27Mosquitoes are important vectors of disease and require sources of carbohydrates for 28 reproduction and survival. Unlike host-related behaviors of mosquitoes, comparatively 29 less is understood about the mechanisms involved in nectar-feeding decisions, or how 30 this sensory information is processed in the mosquito brain. Here we show that Aedes 31 spp. mosquitoes, including Aedes aegypti, are effective pollinators of the Platanthera 32 obtusata orchid, and demonstrate this mutualism is mediated by the orchid's scent and 33 the balance of excitation and inhibition in the mosquito's antennal lobe (AL). The P. 34 obtusata orchid emits an attractive, nonanal-rich scent, whereas related Platanthera 35 species -not visited by mosquitoes -emit scents dominated by lilac aldehyde. Calcium 36 imaging experiments in the mosquito AL revealed that nonanal and lilac aldehyde each 37 respectively activate the LC2 and AM2 glomerulus, and remarkably, the AM2 glomerulus 38 is also sensitive to DEET, a mosquito repellent. Lateral inhibition between these two 39 glomeruli reflects the level of attraction to the orchid scents: whereas the enriched 40 nonanal scent of P. obtusata activates the LC2 and suppresses AM2, the high level of 41 lilac aldehyde in the other orchid scents inverts this pattern of glomerular activity, and 42 behavioral attraction is lost. These results demonstrate the ecological importance of 43 mosquitoes beyond operating as disease vectors and open the door towards 44 understanding the neural basis of mosquito nectar-seeking behaviors. 45 46 Significance Statement47Nectar-feeding by mosquitoes is important for survival and reproduction, and hence 48 disease transmission. However, we know little about the sensory mechanisms that 49 mediate mosquito attraction to sources of nectar, like those of flowers, or how this 50 information is processed in the mosquito brain. Using a unique mutualism between 51Aedes mosquitoes and Platanthera obtusata orchids, we reveal that this mutualism is 52 mediated by the orchid's scent. Furthermore, lateral inhibition in the mosquito's antennal 53(olfactory) lobe -via the neurotransmitter GABA -is critical for the representation of the 54 scent. These results have implications toward understanding the olfactory basis of 55 mosquito-nectar-seeking behaviors. 56 57 58 59 combination of video recordings and focal observations by trained participants, more than 100 581 P. obtusata flowers were observed for a total of 47 h, with 57 floral feeding events by 101 mosquitoes. During our observations, flowers were almost solely visited by various 102 mosquito species (both sexes) that mainly belonged to the Aedes group (Fig. 1A,B; Table 103 S2), with the only other visitor being a single geometrid moth. Mosquitoes quickly located 104 these rather inconspicuous flowers, even on plants that were bagged and thus lacked a 105 visual display. After landing on the flower, the mosquito's probing of the nectar spur 106 resulted in pollinia attachment to its eyes ( Fig. 1A; Movies S1,2). Most of...
