Bumble bees (Hymenoptera: Apidae: Bombus terrestris) collecting honeydew from the giant willow aphid (Hemiptera: Aphididae)

Only rarely have bumble bees (Bombus) been observed collecting honeydew from aphids (Aphididae) feeding on phloem sap. This behavior may be rare because the percentage of sugar in honeydew egested from aphids is generally well below the sugar concentration in floral nectars preferred by bumble bees. Nonetheless, in August 2018, near St. Buryan, Penzance, Cornwall, UK (56.0602°N; -5.6034°W) we observed large numbers of wild Bombus terrestris (Linnaeus) collecting honeydew from a colony of the giant willow aphid Tuberolachnus salignus Gmelin feeding on the stems of the willow Salix alba. Unlike aphid-tending ants, who glean fresh honeydew directly from the aphid anal opening, the bumble bees were collecting honeydew from leaf litter below the aphid colony. We hypothesized that honeydew collected from exposed ground surfaces was more concentrated due to evaporation under ambient conditions than that released directly from the anus (fresh honeydew). We thus monitored sugar concentrations of fresh honeydew and compared them with the concentrations of the crop contents of worker bumble bees foraging from the leaf litter. Our data show that the concentration of sugar in fresh honeydew was as much as 10% w/w lower than that collected from leaf surfaces, as measured from the crop contents of foragers. The unusually hot, dry weather in Cornwall may have enhanced evaporative concentration of honeydew while restricting floral nectar sources, thus favoring honeydew collection by B. terrestris, a generalist bumble bee forager.