Floral visitors of Helianthus verticillatus, a rare sunflower species in the southeastern United States

Whorled sunflower (Helianthus verticillatus) is an endangered species of aster found exclusively in the southeastern United States. Evidence suggests that this species is self-incompatible and reliant on insect pollination for seed production. However, little is known about the general biology of this species, including the identity of probable pollinators. Floral visitors were collected and identified during September of 2017 and 2018. Forty-one species of visitor species, including 29 hymenopteran, 6 dipteran, 1 lepidopteran, and 5 other miscellaneous insects were trapped during seven collection days at one site in Georgia and two sites in Tennessee. Within a collection day (7:45 to 18:15), there were either 5 or 6 discrete half-hour time periods when insects were trapped. Insect visitor activity peaked during the 11:45-12:15 and 13:45-14:15 collection periods and was least during the 7:45- 8:45 and 9:45-10:15 periods at all three locations. Visitors were dentified to genus and species using morphological keys and some with sequences of the COX-1 mitochondrial gene. A rarefaction analysis using the iNext Online package was used to assess species richness, while Simpson's Diversity Index was used to assess species diversity within and across each location. The most common visitors at all locations were Bombus spp. (bumblebees), while Ceratina calcarata (a carpenter bee) and members of the halictid bee tribe Augochlorini were second and third most common at the two Tennessee locations. Pollen on visitors was identified as belonging to the Helianthus genus via direct PCR of DNA using Helianthus-specific microsatellites. Pollen grains were obtained from the 10 most common visitors and Apis mellifera (honey bee) and counted using a hemocytometer. Of these visitors, Bombus spp., Halictus ligatus (a sweat bee), and Melissodes spp. (long-horned bees) carried the most Helianthus pollen grains. These visitors are the most likely candidates to be the primary pollinators of H. verticillatus flowers.