POLLINATION EFFECTIVENESS AND POLLINATION EFFICIENCY OF INSECTS FORAGING PROSOPIS-VELUTINA IN SOUTH-EASTERN ARIZONA

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 Stat es and northern Mexico. Its flowers are an important food source for s everal native insects, as well as Apis mellifera used in the honey ind ustry. 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 inf lorescences with mesh bags with 3 mm pore size, was equal to that on c ompletely open-pollinated inflorescences. Pollination effectiveness of nocturnal insects, determined by exposing inflorescences to visitatio n only at night, was nonsignificant. However, actual activity of noctu rnal insects was not monitored. 3. Pollination efficiency is defined a s the relative ability of an insect to pollinate flowers effectively, as measured by fruit production per some unit of measure (i.e. per vis it). Three measures of pollination efficiency based on pod production per visit (Spears' PE), per minute on the inflorescence (PE(t)), and p er unit of distance traversed on the inflorescence (PE(d)), were deter mined for A. mellifera, Chalicodoma spp., Colletidae, Perdita spp. and Volucella spp. Chalicodoma spp. were most efficient according to Spea rs' PE and PE(t). Perdita spp. and Volucella spp. were equally as effi cient as A. mellifera. Colletidae had the lowest values for all three measures of PE. The results are discussed in regard to interactions be tween floral development and insect behaviour. 4. When considering the number of visits required by a particular insect to effect pod produc tion equal to that of open-pollinated inflorescences, native Chalicodo ma 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 utili zation of insects in pod or seed production orchards, are discussed.