Nectar plant preferences of Hemaris (Sphingidae) on co-occurring native Cirsium and non-native Centaurea (Asteraceae) inflorescences

Abstract

Preferences for certain nectar plants by moth pollinators have not been extensively studied, particularly seasonal switches in nectar diet. The genus Hemaris, found throughout North America and parts of other continents, is a useful new system to address questions of flower-insect interactions as the moths are relatively common and day-flying. In this study we address the following questions with a group of three Hemaris species. (1) Does Hemaris display a preference for foraging on the native swamp thistle, Cirsium discolor, over the non-native invasive knapweeds, Centaurea spp.? (2) What factors might be driving Hemaris’ preferences? The bloom periods of C. discolor and Centaurea overlap in the northeastern United States, with Centaurea blooming from approximately June through September and C. discolor blooming for approximately the first three weeks of August. Hemaris typically ecloses in the first week of July and fed on Centaurea until C. discolor bloomed. We tracked and recorded Hemaris visits to both plant species. During the co-blooming period of C. discolor and Centaurea, Hemaris visits significantly more C. discolor inflorescences, demonstrating a possible preference for the native species. Hemaris reverts to nectaring at the non-native Centaurea after native C. discolor’s bloom period ends. The nectar of C. discolor has a significantly higher sugar concentration than does the nectar of Centaurea; this difference may account for part of the moths’ preference for C. discolor.