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Using the pollen loads carried by floral visitors to infer their floral visitation behavior is a powerful technique to explore the foraging of wild pollinators. Interpreting these pollen records, however, requires assumptions about the underlying pollen dynamics. To compare visitor foraging across flower species, the most important assumption is that pollen is picked up and retained on the visitor at similar rates. Given differences in pollen presentation traits such as grain number or stickiness even among flowers with similar morphologies, however, the generality of this assumption is unclear. We investigated pollen accumulation on the hawkmoth Manduca sexta, testing the degree to which accumulation differed among flower species and how pollen stickiness affected this accumulation. In no-choice floral visitation assays to six plant species visited by long-tongued hawkmoths in the wild, M. sexta individuals were allowed to visit flowers 1, 2, or 5 times, after which the pollen on their proboscises was removed and counted.  We found that the six plant species varied orders of magnitude in the number of pollen grains deposited on the moths, with some placing thousands of grains after a single visit and other placing none after five. Plant species with sticky pollen adhesion mechanisms placed more pollen on the moths and had relatively less pollen accumulation over successive visits than non-sticky plants. Intriguingly, moths carried fewer pollen grains after 5 visits than after 2 visits, suggesting that both sticky and non-sticky pollen was lost during foraging. Together, our results suggest that interpretation of pollen load data should be made cautiously, especially when comparing across plant species. 

1. Precise pollen placement on floral visitors can improve pollen transfer, but in many plant species, pollen is deposited onto the flexible proboscises of long-tongued insects. These proboscises are curled and uncurled between floral visits, potentially causing pollen to be lost or displaced. Rates of pollen movement and loss resulting from proboscis curling, and hence the potential quality of long-tongued insects as pollinators, are unknown. 2. Here, pollen loss and movement on the proboscises of Manduca sexta (Sphingidae) hawkmoths was experimentally measured. It was predicted that (i) proboscis curling causes pollen loss; (ii) pollen that is not lost is displaced from its deposition site; and (iii) repeated curls result in more displacement. Pollen from Datura wrightii, an important nectar plant for M. sexta, was placed distal to the knee bend on M. sexta proboscises, and the number and location of grains was recorded after proboscis curls. 3. Consistent with the hypotheses, proboscis curling caused significant pollen loss. (i) A single curl resulted in the loss of almost 75% of the pollen from the placement site; after repeated curling, 98% of grains were lost from this site. (ii) A single curl was also sufficient to displace pollen across all surfaces of the proboscis, but (iii) further curling did not affect its distribution across surfaces. 4. Together, these results suggest that precise pollen placement on the proboscises of hawkmoths would be unlikely to increase pollen transfer success. Strategies by which flowering plants might mitigate the effects of pollen loss from visitors with flexible pollen-pickup structures are discussed.