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Animals are often assumed to follow a strategy of energy maximization, and therefore should evaluate feeding options based on energy intake rates. However, at the proximal level, a direct estimate of energy intake rates, if that is possible at all, might require postabsorptive senses with relatively longer processing times, whereas an indirect estimate of energy intake through proxies like pre-absorptive senses of different sensory food properties might support rapid foraging decisions. Here, we show that nectar sugar concentration (sweetness) and nectar viscosity (resistance) drive preferences of bumble bees, Bombus terrestris , classical models for economic and foraging decision making. Using a tasteless/odourless biopolymer (Tylose), we created feeding options that differed in sweetness and resistance, properties that affect energy intake rate and can be immediately sensed. When energy intake rates were similar, bumble bees developed preferences based on sweetness and resistance. When energy intake rates were different, but sweetness and resistance were balanced against each other, bees developed no preferences. Decision dynamics during training indicated that bumble bees simultaneously evaluated sweetness and resistance to make decisions quickly (in seconds). These results suggest that bumble bees' food preferences are jointly affected by the immediate sensation of nectar sweetness and resistance as positively and negatively reinforcing properties, respectively, indicating a pre-absorptive proximate mechanism for rapid energy-sensitive decisions in bumble bees. • Bumble bees develop preferences based on nectar sugar concentration and viscosity. • Bumble bees quickly sample and compare the sensory properties of feeding options. • Pre-ingestive information contributes to bumble bees' floral preferences. [ABSTRACT FROM AUTHOR]

Crithidia bombi is a trypanosomatid parasite that infects several species of bumble bees (Bombus spp.), by adhering to their intestinal tract. Crithidia bombi infection impairs learning and reduces survival of workers and the fitness of overwintering queens. Although there is extensive research on the ecology of this host-pathogen system, we understand far less about the mechanisms that mediate internal infection dynamics. Crithidia bombi infects hosts by attaching to the hindgut via the flagellum, and one previous study found that a nectar secondary compound removed the flagellum, preventing attachment. However, approaches that allow more detailed observation of parasite attachment and growth would allow us to better understand factors mediating this host-pathogen relationship. We established techniques for genetic manipulation and visualization of cultured C. bombi. Using constructs established for Crithidia fasciculata, we successfully generated C. bombi cells expressing ectopic fluorescent transgenes using two different selectable markers. To our knowledge, this is the first genetic modification of this species. We also introduced constructs that label the mitochondrion and nucleus of the parasite, showing that subcellular targeting signals can function across parasite species to highlight specific organelles. Finally, we visualized fluorescently tagged parasites in vitro in both their swimming and attached forms, and in vivo in bumble bee (Bombus impatiens) hosts. Expanding our cell and molecular toolkit for C. bombi will help us better understand how factors such as host diet, immune system, and physiology mediate outcomes of infection by these common parasites. [Display omitted] • First described method for genetic modification of the bee parasite Crithidia bombi. • Ectopic transgenes can be introduced using multiple selectable markers. • Cytoplasmic and organelle markers can be visualized in swimming and attached developmental forms. • Simplified method for culture adaptation of parasites from bees for subsequent modification. • Imaging of fluorescent C. bombi parasites in vivo in the bumble bee Bombus impatiens. [ABSTRACT FROM AUTHOR]