Most nocturnal Lepidoptera can be attracted to artificial light sources, particularly to those that emit a high proportion of ultraviolet radiation. Here, I describe a newly developed LED lamp set for the use in the field that is lightweight, handy, robust, and energy efficient. The emitted electromagnetic spectrum corresponds to the peak sensitivity in most Lepidoptera eye receptors (ultraviolet, blue and green). Power LEDs with peaks at 368 nm (ultraviolet), 450 nm (blue), 530 nm (green), and 550 nm (cool white) are used. I compared the irradiance (E_e) of many commonly used light-trapping lamps at a distance of 50 cm. Between wavelengths of 300 and 1000 nm, irradiance from the new lamp was 1.43 W m^-2. The new lamp proved to be the most energy efficient, and it emitted more radiation in the range between 300 and 400 nm than any other lamp tested. Cold cathodes are the second most energy-efficient lamps. Irradiation from fluorescent actinic tubes is higher than from fluorescent blacklight-blue tubes. High-wattage incandescent lamps and self-ballasted mercury vapour lamps have highest irradiance, but they mainly emit in the long wave spectrum. The use of gauze and sheets decreases the proportion of UV radiation and increases the share of blue light, probably due to optical brighteners. Compared with sunlight, UV irradiance is low at a distance of 50 cm from the lamp, but (safety) glasses as well as keeping sufficient distance from the lamp are recommended. In field tests, the new LED lamp attracted large numbers of Lepidoptera in both the Italian Alps and in the Peruvian Andes.

Light-trapping has long been known as an efficient method for collecting of nocturnal insects in general and Lepidoptera in particular (e.g. Taylor and French 1974; Holloway et al. 2001; Infusino et al. 2017). Early on, it was observed that moths can be attracted to the light of fire or candlelight and might even get burned – the family name Pyralidae probably relates to this observation (Emmet 1991). Light-trapping, either manual or with automatic traps, has become a standard and widespread method in ecology, taxonomy, and Lepidoptera monitoring schemes, and it is supposed to represent the only method allowing a large number of clades to be sampled quantitatively in large numbers (Holloway et al. 2001). Light sources with a high proportion of ultraviolet (UV) radiation tend to attract a greater number of individuals and more taxa (van Langevelde et al. 2011). A research focus in recent years has been to investigate the impact of modern street lighting on insects (“light pollution”: e.g. Huemer et al. 2011; van Langevelde et al. 2011; Somers-Yeates et al. 2013; Pawson and Bader 2014; van Grunsven et al. 2014; Macgregor et al. 2016), including implications of anthropogenically driven selection on flight behaviour in urban areas (Altermatt and Ebert 2016).

A wide range of lamp and trap types for light-trapping has been used in entomological research. Although standardisation is desirable, the availability of new designs and lamps has continually led to changes in the lamp set-ups used. Depending on the requirements of research, it is (a) either more important to stress continuity and use a standard method that has been used in previous studies, or it is (b) more important to apply the most efficient and best available technology. A good example of (a) are Rothamsted traps (Williams 1948) that are operated with strong incandescent lamps with a tungsten filament. The use of this ‘old-fashioned’ technology can be justified in long-term monitoring programmes that are intended to be continued without a substantial methodological change (Southwood et al. 2003). Established methods such as the use of incandescent or high-pressure mercury vapour (MV) self-ballasted lamps also offer the advantage of long-term experience and published comparative studies on their performance (e.g., Intachat and Woiwod 1999).

The use of established light trapping methods does, however, have some disadvantages. For example, incandescent lamps have largely been abandoned in Europe because they are primarily producing long-wave radiation including a large proportion of invisible infrared radiation (Fig. 5a) that contributes relatively little to attracting insects (e.g. Cowan and Gries 2009), while the lifespan of such lamps is rather limited (Infusino et al. 2017). MV lamps emit a more favourable spectrum of radiation (Fig. 5a), but the longevity of the commonly employed self-ballasted type is similarly limited as incandescent lamps (Infusino et al. 2017). Moreover, high pressure MV lamps are being phased out due to their content of toxic mercury, which is banned by new legislations in many countries. Both types of lamps require high voltage, which means that during field work heavy and bulky generators are required. Despite containing mercury, fluorescent tubes of all types are still widely used (e.g. ‘energy-saving lamps’). For insect collectors, particularly popular types of fluorescent tubes emit large proportions of UV radiation, including actinic / blacklight (BL) tubes as well as blacklight-blue (BLB) tubes – the latter with a dark-blue filter coating that absorbs most light. More recently, cold cathodes have become available through their use as backlighting of monitors and as decorative illumination in computer cases. These vary in their wavelengths and one can therefore choose those that include the UV range. However, little seems to be known about their performance in light-trapping so far. The use of LEDs is now increasingly common in light-trapping (Green et al. 2012; Price and Baker 2016; Infusino et al. 2017). LEDs have also been employed in experimental studies because a wide range, with different radiation peaks, is available (e.g. Cowan and Gries 2009; Kadlec et al. 2016).

