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 we experimentally measure pollen loss and movement on the proboscises of Manduca sexta (Sphingidae) hawkmoths. We predicted that 1) proboscis curling causes pollen loss; 2) pollen that is not lost is displaced from its deposition site; and 3) repeated curls result in more displacement. We placed pollen from Datura wrightii, an important nectar plant for M. sexta, distal to the knee bend on M. sexta proboscises, and recorded the number and location of grains after proboscis curls.

3. Consistent with our hypotheses, proboscis curling caused significant pollen loss. 1) 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. 2) A single curl was also sufficient to displace pollen across all surfaces of the proboscis, but 3) 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. We discuss strategies by which flowering plants might mitigate the effects of pollen loss from visitors with flexible pollen-pickup structures.

All dataset collection details are available in the associated manuscript.