Dying of thirst: Osmoregulation by a hawkmoth pollinator in response to variability in ambient humidity and nectar availability

Abstract

Climate-induced alterations in flowering phenology can lead to a temporal mismatch between pollinators and the availability of floral resources. Such asynchrony may be especially impactful in desert ecosystems, where flowering time and pollinator emergence are particularly sensitive to rainfall. To investigate the osmoregulation of a desert-living hawkmoth pollinator Manduca sexta, we sampled hemolymph osmolality of over 1000 lab-grown moths held at 20%, 50%, and 80% ambient humidity. Under starvation, the hemolymph osmolality of moths remained within a healthy range from days 1-3, regardless of ambient humidity. However, osmolality levels increase steeply from a baseline of 360-370 mmol/kg to 550 mmol/kg after 4-5 days in low and intermediate humidity and after 5 days in high humidity. Starved moths exposed to low humidity conditions died within 5 days, whereas those in high humidity conditions lived twice as long. Moths fed either synthetic Datura wrightii nectar, synthetic Agave palmeri nectar, or water, maintained osmolality within a healthy baseline range of 350-400 mmol/kg. The same was true for moths that fed on authentic floral nectars. However, moths consumed higher amounts of synthetic nectar, likely due to the non-sugar nectar constituents in the authentic nectar. Finally, simulating a 4-day mismatch between pollinator emergence and nectar availability, we found that a single nectar meal can osmotically rescue moths in dry ambient conditions. Our findings indicate that hemolymph osmolality provides a rapid and accurate biomarker for assessing both the health and relative hydration state of insect pollinators.