Although lamp emission data are sometimes provided by the manufacturers, standardized comparisons of the emission or irradiation of different lamps are rare in the entomological literature. A comparison of six light sources with an emphasis on street lighting was given by van Grunsven et al. (2016). Papers can also easily be overlooked if published in journals or in languages with limited readership, as exemplified by a paper by Steidel and Plontke (2008) that graphically shows the qualitative emission spectra of various lamps.

Here, I describe a new LED lamp design intended for use in light trapping under field conditions, including remote tropical locations. The lamp was developed with the aim to minimize weight and size and to maximize energy efficiency and longevity. The aim was to be able to power this lamp with cheap and widely available 5 V lithium batteries (‘powerbanks’), as well as the option of using 12 V batteries. Overall emission was intended to be of comparable or higher quantity than fluorescent BL and BLB tubes used in many previous field studies (e.g. Brehm and Axmacher 2005), and to provide a higher output than in previously described LED-based designs (Green et al. 2012; Price and Baker 2016; White et al. 2016; Infusino et al. 2017).

The spectral composition of the lamp is orientated towards the peak sensitivity of lepidopteran eye receptors as suggested e.g. by Steidel and Plontke (2008), Mobbs (2016), and Price and Baker (2016). The available data on lepidopteran eye receptor sensitivity is still limited but includes a broad range of taxa (Briscoe and Chittka 2001, Fig. 1a). These data suggest that three types of receptors are commonly found in moths, exemplified in the hawkmoth Deilephila elpenor (L.) (Johnsen et al. 2006): one in the ultraviolet, one in the blue, and one in the green range (Fig. 1a). Synanthedon myopaeformis (Borkhausen) (Sesiidae) is sensitive both in the ultraviolet and green range (Eby et al. 2013). Further receptors can be present and are possibly even widespread, such as red receptors known from the noctuids Spodoptera exempta (Walker) and Mamestra brassicae (L.) (Fig. 1a). As an extreme case, photoreceptors of 15 distinct spectral sensitivities were found in the butterfly species Graphium sarpedon (L.) (Papilionidae) (Chen et al. 2016). Given the large empirical success of lamps with a high proportion of UV radiation (including MV lamps, fluorescent tubes, cold cathodes, and UV LEDs), the emission of this short wave radiation was considered to be particularly important.

Figure 1. 

a. Values of maximum spectral sensitivity of Lepidoptera eyes, modified from Briscoe and Chittka (2001) and Johnsen et al. (2006), and sensitivities of the photoreceptors of the hawk moth Deilephila elpenor as an example, with peak absorption wavelengths of 350, 440, and 525 nm (Johnsen et al. 2006). b. The spectral composition of the new LED lamp (operated at 350 mA) is oriented towards the spectral sensitivity of moth eye receptors (background grey bars). A transparent acrylic cylinder has only a minimal influence on the irradiation from the lamp whereas a matt acrylic cylinder (dashed white line) slightly decreases the performance of the lamp, see also Table 1.

The emission of the new lamp is described in detail and quantitatively compared with a range of lamps commonly used by entomologists. Measurements include transparent clear and matt protective acrylic glasses, sheets, and gauze. Lamp emissions at different distances are compared with sunlight and the roles of spectacles and sun spectacles as eye protection are discussed briefly. Finally, the new LED lamp was tested under field conditions in more than 50 sampling events in the Italian Alps and Peruvian Andes, to confirm that nocturnal Lepidoptera were indeed attracted to the lamp and opening perspectives for further research.

The new LED lamp was constructed with the aims of being lightweight, handy, robust, and energy efficient, and these aims were clearly fulfilled. First field tests have demonstrated that the lamp is very attractive to nocturnal Lepidoptera (Fig. 7), and a detailed analysis of the samples will be published in due course. The measurements carried out concentrate on irradiance rather than on total emission of the lamp, first because the required measuring device, a two-meter diameter Ulbricht sphere, was not available in Jena. Second, the chosen approach allowed comparison of irradiance from lamps in combination with gauze and sheets, as well as with incoming sunlight. Generally, comparisons between different lamps are never simple because lamps differ in their design, in their dimensions, and in the way radiation is emitted. All of these factors could possibly influence moth behaviour, and therefore measurement results should be regarded as an approximation of potential moth attractiveness, to be supplemented by field studies and physiological measurements.

The age and the cumulative operating hours of the lamps could have an impact on their performance, but it was beyond the scope of this paper to explore this effect in detail. For example, the emission of fluorescent tubes drops with age to ca. 80% in new-generation lamps (Sylvania BL 368 nm) and to ca. 50% in old-generation tubes (e.g. Sylvania BL 350 nm) (Havells-Sylvania 2012). Decreases are also expected to occur in LEDs, accelerated by high temperatures and high currents. For this reason, LEDs in the lamp are not being operated at the maximum possible current (700 mA) but only at 350 mA, aided by an efficient cooling system. Ageing of acrylic glass and other materials possibly also influences the radiation flux.

