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SUMMARY
To estimate Osmia cornuta densities required for pollination of ‘Braeburn’ apple, trees were caged and supplied with different bee densities in 2000 and 2001. The following pollination treatments were compared: (1) one O. cornuta pair per five trees: 5 adjacent trees, 2 ‘Braeburn’, 1 ‘Granny Smith’ (pollenizer) and 2 ‘Braeburn’, caged with 1 female and 1 male O. cornuta; (2) one O. cornuta pair per tree: 5 trees as above, caged with 5 female and 5 male O. cornuta; (3) open pollination: 5 trees as above, uncaged; (4) no pollinators: 5 trees as above, caged without pollinating insects. Fruitlet-set (fruitlets retained on the tree before June drop), misshapen fruit-percentages (percent fruits with at least one empty carpel), and seed-set (number of seeds per fruit) were measured. Cages with no pollinators had the lowest fruitlet-set (2000: 11.5%; 2001: 7.7%) and seed-set (2000: 1.6; 2001: 1.9), and the highest proportion of misshapen fruits (2000: 96.8%; 2001: 96.3%) in both years. Values obtained in cages with one O. cornuta pair per five trees were not significantly different from those obtained in open-pollinated trees. Cages with one O. cornuta pair per tree had fruitlet-set (2000: 37.9%; 2001: 20.4%) similar to cages with one pair per five trees (2000: 33.0%; 2001: 18.6%) or to open-pollinated trees (2000: 31.0%; 2001: 21.7%), but had significantly higher seed-set (2000: 7.5 vs. 5.7 and 6.0, respectively; 2001: 5.6 vs. 4.3 and 4.0, respectively) and lower misshapen fruit incidence (2000: 27.5% vs. 54.0% and 56.5%, respectively; 2001: 54.8% vs. 71.5% and 76.1%, respectively) in both years. One O. cornuta pair per five trees can provide commercially acceptable fruitlet-set, whereas one O. cornuta pair per tree can ensure maximum seed-set and thus high-quality yields on ‘Braeburn.’