In 2016, I wrote a short blog post concerning the construction of a new LED lamp for the attraction of moths. The rationale was based upon the idea that any lamp designed specifically for the attraction of moths should use LEDs ‘tuned’ to the wavelengths to which the moth’s eye are maximally sensitive. Since the peak absorbance of moth photoreceptors (Manduca sexta) are found in the UV, green and blue (λmax 350 (UV), 450 (blue), 490 (blue/green), and 530 nm (green)) I chose a combination of UV, green and blue LEDs, later adding cool white which adds significantly to the light emitted by the lamp at 450nm but also at longer wavelengths. In 2017, Gunnar Brehm published a comprehensive paper concerning the construction of an LED-based moth lamp “A new LED lamp for the collection of nocturnal Lepidoptera and a spectral comparison of light-trapping lamps” (Nota Lepi. 40(1) 2017: 87–108). His paper contains a great deal of valuable data concerning the spectral properties of a variety of light sources used to trap moths and also explains the rationale for his development of a ‘LepiLED’ – a lamp designed to be as effective as possible in attracting moths. His light sources were chosen to be near matches the peak spectral sensitivities of the receptors found in moth eyes. He chose 8 by 3W LEDs; 4 x UV, 2 x royal blue , 1 x green, and 1 x Cool White. The product details for the LepiLED suggest that the LEDs are under-run when powered from a 5V source and the input power with all the LEDs illuminated is limited to 13W. While the balance of the emitted light in Brehm’s lamp is oriented toward the UV, half of the output is at longer wavelengths. However, these longer wavelength LEDs can be switched off and the lamp used purely as a source of UV. The product was designed to be portable and to be powered from batteries.
As Brehm observes, particularly with respect to the input power of high pressure mercury (Hg) vapour bulbs, LEDs are much more efficient. His paper includes data showing that where UV is concerned the irradiation from his lamp with 4 x 3W UV LEDs is greater than that from a 160W Hg bulb. However, a 160W Hg bulb produces a great deal more light at longer wavelengths than do the visible light LEDS in the LepiLED, including a significant output in the infra-red (heat). When the LepiLED and my lamp were produced, little was known about the neural mechanisms responsible for the behaviour that causes moths to be attracted to light or their action spectrum .
More recently, Brehm and co-workers (Insect Conservation and Diversity (2021) 14, 188-198) have shown that UV light is the most effective in attracting moths; blue LED light outperforming white, green and red light, with UV light being more effective than all the other wavelengths. Their work indicates not only that UV light sources are much the best for attracting moths for the purposes of surveying their populations but also that from a conservation point of view, artificial light sources used for outdoor lighting should be pitched towards longer wavelengths. Thus, outdoor light sources should not use cool white LEDs which emit significant amounts of blue light and that ‘warmer’ light sources should be used instead. I would add that the ‘blue’ xenon headlamps used in some cars are likely to attract moths in greater numbers than tungsten filament lamps, the spectrum of which is richer in longer wavelengths.
Insect vision at night is very different to that of humans. Nocturnal insects such as moths have compound eyes in which rather than single facets viewing a narrow angle such as are found in day-flying insects like the honeybee, have superposition optics which enable light to reach the receptors in one ommatidium through several hundred facets. The large facets, wide rhabdoms (the structures in which the visual pigments are located) and a reflective layer, the tracheal tapetum which reflects photons that pass through the rhadhoms back towards them, mean that at the same light intensity, a single photoreceptor in the moth eye absorbs over 4000 times as many photons as a photoreceptor in a honeybee’s eye or the human retina. However, while the moth eye’s absolute sensitivity could be further enhanced by expressing a single visual pigment, most nocturnal insects including moths have the three spectral types of photoreceptors with the absorbance maxima given above. In-so-far-as moth behaviour at night has been examined, some if not all species are able to use colour information even at the low light intensities found under the night sky. It is interesting to note that the light from the sky undergoes a significant blue shift after sunset though the light from the moon has a spectrum similar to that of daylight. While the neural mechanisms involved in steering a moth toward an artificial light source remain poorly understood, significant insights can be drawn from 3D kinematics of insects flight around a light. It appears from the work of Fabian et al. (2024, Nature Communications 15, no 689) that nocturnal insects maintain their flight attitude by keeping their dorsum oriented toward a light source – which would usually be part of the celestial hemisphere. However, near an artificial light the dorsal orientation toward it causes the insect to circle or even to invert and crash. Fabian et al. used a variety of UV light sources in their recordings but they did not systematically examine the action spectrum of the responses they observed. It remains unclear whether this dorsal light response is driven by the photoreceptors in the moth’s compounds eyes or their ocelli – the latter also containing UV receptors and being known to play an important role in flight stabilisation in day-flying insects.
The implications of the above for the design of a ‘moth lamp’ would appear to be that on a watt-for-watt basis an LED-based design would be best to use entirely 365nm UV light. Further, in terms of the UV light available from Hg bulbs which are now banned for general use, UV LEDs far outperform them. However, because Hg bulbs emit large amounts of light at longer wavelengths, and because these will albeit to a far lesser extent, also be absorbed by the shorter wavelength pigments in the moth eye, it is difficult to assess the wattage required for a UV LED lamp designed to outperform a much higher power Hg bulb. That said, according to Brehm’s figures, on a per watt basis much lower energy UV LEDs generate a great deal more UV light than Hg bulbs. Further, UV LEDs have been shown to be highly effective in attracting moths. The safety aspects of UV light sources cannot be ignored. A powerful Hg vapour bulb is a signifcant hazard and should never be viewed directly without UV blocking spectacles. The problem with pure UV LED light sources is that unlike their Hg vapour counterparts, the light they emit is barely visible to the human eye and while a high wattage UV LED lamp may be very attractive to moths, it also represents a significant safety hazard. With the newly available data on the effectiveness of LED UV light sources for light-trapping in mind, it would be interesting to generate a moth light that while emitting significantly more 365nm light than the LepiLED or a much higher power Hg bulb, could be powered from a mains PSU, a 12 or 5V battery. Only experiment can determine the effectiveness of such a pure 365nm UV lamp.