Efficiency and effectiveness of native bees and honey bees as pollinators of apples in New South Wales orchards

Abstract

Relatively little is known about regional variation in the native insects that visit and pollinate apples - a pollination-dependent and economically valuable global crop - in Australia. We undertook three studies on Pink Lady apple crops in two regions of Australia to 1) quantify the amount of apple pollen carried on the bodies of insects; 2) evaluate differences in the amount of apple pollen deposited on the stigmas by different insects (pollinator efficiency); 3) experimentally assess fruit set to ascertain baseline pollination (open treatment), maximum pollination (hand pollination treatment), pollination after a single visit by an insect (single visit treatment) and no insect pollination (closed control); and 4) quantify regional contributions to pollination services provided by each taxon by calculating pollinator effectiveness (efficiency * visitation rate). The most common apple flower visitors included introduced honey bees (Apis mellifera), native meliponine, allodapine, and halictine bees, hoverflies, and beetles. Native stingless bee, Tetragonula carbonaria, foragers carried the most loose apple pollen on their bodies, followed by honey bees, but most individual insects (51.6 %) visiting flowers carried no apple pollen at all. Comparisons at higher taxonomic levels showed that all bees (pooled) carried more apple pollen on their bodies than all non-bee taxa (pooled). In addition, when contacting flower stigmas, bees deposited more pollen per visit than non-bees. The most pollen grains per single contact visit were deposited by native Exoneura bees (x̅ = 231.25), but variation was high (s.e. = 179.75). Irrespective of pollinator identity, pollen deposition from a single visit increased with time spent on the flower. However, across species, the amount of loose body pollen did not predict the quantity deposited; in particular, honey and stingless bees carried more pollen but deposited less than halictine (Lasioglossum) and allodapine (Exoneura) bees. Fruit set experiments confirmed that Pink Lady apples require pollination, because closed controls set zero fruits. Fruit set also required repeated pollinator visits, because flowers restricted to a single insect visit rarely set fruit. Pollination deficits were detected in both study regions, wherein open treatments set fewer fruits than hand-pollination treatments. Pollination services were predominantly provided by honey bees (97 %) in one region (Orange), but in a second region (Bilpin), honey bees (60 %) were complemented strongly by wild stingless bees (35 %). Hence, in Bilpin, native bee pollination services may buffer predicted decreases in honey bee populations, should the Varroa mite become established in Australian agro-environments.