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1. The genus Prosopis consists of woody leguminous trees and shrubs of interest for fuelwood and agroforestry purposes in arid and semi-arid regions. Prosopis velutina is common to the south-western United States and northern Mexico. Its flowers are an important food source for several native insects, as well as Apis mellifera used in the honey industry. This paper examines plant-pollinator interactions, and provides the first quantification of pollination effectiveness and pollination efficiency of several groups of insects that forage on P. velutina in the Sonoran Desert near Tucson, Arizona.

2. Pollination effectiveness is defined as a measure of an insect’s potential as a pollen vector, as reflected by its ability to effect fruit production significantly. Pollination effectiveness of small insects, determined by covering inflorescences with mesh bags with 3 mm pore size, was equal to that on completely open-pollinated inflorescences. Pollination effectiveness of nocturnal insects, determined by exposing inflorescences to visitation only at night, was nonsignificant. However, actual activity of nocturnal insects was not monitored.

3. Pollination efficiency is defined as the relative ability of an insect to pollinate flowers effectively, as measured by fruit production per some unit of measure (i.e. per visit). Three measures of pollination efficiency based on pod production per visit (Spears’ PE), per minute on the inflorescence (PE(t)), and per unit of distance traversed on the inflorescence (PE(d)), were determined for A. mellifera, Chalicodoma spp., Colletidae, Perdita spp. and Volucella spp. Chalicodoma spp. were most efficient according to Spears’ PE and PE(t). Perdita spp. and Volucella spp. were equally as efficient as A. mellifera. Colletidae had the lowest values for all three measures of PE. The results are discussed in regard to interactions between floral development and insect behaviour.

4. When considering the number of visits required by a particular insect to effect pod production equal to that of open-pollinated inflorescences, native Chalicodoma spp., Perdita spp. and Volucella spp, were more efficient than the introduced A. mellifera.

5. Implications of the results with regard to conservation of biological diversity in natural ecosystems, and utilization of insects in pod or seed production orchards, are discussed.