Wing wear, but not asymmetry in wear, affects load-lifting capability in bumble beesBombusimpatiens

Abstract

Wing wear is widespread in flying insects, but its effects on flight are controversial. In this research, we examine the separate and combined effects of wing area and wing area asymmetry on maximum load-lifting capability in bumble bees Bombus impatiens Cresson, 1863. Individual bees with experimentally induced forewing wear (0%–24% forewing area loss, 0%–38% forewing area asymmetry) were harnessed with a string to which small bead groups were attached and tested in a flight chamber to measure the maximum weight that they could lift incrementally. Wing wear significantly decreased load-lifting ability: the higher the mean wing area loss, the less mass a bee could lift (2.66 mg load reduction per 1% forewing area loss, which represents ∼1.6% of mean body mass or ∼5.2% of expected mean nectar load). However, wing area asymmetry, both alone and in combination with area loss, had no detectable effect on maximum lift. The clear cost of wing wear for bumble bees is a linear reduction in weight-lifting capability through loss of wing area. This relatively strong diminution of load lifting by wing wear, observed over the range of wing area losses naturally accrued by wild bees, provides a potential mechanism for declining foraging ability and survivorship of worker bees with wing wear. What remains to be explained is the utter insensitivity of maximum load lifted to forewing asymmetry.