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Conspecific Pollen on Insects Visiting Female Flowers of Phoradendron juniperinum (Viscaceae) in Western Arizona

Abstract 
 Phoradendron juniperinum (Viscaceae) is a dioecious, parasitic plant of juniper trees ( Juniperus [Cupressaceae]) that occurs from eastern California to New Mexico and into northern Mexico. The species produces minute, spherical flowers during early summer. Dioecious flowering requires pollinating insects to carry pollen from male to female plants. I investigated the pollination of P. juniperinum parasitizing Juniperus osteosperma trees in the Cerbat Mountains in western Arizona during June–July 2016. I examined pollen from male flowers, aspirated insects from female flowers, counted conspecific pollen grains on insects, and estimated floral constancy from proportions of conspecific pollen in pollen loads. The tricolpate pollen from P. juniperinum was subangular in shape with open furrows in polar view and oval-spherical in shape in equatorial view. Most insects on flowers were small (<3 mm in length). Insects carrying conspecific pollen to female flowers included 6 species of Hymenoptera in 6 families and 6 species of Diptera in 5 genera and 4 families. Conspecific pollen was found on 41% of the insects, with only 18% carrying >1 grain, and numbers of grains on insects did not differ among the species collected. Phoradendron juniperinum was most likely pollinated by 2 species of flies in Hippelates (Chloropidae) and Desmometopa (Milichiidae) and 1 species of parasitic wasp in Apanteles (Braconidae). Insects in these genera were widespread and frequently collected, with half of specimens carrying conspecific pollen that comprised most of the pollen load. Other Diptera carrying conspecific pollen included 1 species in Olcella (Chloropidae), 2 species in Mythicomyia (Mythicomyiidae), and 1 species in Scatopsidae. Other Hymenoptera with conspecific pollen included 1 species of bee in Lasioglossum (Halictidae), 1 species of wasp in Parancistrocerus (Vespidae), and 3 species of parasitic wasps in Chalcidoidea. Minute flies, such as Tephritidae or Chloropidae, are the most common pollinators among P. juniperinum and the 2 other examined species of Phoradendron (P. californicum and P. coryae). Pollination of Phoradendron likely differs among species due to different host plants, plant communities, and flowering seasons.