Scott Dobrin

Honey bees are crucial to human survival in their role as generalist pollinators. However, humans also impact honey bees through the use of agricultural chemicals, typically aimed at increasing crop yield. Pesticide exposure to honey bees has been associated with a decrease in colony growth and fewer queens being born. However, the safety of pesticides is derived from acute feeding studies that focus on mortality. Pesticides are now ubiquitous in the environment and honey bees are constantly exposed. Studies utilizing long-term exposure better represent the lifetime experience of a typical honey bee. Here we measured changes in associative learning following 1 week of exposure to one of two sub-lethal, field-relevant doses of imidacloprid. While neither concentration of pesticide altered mortality relative to control, learning acquisition decreased in the high compared to control groups. These data demonstrate that chronic exposure to field-relevant doses of imidacloprid impact the ability for honey bees to form memories. The foraging success of honey bees relies on remembering the location of the hive and potential forage locations.

Abstract As part of a quantitative survey of Maine’s bumble bee fauna (Butler et al. 2021), we compared and contrasted genetic diversity, parasite and pathogen burdens, and pesticide exposure of the relatively common Bombus ternarius Say, 1937 and the spatially rare Bombus terricola Kirby, 1837. We recorded 11 Bombus species at 40 survey sites across three Maine ecoregions, and B. ternarius was the most common species, while B. terricola was spatially rare. Nonmetric multidimensional scaling indicated that B. terricola was associated with higher elevation sites in Maine, while B. ternarius was more broadly distributed in the state. Pollinator networks constructed for each bee indicated B. ternarius foraged on more plant species than B. terricola, but that there was considerable overlap (73%) in plant species visited. Genetic diversity was greater in the spatially restricted B. terricola, whereas the widely distributed B. ternarius was characterized by greater genetic differentiation among regions. Bombus terricola had higher molecular marker levels of the microsporidian fungi Nosema spp. and the trypanosome Crithidia spp., and both species had high levels of Trypanosoma spp. exposure. No Western Honey Bee (Apis mellifera, Linnaeus, 1758) viruses were detected in either species. Pesticides were not detected in pollen samples collected from workers of either species, and B. ternarius worker tissue samples exhibited only trace levels of diflubenzuron.

Global declines of bumble bees place natural and agricultural ecosystems at risk. Given bumble bees importance to Maine's major agricultural crops, we conducted a statewide, quantitative survey of bumble bee species seasonal and ecoregional abundance, richness, diversity, and floral resource use. We recorded 11 Bombus species at 40 survey sites across Maine's three ecoregions, with Bombus ternarius Cresson, 1863 and Bombus impatiens Cresson, 1863 being the most common and Bombus citrinus Smith, 1854 the least commonly encountered. Bumble bee species richness did not differ as a function of ecoregion, but did decline over the season, while species diversity differed by ecoregion and also declined over the season. Multiple response permutation procedure (MRPP) indicated ecoregional differences in species composition of bumble bee assemblages and nonmetric multidimensional scaling produced a stable ordination suggesting assemblage differences were associated with survey site variables including forage plant cover, forage plant richness, elevation, development, and deciduous forest cover. Both MRPP and correspondence analysis also revealed differences in the floral resources utilized by bumble bee species in each ecoregion. Low connectance and nestedness levels indicated low stability pollinator networks in each ecoregion, suggesting Maine bumble bee assemblages may be at risk of decline in response to additional external perturbations.

A restrained honey bee can be trained to extend its proboscis in response to the pairing of an odor with a sucrose reward, a form of olfactory associative learning referred to as the proboscis extension response (PER). Although the ability of flying honey bees to respond to visual cues is well-established, associative visual learning in restrained honey bees has been challenging to demonstrate. Those few groups that have documented vision-based PER have reported that removing the antennae prior to training is a prerequisite for learning. Here we report, for a simple visual learning task, the first successful performance by restrained honey bees with intact antennae. Honey bee foragers were trained on a differential visual association task by pairing the presentation of a blue light with a sucrose reward and leaving the presentation of a green light unrewarded. A negative correlation was found between age of foragers and their performance in the visual PER task. Using the adaptations to the traditional PER task outlined here, future studies can exploit pharmacological and physiological techniques to explore the neural circuit basis of visual learning in the honey bee.

Foraging experience is correlated with structural plasticity of the mushroom bodies of the honey bee brain. While several neurotransmitter and intracellular signaling pathways have been previously implicated as mediators of these structural changes, none interact directly with the cytoskeleton, the ultimate effector of changes in neuronal morphology. The Rho family of GTPases are small, monomeric G proteins that, when activated, initiate a signaling cascade that reorganizes the neuronal cytoskeleton. In this study, we measured activity of two members of the Rho family of GTPases, Rac and RhoA, in the mushroom bodies of bees with different durations of foraging experience. A transient increase in Rac activity coupled with a transient decrease in RhoA activity was found in honey bees with 4 days foraging experience compared with same-aged new foragers. These observations are in accord with previous reports based on studies of other species of a growth supporting role for Rac and a growth opposing role for RhoA. This is the first report of Rho GTPase activation in the honey bee brain. (C) 2011 Elsevier Ltd. All rights reserved.

The experience of foraging under natural conditions increases the volume of mushroom body neuropil in worker honey bees. A comparable increase in neuropil volume results from treatment of worker honey bees with pilocarpine, an agonist for muscarinic-type cholinergic receptors. A component of the neuropil growth induced by foraging experience is growth of dendrites in the collar region of the calyces. We show here, via analysis of Golgi-impregnated collar Kenyon cells with wedge arborizations, that significant increases in standard measures of dendritic complexity were also found in worker honey bees treated with pilocarpine. This result suggests that signaling via muscarinic-type receptors promotes the increase in Kenyon cell dendritic complexity associated with foraging. Treatment of worker honey bees with scopolamine, a muscarinic inhibitor, inhibited some aspects of dendritic growth. Spine density on the Kenyon cell dendrites varied with sampling location, with the distal portion of the dendritic field having greater total spine density than either the proximal or medial section. This observation may be functionally significant because of the stratified organization of projections from visual centers to the dendritic arborizations of the collar Kenyon cells. Pilocarpine treatment had no effect on the distribution of spines on dendrites of the collar Kenyon cells. ► Forager honey bee Kenyon cell dendritic arbors are made more complex by muscarinic cholinergic activation. ► Muscarinic signaling alters dendritic complexity of pre-forager Kenyon cells. ► Spine density and distribution are not influenced by muscarinic signaling changes.