting and avoiding toxins, providing data for land managers and policy makers regarding the risks of specific agrochemicals to bees when they are sprayed or delivered to flowering crops. In the future, this information could also be used to design new pesticides that permit bees to detect and avoid these flowers with nectar containing these compounds when such compounds are used to protect crops.

Most animals eat foods that are complex mixtures of chemical compounds. The sense of taste permits the detection of several types of compounds to create a representation of food's relative value. In insect mouthparts, activation of populations of gustatory receptor neurons (GRNs) drives motor programs involved in feeding and in food rejection. Although it is critical to understanding how insects evaluate food quality, few studies have characterized how the GRN population responds to complex taste stimuli. Bees are important pollinators that have specialized mouthparts which allow them to feed on floral nectar. Modern agriculture increasingly exposes bees to toxins like pesticides in nectar, but research thus far indicates that bees have difficulty detecting these compounds. In this proposal, we have designed experiments to identify how the GRNs of the mouthparts of the buff-tailed bumblebee (Bombus terrestris) form a representation of the relative value of food containing potential toxins. We will make extracellular recordings from gustatory sensilla and perform detailed behavioural assays to identify the classes of non-nutrient compounds bumblebees detect. To measure the GRN population response, we will perform multichannel extracellular recordings from the maxillary nerve of B. terrestris during stimulation with mixtures of sucrose and non-nutrient compounds. Combined, these data will reveal how the spiking activity of the GRN population directs food acceptance or rejection in bees. A subset of these experiments will be recapitulated in other bee species (the honeybee, Apis mellifera, the long-tongued, garden bumblebee, Bombus hortorum, and the red mason bee, Osmia bicornis). By identifying the detection thresholds of bees for pesticides in nectar, we can provide the public with information regarding the potential hazards that specific classes of agrochemicals pose to foraging bees.

The proposed research has the potential to impact industry, government policy making, agriculture and land management, and the public. Billions of pounds are spent annually combating insect pests. Taste is an important mechanism that guides insect food selection. Our proposed experiments have the potential to make a significant advance in our understanding of the way that insects detect and avoid toxins in food. We expect that what we learn about the mechanisms bees possess for the detection of toxins will reveal the basic principles that govern how insects perceive the complex mixtures of compounds that make up their food. For this reason, our project will provide deeper understanding of the basic principles of insect gustation and chemosensation that will be valuable to many fields from pest management in agriculture to insect nutrition.

Our research could also have a potentially lasting positive impact on bee species. Insect pollinators, including bees, provide important pollination services to at least 87% of the world's plants. Through their action as pollinators, bees contribute billions of pounds annually to the production of human foods. Wild bee populations are in decline due to many factors that include exposure to pesticides and other agrochemicals. Our research will provide direct evidence for the thresholds of domesticated and wild bee species towards the detection of specific classes of pesticides in substances like floral nectar. Our research should also provide the groundwork for understanding how the structure of a compound influences whether bees detect it as a potential toxin. This information could be used by chemists involved in the development of pesticides to create compounds or mixtures of compounds that bees could detect and avoid. In addition, our data could be used to predict the relative risk of exposure of bumblebees, honeybees, and mason bees to specific pesticides when they are used on flowering crops. If farmers know that bees can detect and avoid certain pesticides, they could safely use these pesticides to protect flowering crops. For this reason, our research could be used to guide government policy making when combined with information about the mortality risks to individuals and to colonies of bees from field and lab studies. Worldwide, people take a general interest in bees and their behaviour. Our data will be the first to study the bee's taste system in detail. We expect that the information generated by our research will intrigue many and fuel public understanding of bees and their biology.