Are New York apples pollen limited?

Apples are an economically important crop in the United States which rely on pollination services provided by both the European honey bee, Apis mellifera, as well as a surprisingly diverse community of native, wild bees. Although we know that apples depend on insect pollination the relationship between pollen limitation in apples and fruit set (a measure of orchard yield), is still not understood. Here we conducted a series of experiments to examine the impact of pollen limitation on fruit set at two experimental apple orchards. We evaluated fruit set on 120 trees (60 trees per orchard, 20 trees per variety, three varieties per orchard). On each tree we selected two branches of similar diameter, orientation, and position on the tree. Each branch was randomly assigned one of two treatments: (1) hand pollination or (2) open (natural bee) pollination. We applied maximal amounts of crabapple pollen to all blossoms per cluster on hand pollination branches and left open treatment blossoms exposed to bee pollination. Fruit diameter and number of seeds were evaluated after initial (two weeks past petal fall) and final (harvest) fruit set. We found the number of seeds per fruit from hand pollinated blossoms were 15% higher than open (bee) pollinated blossoms. Understanding the relationship between pollen limitation and fruit set is essential for advising growers on how to effectively manage the local bee fauna and in determining under what conditions native bees can provide effective pollination in commercial apple orchards.