Clearly, a cross calibration study with other lamps is desirable. Such comparative studies have regularly shown that even lamps with fundamentally different light spectra attract similar moth assemblages. For example, Geometroidea samples attracted to an incandescent and a MV lamp were surprisingly similar (Intachat and Woiwod 1999; Infusino et al. 2017; Jonason et al. 2014). On the other hand, noctuid moths were more attracted to short wave radiation than geometrid moths (Somers-Yeates et al. 2014), so certain differences in samples obtained with different methods must be expected.

An unexpected result was the appearance of a blue peak at ca. 440 nm when ‘light tower’ gauze and a white sheet were used in combination with various lamps. In all cases, a part of the UV radiation is absorbed and re-emitted by the textile as blue light, caused by commonly used optical brighteners in textile production and in washing powders. This means that supposedly ‘pure’ UV sources such as BLB tubes and UV LEDs combined with a textile also emit a certain amount of blue light. This lowers energy efficiency to some extent, but the additional blue light possibly increases the attractiveness to insects.

The lamp itself has a weight of less than 500 g, and it can be operated for five to six hours with a standard powerbank, e.g. an Easy Acc battery (5 V, 26 Ah, 400 g). Since powerbank batteries are a mass product on the market used for mobile phones etc., their prices are reasonable, they can easily be transported in carry-on baggage and recharged with mobile solar panels in remote areas. The total equipment, including the lamp, powerbank and charging device (220 V AC to 5 V DC USB charger) weighs less than 1 kg. In comparison, any equipment operated with generators is far heavier because a generator alone weighs ca. 13 kg. Equipment operated with 12 V is usually connected to (heavy) lead batteries. For example, field work in Ecuador and Costa Rica (Brehm and Axmacher 2005; Brehm 2007) was undertaken with a 15 W actinic BL tube and a 15 W BLB tube, operated with a lead battery (12 V, 7 Ah, 2 kg). Together with the charging device, the equipment weighed ca. 4 kg. The size of the new LED lamp (6 x 14 cm) is also small, so that it easily fits into travel bags and backpacks. The lamp has furthermore proven to be robust in the field. In one case, the axial fan broke when a gauze tower was blown down in a thunderstorm. However, the lamp remained fully functional without the fan, but since a working fan reduces the temperature of the LEDs (by ca. 10° C), which leads to a longer lifetime of the diodes, it is recommended that a broken fan is replaced when it is practical to do so (it takes only a few minutes).

In terms of energy efficiency, the new LED lamp outperformed every other lamp that was tested (Table 1, Appendix 1), and total irradiance between 300 and 400 nm was greater than from any other tested lamp, even including strong, self-ballasted MV bulbs. If efficiency is to be maximized, the use of 12 V batteries is recommended but from a weight-optimising point of view, 5 V powerbank batteries are the better choice. The second most energy-efficient tested lamps are cold cathodes with an input voltage of 12 V (Table 1). Their use nonetheless requires heavier batteries or a step-up converter that lowers energy efficiency. Cold cathodes (especially blue ones) are very lightweight and appear to have the best price / performance ratio.

The new LED lamp emits the desired spectrum of different wavelengths (UV, blue, and green). Half of the LEDs are UV diodes because UV is particularly attractive to moths. However, the additional diodes are expected to contribute further to the attractiveness of the lamp, and to stimulate eye receptors sensitive to longer waves. When MV lamps are compared with BL and BLB tubes, MV lamps usually attract more moth species and individuals (e.g., Jonason et al. 2014; Tikoca et al. 2016). A possible reason is that MV lamps emit not only more UV radiation, but also a much broader spectrum, than fluorescent tubes. A major advantage of LEDs is that a light mix can experimentally be assembled from a wide range of available diodes (Cowan and Gries 2009; Kadlec et al. 2016). Future studies could assess whether a maximisation of UV radiation on the one hand versus a mixture of wavelengths on the other hand, results in a higher number of attracted moth species and individuals. The new LED lamp could easily be used for such experiments, and mixtures of different LEDs could be tested. In principle, the lamp could also be modified in such a way that more diodes, e.g. 12, 16, or 20, are mounted on an extended design.

I am indebted to Jan Axmacher, Konrad Fiedler, Jeremy Holloway, Luisa Jaimes, Bernard Landry, Rolf Mörtter, and Eric Warrant for providing helpful comments, hints to literature, and linguistic assistance with the manuscript. Rainer Bark, Tobias Borchert, and Daniel Veit helped me to technically realize the project, B. Kühn checked the manuscript with regard to physics, and D. Veit kindly provided the photospectrometer for multiple measurements. Jan Axmacher, Konrad Fiedler, Egbert Friedrich, Kai Grajetzki, Marco Infusino, Rolf Mörtter, Rando Müller, Stefano Scalercio, and Axel Steidel kindly provided lamps and Jürgen Schmidl (Bioform) gauze for measurements. Field work in Peru was conducted together with Daniel Bolt, Juan Grados, Gerardo Lamas, and Matthias Nuß. Stella Beavan is thanked for the final language check of the manuscript.The Peru project was supported by the Deutsche Forschungsgemeinschaft (BR 2280/6-1).