Pollinators with bad PR

By Athayde Tonhasca

When member of the serial killer community ‘Buffalo Bill’ decided to leave a message to his FBI pursuers, he stuffed the pupa of a black witch moth (Ascalapha odorata) down the throat of his last victim (The Silence of the Lambs, by Thomas Harris). Buffalo Bill may have been a psychopath, but he knew his insects: a target for superstition and ignorance, the black witch is considered a harbinger of doom in many countries. But the makers of the film version of Harris’ story wanted something even more dread-inducing: they substituted the black witch for the African death’s-head hawk moth (Acherontia atropos), another species with a long tradition of spookiness. 

An African death’s-head hawk moth © Muséum de Toulouse, Wikimedia Commons.

With a bit of imagination, you can see a human skull on the moth’s thorax, a feature that has tied the harmless creature to all sorts of legends and misconceptions. Starting with its name, A. atropos, and the names of the other two death’s-head hawk moths from Asia,  A. lachesis and A. styx, which are homages to Greek myths about mortality.

Atropos and Lachesis were two of the three Moirai, the goddesses of destiny, and Styx is the river that divides Earth from the underworld. Adding to the African death’s-head hawk moth’s capacity to awe and alarm, it can chirp when disturbed. You can listen to it here (volume up!) 

The three Moirai, by Alexander Rothaug (1870-1946).

Artists and writers have been inspired by the hawk moths’ mystique, from Edgar Allan Poe, John Keats and Bram Stoker, to stories about its nefarious influence on the mental health of King George III. The University of Cambridge’s Museum of Zoology holds a specimen taken from the king’s chambers by one of the royal physicians.

Dracula, by Bram Stoker (1847-1912), 1919 edition © British Library, image in the public domain.

All these loopy tales distract us from the tangible facts about the death’s-head hawk moth. Which is a pity, because this species is remarkable in many ways. It migrates seasonally from Africa to southern Europe, venturing now and then north of the Alps and into Britain, sometimes as far as the Shetlands, from August to October. The moth travels at night for up to 4,000 km, maintaining a straight path by adjusting its flight plan according to wind conditions. How do we know that? By fitting moths with miniscule transmitters and following them in an aeroplane (Menz et al., 2022). 

The moth’s large, unmistakable caterpillars feed on potato-related plants (Solanum spp.), and can be abundant in potato fields during years of high migration. The feeding habits of adults are less well known. Most adult hawk moth species have an extended proboscis suitable for taking nectar from flowers with long corollas. But the death’s-head’s proboscis is shorter, and it cannot reach the nectar of many flowers. So it adds to its diet by feeding on rotting fruit, tree sap, and, remarkably for a moth, honey. The death’s-head sneaks into colonies of honey bees (Apis spp.) to pilfer their honey – thus the moth’s alternative epithet: ‘bee robber’. It manages to survive such a daring raid thanks to chemical camouflage: moth and bees share some of the cuticular hydrocarbons that bees use to identify nestmates. The moth’s short legs and stout body also help it wiggle in and out of the hive. But these ruses don’t work all the time: beekeepers often find moth remains in their hives.   

A death’s-head hawk moth caterpillar © Erik Streb, and imago (adult stage) © Stahre, Wikimedia Commons.

The death’s-head hawk moth is a poor pollinator candidate, even though we know little about its ecology. At a first glance the other 1,400 or so hawk moths, also known as hummingbird moths, sphinx moths or sphingids (family Sphingidae) – mostly from Africa and the Americas – don’t look promising either. They don’t have pollen-carrying apparatus, and most of them are very good at keeping their distance from flowers. They usually feed by hovering in front of a flower, probing it with their straw-like proboscis. Hovering, a trait shared with hover flies, hummingbirds and some bats, is an energy-demanding flight mode, but it allows the moth to dart from approaching enemies and stay clear of spiders, frogs and other predators lurking on the flower. 

A convolvulus hawk moth (Agrius convolvuli) © Charles J. Sharp, Wikimedia Commons.

Plants however tell a different story. Flowers visited by hawk moths have characteristics that meet their visitors’ needs: in general, they open at night, produce copious volumes of sugar-rich nectar, are of white or pale colours, have long nectar tubes, and lack unnecessary landing zones. This fine-tuning between hawk moths and plants suggests adaptations for sphingophily, or pollination by hovering moths (a development from phalaenophily, which is pollination carried by flower-alighting moths). For plants, a smaller pool of specialised nocturnal visitors has the advantage of reducing the chances of pollen transfer between the wrong species. As a result, many plants are pollinated by hawk moths, of which certainly the most famous is the Darwin’s orchid (Angraecum sesquipedale). 

Two species pollinated by hawk moths: the greater butterfly-orchid (Platanthera chlorantha) © Jörg Hempel, and jasmine tobacco (Nicotiana alata) © Carl E Lewis, Wikimedia Commons.

Hawk moths are among the strongest flying insects; some are capable of commuting for several kilometres in search of flowers, sustaining speeds of over 19 km/h. They may carry only a few pollen grains attached to their proboscis or other body parts, but this pollen can be dispersed over large areas, which is advantageous for plants’ genetic diversity. Long-distance distribution may reduce the impacts of habitat fragmentation, which is the inevitable result of human occupation of natural areas. Indeed, Skogen et al. (Annals of the Missouri Botanical Garden 104: 495-511, 2019) showed that pollen dispersal by the white-lined sphinx or hummingbird moth (Hyles lineata) promotes gene flow between populations of the endemic Colorado Springs evening primrose (Oenothera harringtonii). You can watch hawk moths at work here.

A white-lined sphinx © Larry Lamsa and Colorado Springs evening primrose © Juanita A. R. Ladyman, Wikimedia Commons.

The role of moths as pollinators is poorly understood, but this gap in our knowledge is understandable. It is not easy to study pollination in daytime: at night, data collection is downright difficult. But we are learning more and more about moths’ contribution to the pollination of wild plants and crops. Nocturnal moths comprise around 90% of the 180,000 or so known species of Lepidoptera, so there is much to be discovered about busy but secretive hawk moths. 

The Hireling Shepherd, by William Holman Hunt (1827-1910). The cheeky shepherd boy shows a death’s head hawk moth to the girl, who looks unimpressed.