Conspecific Pollen Loads on Insects Visiting Female Flowers on Parasitic Phoradendron Californicum (Viscaceae)
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Western North American Naturalist Volume 76 Number 1 Article 11 3-31-2016 Conspecific pollen loads on insects visiting emalef flowers on parasitic Phoradendron californicum (Viscaceae) William D. Wiesenborn [email protected] Follow this and additional works at: https://scholarsarchive.byu.edu/wnan Part of the Anatomy Commons, Botany Commons, Physiology Commons, and the Zoology Commons Recommended Citation Wiesenborn, William D. (2016) "Conspecific pollen loads on insects visiting emalef flowers on parasitic Phoradendron californicum (Viscaceae)," Western North American Naturalist: Vol. 76 : No. 1 , Article 11. Available at: https://scholarsarchive.byu.edu/wnan/vol76/iss1/11 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Western North American Naturalist 76(1), © 2016, pp. 113–121 CONSPECIFIC POLLEN LOADS ON INSECTS VISITING FEMALE FLOWERS ON PARASITIC PHORADENDRON CALIFORNICUM (VISCACEAE) William D. Wiesenborn1 ABSTRACT.—Desert mistletoe, Phoradendron californicum (Viscaceae), is a dioecious parasitic plant that grows on woody legumes in the Mojave and Sonoran Deserts, produces minute flowers during winter, and is dispersed by birds defecating fruits. Pollination of desert mistletoe has not been examined despite the species’ reliance on insects for trans- porting pollen from male to female plants. I investigated the pollination of P. californicum parasitizing Acacia greggii (Fabaceae) shrubs at 3 sites at different elevations in the Mojave Desert of southern Nevada during February 2015. I examined pollen from male flowers, aspirated insects landing on female flowers, and counted pollen grains in insect pollen loads. Desert mistletoe’s tricolpate pollen differed from a previous description by being oblate instead of subpro- late in equatorial view. Female flowers were visited by 13 species of Diptera in 10 genera and 6 families and 3 species of Hymenoptera in 3 families. Almost all (98.5%) of the pollen carried by insects was from desert mistletoe. Five species of phytophagous fruit flies in Tephritidae were frequently found on flowers, comprising 53% of the insects collected, but carried low amounts of P. californicum pollen. Two species of blow flies in Calliphoridae, both larval decomposers of animals, were also relatively abundant on flowers and carried moderate to high pollen loads. Flies in Syrphidae, 2 preda- tors and 1 plant-decomposer, carried varying amounts of conspecific pollen. Conspecific pollen loads also varied on a species of native bee in Halictidae and on naturalized honey bees in Apidae. Desert mistletoe appears to be pollinated mostly by tephritids, due to their abundance on female flowers, and by calliphorids. Blow flies would be more likely than fruit flies to carry pollen between male and female plants on different host shrubs due to their larger size and stronger flight. Parasitic, dioecious P. californicum plants appear to rely on a web of mutualism between fruit-eating birds and flower-fertilizing insects. RESUMEN.—El muérdago del desierto, Phoradendron californicum (Viscaceae), es una planta parásita dioica que crece en leguminosas leñosas en los Desiertos de Mojave y de Sonora. Esta planta produce flores diminutas durante el invierno y son dispersadas por aves que defecan las semillas. La polinización del muérdago del desierto no ha sido estu- diada, a pesar de la dependencia a los insectos para transportar polen de plantas macho a plantas hembras. Estudié la polinización de P. californicum que parasita los arbustos Acacia greggii (Fabaceae) en tres sitios a diferentes elevaciones en el Desierto de Mojave al sur de Nevada, durante febrero del 2015. Examiné el polen de flores macho, insectos suc- cionados al aterrizar en flores hembras y conté los granos de polen en las cargas de polen de los insectos. El polen tricol- pado del muérdago del desierto difirió de una descripción anterior por ser achatado en lugar de subprolado en vista ecuatorial. Las flores hembra fueron visitadas por 13 especies de dípteros (Diptera) en 10 géneros y 6 familias, y 3 especies de himenópteros (Hymenoptera) en 3 familias. Casi todo el polen (98.5%) llevado por los insectos pertenecía al muérdago del desierto. Encontré con frecuencia cinco especies de moscas de la fruta fitófagas en las flores, comprendiendo el 53% de los insectos colectados, pero llevaban cantidades bajas de polen de P. californicum. Dos especies de moscas verdes Calliphoridae, ambas descomponedoras de animales en etapa larval, fueron relativamente abundantes en las flores y transportaron tanto cargas