by Sandra Tuszynska
Moths are often overlooked as pollinators, as many are only active by night and perhaps we often don’t find them as attractive as butterflies. However, moths belong to the same order as butterflies called Lepidoptera and are equally important as pollinators of plants.
Many moth species take over the night shift in pollinating plants. Nocturnal hawkmoths are highly sensitive to the scent of flowers and learn flower odours rapidly. Besides olfactory (scent) senses, like bees, moths also use colour vision to identify and discern suitable flowers to forage for nectar (Kelber et al., 2003). However, while bees and butterflies are colourblind by night, just as we are, nocturnal moths can discriminate between flowers at starlight intensities (Kelber et al., 2003).
Diurnal hawkmoths (active during the day) and nocturnal hawkmoths, of the family Sphingidae, see colours and have excellent vision, which they use to hover in front of flowers. The diurnal hummingbird hawkmoth, Macroglossum stellatarum, strikingly resembles the flight, sound and appearance of a hummingbird. It uses its very long tongue (probosis), to reach deep inside some of the elongated flowers hiding their nectar.
Diurnal hawkmoths visit a variety of differently coloured, shaped and sized flowers. In contrast, their nocturnal cousins prefer white, cream and brightly yellow flowers, which produce strong intensity (achromatic) contrast to the surrounding darkness (Balkenius and Kelber, 2004). Nocturnal hawkmoths are thus sensitive to the intensity or brightness of flowers, rather than their actual colour (chromatic) properties.
Moths possess trichromatic or tipple colour-sensitive vision with an ultraviolet, blue and green-sensitive receptor types. Both diurnal and nocturnal moths can learn to memorise colours rapidly.
Learning to Discriminate Colours
Like bees moths can learn to discriminate colours based on the reward given to them. Animals can be trained to associate one colour with a food reward and another colour with no reward. When relative intensities of light are changed and the animal chooses the stimulus of the learned wavelength, the animal uses colour to identify it’s reward. If the animal change its choice behaviour based on intensity-related cue, the animal possesses achromatic (brightness) and not colour based vision.
In 1914, von Frisch studied colour vision in the honeybee. Animals are trained to associate a reward with a specific colour, but instead of one single unrewarding stimulus, many shades of grey are used. If the trained animal can distinguish the colour from all the shades of grey, then it is most likely to use coulour (chromatic) vision.
It was shown that both nocturnal and diurnal hawkmoth species can learn to associate colours of flowers, which are not typical colours they associate with food. For example M. stellatarum have an innate preference for blue and yellow, however they rapidly learn to accept green as a reward flower (Kelber et al., 2003).
The moths migrate from the Mediterranean to central and northern Europe in summer and must rapidly learn to find new food sources along the long journey. In fact, the hummingbird moth (M. stellatarum), can be trained to a new pair of colours every day, for several days in a row (Kelber et al., 2003). These moths have exceptional colour vision. They choose the correct colour, even if one of the colours is made ten times brighter or a hundred times dimmer than the other, under different wavelengths of light (Kelber et al., 2003).
Strictly diurnal moths will stop visiting flowers under dim conditions and resume feeding as soon as light is available. Hyles lineata, White-lined Sphinx moth is active during the night and the day, so it continues feeding in very dim conditions. These amazing creatures possess day and night colour vision, adapted to an incredibly wide range of light intensities, which still baffles scientists (Kelber et al., 2003).
Dawn and dusk are the two times of the day when the colour of light changes considerably. Colour constancy is the ability to recognise a specific colour regardless of the illumination, which may change the shade or intensity of a colour. While the spectra reflected from an object reaching our eyes differ under the sun, compared to the same object being in the shade, our brain recognises the colour as the same (Balkenius and Kelber, 2004). Constancy is especially important to hawkmoths as many species are active at dawn and dusk, when the spectrum of illumination changes the most. Their colour-constant visual system, allows moths to find and recognise rewarding flowers, despite of the diurnal changes in colour of the illuminating light.
Moths are also able to learn illumination. Balkenius and Kelber (2004) found that when the illumination is changed from the illumination used during training, the moths are initially disoriented. They explore the flowers more to identify the correct or reward producing flower colour. However, within a few days the moths adjust to the different illumination and are back on track visiting the rewarding flowers.
Another test showed that moths can learn to discriminate between illumination (Balkenius and Kelber, 2004). They were not rewarded when yellow illumination was used and rewarded when white illumination was used. The moths learnt to only forage during white illumination and did not bother to forage under yellow illumination, as this would not result in a food reward (Balkenius and Kelber, 2004). This intelligence possibly becomes handy for pollinating insects to learn exactly when flowers produce the highest quality nectar, which changes throughout the day.
Kelber et al. (2003) tested constancy in D. elpenor and M. Stellatarum hawkomoths trained to green and turquoise stimuli, under white and yellow illumination. Under yellow illumination, turquoise is perceived as green under white illumination, by the blue and green photoreceptors. Moths trained to associate a food reward with green colour under white illumination, could easily distinguish it from the turquoise colour, under both white and yellow illumination (Kelber et al., 2003). The moths thus show colour constancy, as they did not confuse these two colours, always choosing the rewarding colour despite the confusing illumination. For nocturnal insects, the contrast produced by white or yellow flowers against the dark vegetation is vital.
The achromatic aspect of colour is its intensity or brightness. While bees use achromatic vision for pattern recognition and motion vision, their colour vision is not sensitive to achromatic properties such as brightness.
The diurnal hawkmoth M. stellatarum is sensitive to achromatic cues. At very dim light intensities, it is unable to see colour as only the moth’s green receptor is sensitive. In dim conditions M. stellatarum, naturally prefers flowers that are brighter, producing high achromatic contrast (Kelber et al., 2003).
These moths prefer dark blue compared with light blue colours, when presented on a light grey background. They will also use contrast within the one flower, for example they choose dark blue flowers with a white centre, instead of a black centre, and light blue flower with a black instead of white centre (Kelber et al., 2003).
In contrast the nocturnal Deilephila elpenor, does not discriminate between different shades of blue or yellow. It is postulated that it uses achromatic cues only when chromatic cues are absent (Kelber et al., 2003). In fact, the moths ignore white stimuli on a light grey background, but are highly attracted to the same stimulus on a black background, which produced a highly achromatic (bright) contrast. However, even in dim starlight, nocturnal hawkmoths use chromatic (colour) cues rather than achromatic (brightness) cues in recognising rewarding flowers (Kelber et al., 2003).
Besides recognising flowers by colour, brightness and scent, hawkmoths also possess excellent memory of innate and learnt flower attributes. They hibernate for up to 5 months during the European winter but wake up remembering which flowers provide them with sweet nectar.
Kelber A., Balkenius A. and Warrant E. J. (2003). Colour Vision in Diurnal and Nocturnal Hawkmoths. Integr. Comp. Biol., 43:571-579.
Balkenius A. and Kelber A. (2004) Colour constancy in diurnal and nocturnal hawkmoths. The Journal of Experimental Biology, 207: 3307-3316.