In flowering plants, mechanisms to precisely control how pollen is deposited on visiting animals have evolved in multiple plant taxa. Precise pollen placement can improve subsequent pollen-stigma contact rates, reduce pollen loss through grooming, and reduce heterospecific pollen transfer. This mechanism, however, only benefits the plants when the pollen is unlikely to move after it is deposited on the pollinator. Thus, while pollen can stay in separate sites on a rigid hummingbird bill or bee exoskeleton, pollen placed on the flexible proboscises of long-tongued pollinators may move away from the deposition site before the next conspecific visit.
To examine the potential consequences of proboscis curling behavior on pollen fate, we experimentally placed pollen onto the proboscises of Manduca sexta hawkmoths. The moths were then split into three groups that were allowed to curl their proboscises zero times (control treatment), once (1-curl treatment), or many times by allowing them to forage on artificial flowers for 24 hours (24-hour treatment). After the treatment, the amount of pollen on the tongue was counted in each of 6 discrete sites based on the section (the proximal, central, and distal thirds) and surface (dorsal and ventral) of the proboscis.
Results/Conclusions
We report three key results. (1) Proboscis curling causes significant pollen loss over the whole tongue, and particularly rapid loss from the original deposition site: less than 1/12th of the pollen deposited remained at that site in 24-hour moths. (2) From its original deposition site in the central portion of the tongue, pollen moves extensively and to all sites along the proboscis. (3) While more proboscis curling results in more pollen loss, a single curl was sufficient to distribute pollen grains evenly across the length of the proboscis. Together, these results suggest that precise pollen placement is unlikely to be a successful strategy for increasing the number of pollen grains transferred in hawkmoth-visited flowers. It may, however, improve the probability of transferring at least some grains if visits by the moths are imprecise as the pollen is quickly distributed over a larger surface. Ongoing work is investigating how the location of deposition and the presence of heterospecific pollen impacts pollen movement on moth proboscises, and how this movement is likely to affect pollen deposition on stigmas during flower visits.