moderadas como cargas grandes de polen. Las moscas Syrphidae, dos depredadoras y una descomponedora de plantas, transportaron cantidades variables de polen conespecífico. Las cargas de polen conespecífico también variaron en una especie de abeja nativa Halictidae y en abejas melíferas Apidae. El muérdago del desierto parece ser polinizado principalmente por los tefrítidos, debido a su abundancia en flores hembras y califóridos. Las moscas verdes serían más propensas, que las moscas de la fruta, a transportar el polen entre plantas macho y plantas hembras en diferentes arbustos de refugio debido a su tamaño mayor y a su vuelo más firme. Las plantas parasitas dioicas P. californicum, parecen depender de una red de mutualismo entre aves que se alimentan de frutas e insectos fertilizadores de flores. Phoradendron (Viscaceae) is a large genus eastern Pacific islands to throughout the of perennial plants called mistletoe that para- Caribbean (Kuijt 2003). Phoradendron plants sitize a wide variety of woody plants (Trelease penetrate trunks or branches with their haus- 1916, Kuijt 2003). The genus includes 234 torium, a root-like structure that taps into species found in the Americas from the northern host-plant xylem. Plants obtain water and other United States to northern Argentina and from inorganic nutrients from their hosts and produce 1E-mail: [email protected] 113 114 WESTERN NORTH AMERICAN NATURALIST [Volume 76 organic nutrients from photosynthesis in The unisexual flowers in Phoradendron leaves and stems (Kuijt 1969, 2003). Insects require insects to transport pollen from male and other invertebrates may feed and develop to female flowers. Dioecious flowering in P. on mistletoe as observed in Phoradendron californicum and many other species of mistle- tomentosum Engelmann (Whittaker 1984). toe further requires insects to carry pollen Flowers on Phoradendron (Kuijt 2003) are from male to female plants. Studies on pollina- very small, typically around 1 mm in diame- tion in Phoradendron are lacking despite the ter, and unisexual. Male flowers are spherical genus’s parasitic habit, unusually small flowers, with 3 or 4 petals that enclose an open space, myriad species, wide distribution, and impor- and each petal supports a minute sessile tance to wildlife. Insect pollination in P. cali- anther. Female flowers are similar but contain fornicum is even more obscure due to its winter a short style with an indistinct rounded or flowering. The only observation of the species’ faintly bilobed stigma atop an inferior ovary. pollination has been by Kuijt (2003), who writes Both sexes of Phoradendron flowers contain a that flowers are, “very fragrant, on warm days vestigial nectar gland described by Trelease attracting numerous insects in a season when (1916) as “apparently adapted to pollination few other local plants are in flower.” by such short-tongued insects as flies and Pollination of P. californicum within the small bees.” Mojave Desert in southern Nevada was inves- Phoradendron californicum Nuttall, or desert tigated by examining pollen loads on insects mistletoe, is a dioecious species with scale-like visiting female flowers. I considered the fol- leaves found in the Mojave and Sonoran lowing questions: (1) What species of insects Deserts of the southwestern United States and carry P. californicum pollen to female flowers? northwestern Mexico (Trelease 1916, Kuijt (2) Do individuals of these species carry differ- 2003). The species mainly parasitizes trees ent amounts of P. californicum pollen? (3) How and shrubs in Fabaceae including Acacia, specific are insects to desert mistletoe flowers Cercidium, Olneya, and Prosopis (Hawksworth based on pollen loads? (4) Which taxa of insects and Wiens 2002). Desert mistletoe grows to are more likely to pollinate P. californicum 2 m in diameter and contains hanging clusters female flowers? of branching 0.4–1.0-m-long, grayish or reddish green stems (Hawksworth and Wiens 2002, METHODS Kuijt 2003). Flowers are borne on inflores- cences, 5–25 mm long on male plants and 5–10 The study was conducted in the Eldorado mm long on female plants, that branch oppo- Mountains along State Highway 165 near sitely along stems. Scanning electron micro- Nelson, Clark County, Nevada. Insects were graphs of P. californicum pollen show tricol- collected from P. californicum plants parasitiz- porate grains with a rounded-triangular shape ing Acacia greggii A. Gray (Fabaceae) shrubs in polar view and a subprolate shape in equa- that grew within sand and gravel washes at 3 torial view (Kuijt 2003). Grains measure 31 mm sites at different elevations. The lowest site along the polar axis and 24 mm across the (35.706° N, 114.786° W) was