Pollen accumulation on hawkmoths varies substantially among moth-pollinated flowers

Abstract

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.

A signal-like role for floral humidity in a nocturnal pollination system

“…4h). Flower entry shows commitment to nectar-feeding, increasing the chances of pollen deposition and export, via dose-dependent pollen loading on moth body parts 50 . Plant-pollinator communication often is multimodal [67][68][69] , including traits associated with primary metabolism.…”

“…Moths benefit by reducing their flower handling cost on humid flowers (Fig. 5j-l), whereas, from the plant perspective, multiple entries of moths into Datura flowers increase pollen loading 50 , promote siring success 34 , and cross-pollination 34,64 , satisfying the fourth criterion. Indeed, reduced probing by Manduca sexta due to low nectar presence (and likely lower humidity) has been shown to lower seed set and impact plant fitness in related Petunia plants 82 .…”

“…10). For each treatment, we measured the moth's probing duration (sec) and the number of entries in both flowers, which would benefit plants through increased pollen export and therefore male siring success 50 . Likewise, we measured flower handling time (sec) until nectar discovery by moths and used it to calculate net energy gain and expenditure (J) during floral visits as surrogates for moth fitness 51 .…”