Date of Award:

5-1-1981

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Life Sciences:Biology

Committee Chair(s)

Frank D. Parker

Committee

Frank D. Parker

Committee

George E. Bohart

Committee

William F. Campbell

Committee

Richard J. Shaw

Committee

David L. Turner

Abstract

Pollination and seed production of the commercial onion, Allium cepa, and a native onion, A. acuminatum, were studied from 1978 to 1980. Insect densities on five fertile (male fertile) onion cultivars grown in Utah were high and there was a positive linear relationship between insect density and flower density. Insect densities on the cultivars were significantly different, suggesting differences in attractiveness of the cultivars. At peak bloom, pollination success was high and a mean of 91% of the flower styles examined contained pollen tubes. In four of the cultivars, 60% or greater of the flowers on an umbel set seed. The most abundant bee species visiting the onions were the honey bee (52%) and Halictus farinosis (23%). Onions grown in the greenhouse demonstrated significantly different levels of recently dehisced pollen germination. Mixed age pollen exhibited a diurnal pattern of germination with significantly lower germination at 1400 than at 1000 and 1800 h. In the hybrid onion seed fields studied in Idaho, the honey bee was the only pollinator of significance. In 1979, honey bee densities were significantly higher on fertile than sterile (male sterile) onion rows. This was due to higher densities of pollen-collecting bees on the fertile rows. Honey bee densities and pollination success were observed to decrease during the bloom. The percent of styles bearing pollen tubes decreased from 100% in both fertile and sterile onions on July 2 to 50% and 25% respectively, on July 13. Fertile onions had significantly higher nectar volume and amount of sugar than sterile onions. Flower nectar volume and sugar were observed to increase during the bloom and sugar and potassium concentrations were high (sterile: 60% and 7278 ppm; fertile: 63% and 7797 ppm). The large amounts and high concentrations of nectar in this field are probably the result of nectar acumulation and evaporative concentration in the absence of sufficient flower visitation. This caused the nectar to become an undesirable food for the honey bees. Forty-four percent of the sterile and 41% of the fertile onion flowers set seed. More than 50 species of bees were collected visiting A. acuminatum in the vicinity of Logan, Utah. The major pollinatiors were Halictus tripartitus and metalic species of Osmia. The bees visiting A. acuminatum are polylectic but showed a high degree of constancy during individual foraging episodes. Pollination success on A. acuminatum was very high and more than 90% of the flower styles examined contained pollen tubes. Bees foraging on A. acuminatum visited a mean of 1.5 flowers during each umbel visit. Foraging behavior of the pollinators coupled with protandry probably results in a high degree of out-crossing in A. acuminatum.

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