Abstract
Honey bees learn to associate sugars with odorants in controlled laboratory conditions and during foraging. The memory of these associations can be impaired after exposure to contaminants such as pesticides. The sub-lethal effects of acaricides such as 5-methyl-2-(propan-2-yl)-phenol (thymol) introduced into colonies to control varroa mites are of particular concern to beekeeping, due to detrimental effects of some acaricides on bees. Here we assess whether various odorant/sugar pairs are identically memorized in a differential appetitive olfactory conditioning experiment and whether this learning is affected by thymol exposure. Responses to odorants in retrieval tests varied according to the sugar they were paired with, a property called congruency. Interestingly, congruency was altered by pre-exposure to some thymol concentrations during retrieval tests, although electroantennography recordings showed it left odorant detection intact. This highlights the importance of taking into account subtle effects such as odor/sugar congruency in the study of the effect of pesticides on non-target insects, in addition to the simpler question of memory impairment.
Highlights
In our previous work, bees were trained to associate a single odorant, either 2-hexanol or 1-nonanol, with sucrose as a reward, following classical conditioning procedures[16]
Bees readily learn to associate floral odors with food, and it has been demonstrated that odor/food associations performed while flying freely to natural or artificial sucrose food sources can be transferred to the restrained conditions, and vice-versa[27,28,29,30]
Recent reports indicate that the nature of the sugar used as unconditioned stimulus (US) during training affects the robustness of memory: bees develop little long-term memory when 1-hexanol is associated with fructose[32] whereas sucrose and glucose led to higher conditioned proboscis extension response (PER)
Summary
Bees were trained to associate a single odorant, either 2-hexanol or 1-nonanol, with sucrose as a reward, following classical conditioning procedures[16]. In laboratory conditions tethered bees are often trained to associate alcohols such as 1-hexanol or 1-octanol with a sucrose solution This type of protocol relies on the proboscis extension response (PER): when a sugar solution touches its antennae, the bee reflexively extends its main mouthpart, the proboscis. The terminology of Pavlovian conditioning, and a sucrose reward or unconditioned stimulus (US) Following this training, presentation of the odorant alone becomes sufficient to elicit the PER31. We used either sucrose or fructose and 4 odorants (1-hexanol, 2-hexanol, 1-octanol or 2-octanol) with and without thymol treatment These odorants are present in floral[38,39] and bee pheromone compounds[40]. Fructose could be better associated with the odorants present inside the hive (such as pheromone components 1-hexanol, 1-octanol) than sucrose because it is the main sugar of honey and it is less abundant than sucrose in nectars. It could be envisaged that fructose is congruent with the odorants of the colony and sucrose with floral odorants
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