Ivy: an underappreciated key resource to flower-visiting insects in autumn

Ivy (Hedera helix and H. hibernica) is a common autumn-flowering plant found in Europe, North Africa, Macaronesia and Asia. Here, we use five complementary approaches (pollen trapping, nectar refractometry, local and regional surveys of insects foraging on ivy flowers, local survey of ivy abundance) to evaluate its importance to the honey bee (Apis mellifera) and other flower-visiting insects in Sussex, England. Pollen trapping at six hives in two locations showed that an average 89% of pollen pellets collected by honey bees in the autumn were from ivy. Observations of foraging honey bees on ivy showed that ivy nectar is an even greater target than pollen, as 80% were collecting only nectar. Refractometry of samples from ivy flowers and from honey bees foraging on ivy showed that ivy nectar is rich in sugar, 49% w/w. Surveys showed that the main insect taxa foraging on ivy were honey bees (21%), bumble bees (Bombus spp., 3%), ivy bees (Colletes hederae, 3%), common wasps (Vespula vulgaris, 13%), hover flies (Syrphidae, 27%), other flies (29%) and butterflies (4%). The surveys also showed significant temporal and spatial variation in taxon abundance and proportion. A survey showed that ivy was very abundant on a small scale in both rural and urban areas, being present in 10/10 and 6/10 0.2x0.2km samples within two 4x4km areas respectively. The results show that ivy should probably be considered a keystone species with a high value in the conservation of flower-visiting insects in autumn.