Evidence of introduced honeybees (Apis mellifera) as pollen wasters in orchid pollination

Abstract

Biological invasions threaten global biodiversity, altering landscapes, ecosystems, and mutualistic relationships like pollination. Orchids are one of the most threatened plant families, yet the impact of invasive bees on their reproduction remains poorly understood. We conduct a global literature survey on the incidence of invasive honeybees (Apis mellifera) on orchid pollination, followed by a study case on Australian orchids. Our literature survey shows that Apis mellifera is the primary alien bee visiting orchids worldwide. However, in most cases, introduced honeybees do not deposit orchid pollen. We also test the extent to which introduced honeybees affect orchid pollination using Diuris brumalis and D. magnifica. Diuris brumalis shows higher fruit set and pollination in habitats with both native and invasive bees compared to habitats with only introduced bees. Male and female reproductive success in D. magnifica increases with native bee abundance, while conversely pollinator efficiency decreases with honeybee abundance and rises with habitat size. Our results suggest that introduced honeybees are likely involved in pollen removal but do not effectively deposit orchid pollen, acting as pollen wasters. However, Apis mellifera may still contribute to pollination of Diuris where native bees no longer exist. Given the global occurrence of introduced honeybees, we warn that certain orchids may suffer from pollen depletion by these invaders, especially in altered habitats with compromised pollination communities.

Keywords: Habitat alteration; Introduced honeybees; Invasive species; Native bees; Orchids; Pollination.

Figure 1

Sites where populations of Diuris brumalis (red triangles) and D. magnifica (green triangles) were studied in the metropolitan area of Perth and Perth hills respectively, Southwestern Australia. Geographic coordinates are reported in Datafile S1. Satellite imagery was obtained from Google Maps, and the map was created using QGIS 3.10 (2019) - QGIS Geographic Information System. Open-Source Geospatial Foundation Project: http://qgis.org.

Figure 2

Native and non-native pollinators carrying Diuris (Orchidaceae) pollinia. Pollinia placement on Trichocolletes capillosus, native pollinator for Diuris brumalis (a), on Apis mellifera, pollen remover for D. brumalis and D. magnifica (b); and on Trichocolletes gelasinus, native pollinator of D. magnifica (c); flower morphology of D. brumalis (d) and D. magnifica (e) flowers showing the column where the pollinia is placed at the top and the labellum, insect platform. Scale bar of 5 mm. Credit: Daniela Scaccabarozzi.

Figure 3

Boxplot of co-occurrence (minimum and maximum value) of honeybees and native bees vs. honeybees alone on pollinia removal (a), fruit set (b), and pollination efficiency (c) in Diuris brumalis (Orchidaceae).

Figure 4

Effect of number of native bees and non-native honeybees quantified during transects for Diuris magnifica (Orchidaceae) reproductive success. The number of native bees influences pollinia removal (a), and fruit set (b) and number of non-native honeybees impacts orchid pollination efficiency (c).

Figure 5

Pollination efficiency of Diuris magnifica in relation to bushland reserve area (habitat size).