Nectar Robbery and Thievery in the Hawk Moth (Lepidoptera: Sphingidae)-Pollinated Western Prairie Fringed Orchid Platanthera praeclara

Nectar larcenists target flowers with a long nectar spur. The western prairie fringed orchid, Platanthera praeclara Sheviak and Bowles (Orchidaceae), has a 50-mm-long spur and is pollinated by hawk moth (Lepidoptera: Sphingidae) species with tongues that are shorter than the spur. We placed net traps above inflorescences at P. praeclara sites in southeastern North Dakota to survey potential nectar larcenists, including hawk moth thieves (2004-2014) and bumble bee robbers (2007-2014). We captured two hawk moths, Manduca quinquemaculata (Haworth) and Agrius cingulata (F.), that have tongues over twice the length of P. praeclara's spur. Also captured were eight bumble bee species (genus Bombus), which may produce the holes discovered in P. praecala's spur. Holes triggered loss of the spur and its nectar. In 2007, 2013, and 2014, robbery was observed in 10, 60, and 37% flowers per plant, respectively. At the flower level, robbery had a positive effect on removal of pollinaria and no effect on pollen deposition or seed capsule production. Based on results of an experiment, we propose that P. praeclara deploys variable nectar rewards across the inflorescence to promote a balance of selfing and outcrossing pollination. We expect less harm to this strategy from a robber bee satisfied by feeding on a single flower and more harm from a hawk moth thief that removes nectar from every flower of the inflorescence. Experiments are needed to determine whether nectar larceny is creating problems for conservation and recovery of P. praeclara, a threatened species in the United States and Canada.