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Beetles Attracted to Light


A better title for this article would have been ’insects at light’ because, as is common knowledge, a wide variety of insects are attracted to light; this is obvious to anybody leaving a window open with a light switched on during the summer. Similarly, outside lighting will attract insects which might become annoyingly obvious in gardens during the summer. In this sense almost any light will attract insects e.g. an ordinary tungsten filament bulb in an ornamental garden fixture will soon attract moths and swarms of gnats on warm evenings. And we are not the only ones to be aware of this; light fittings in urban areas, and especially around shopping centres, will invariably have associated webs because spiders soon come to equate the lights with a rich food source. Lights are so attractive to insects that an ordinary light bulb or even a gas lantern left out in an urban garden will produce results, and because many of the species attracted to lights tend to be otherwise difficult to find this simple method can be very rewarding for the novice entomologist. But entomologists soon learn that some lights are better at attracting insects than others.

Types of Light

All kinds of entomologists use light to attract insects. This may be specular, as with an exposed filament bulb, or it may be diffuse, as with a fluorescent tube, and these produce light that decreases with intensity from the source according to inverse square spreading which, to state the painfully obvious, means that a more powerful source will, generally at least, attract insects from further away. The following discussion refers to specular light sources but another type of light can also be very attractive to certain insects. Light reflecting from a flat surface can become partially polarized; this happens with ponds and other still water bodies and may contribute to how aquatic insects disperse between aquatic habitats by flying at night. The same happens with light reflecting off vehicles and wet roofs which may explain why certain beetles e.g. Helophorus or Cercyon may appear in numbers on cars parked close to water in the summer or why the odd Dytiscus might collide with a greenhouse roof on a warm spring evening. Polarized light cannot be used as a source because it is difficult to produce for this purpose but the entomologist should be aware that it can be very attractive to flying insects and that it is likely to be present in light reflected from surfaces, especially when the light strikes a flat surface at a low angle and when it is the right kind of light. This last statement deserves some discussion because when it comes to attracting flying insects some lights are better than others.

Two very good clues about the attractiveness of light to insects are available from everyday life, and these two rather extreme examples sum up much of what is needed to be understood about attracting insects to light. Commercial food preparation areas often have those devices attached to the walls to attract insects away from the food and kill them, these devices include two functional parts; a light source and a high-voltage grid that will do the killing, the light always has a violet colour and this is no accident because it is designed to emit lots of violet and ultra-violet light, both of which are very attractive to insects. Yellow street lights, which are becoming rather old-fashioned now with the advent of powerful LED lights, contain virtually no violet light and so are generally ignored by insects, although some designs may attract a few insects because they rely on mercury to ‘start’ an electric arc that will heat up the system and vaporize the sodium which then emits light at two very specific wavelengths close to 589nm which we perceive as being in the yellow part of the visible spectrum, this is why sodium lights glow red and become more intense before the yellow sodium emissions begin. Light can be described by frequency, colour or wavelength, and all are in common usage but here we will use wavelength in a general way because it is possibly the easiest understood and most widely used. Another general term, amplitude, simply refers (for our purpose) to the intensity of the light; high amplitude means lots of light and vice-versa. The visible light spectrum extends from red to violet, red light is of low energy and long wavelength and is generally ignored by insects, violet light is of high energy and short wavelength and many insects are attracted to this for reasons that remain unknown. Depending upon the wavelength, ultra violet light is classed as UVA, UVB or UVC; UVA has the longest wavelength and UVC the shortest. All are intuitively energetic because UVA, which is abundant in normal sunlight, gives us sun tans and cancer and generally damages our skin as we get older while the more energetic UVB and UVC, which also damage our skin, are mostly filtered out in the upper atmosphere. Red light has no effect because it is not energetic although can be extremely damaging at very high amplitudes as it tends to cause combustion to nearby materials, a very good way of understanding this is to imagine a cooker ring glowing red, but however high the amplitude this type of light is generally ignored by insects. The yellow light emitted by sodium street lamps lies towards the red-end of the spectrum and is not generally energetic enough to attract insects. And so to optimize light trapping we need to capitalize on the attraction of high energy light to insects.

Any discussion of individual bulbs is pointless because the market is both vast and changing very rapidly, but the old classic 125W Mercury Vapour discharge bulb is worth a mention (although it may soon be banned from sale for various reasons) as it remains among the most powerful and, so far as insects are concerned, attractive light sources available. This warrants some discussion. The MV bulb is an electrical discharge unit that sends a spark between electrodes through mercury vapour, as with sodium lamps they start conducting then warm up and become intensely bright and, because of the properties of mercury, produce huge amounts of ultra violet light, so much that the discharge bulb is enclosed in an outer pearl glass bulb that filters out much of this UV, but even so the design of these bulbs has changed over the years to reduce the UV output and eliminate any danger posed by these emissions (and I swear that older discharge lamps from the 1960s and 1970s were far more attractive and produced far larger samples than more modern discharge lamps but this might simply reflect the huge decline in insect numbers since those times.) Even so there were a few years during the 1990s and beyond when UV-filtering goggles were popular among entomologists through fears of UV emissions which can be very damaging to eyes (and anyway entomologists soon become aware of how fragile the eyes are after a few years of peering through microscopes and using lenses in the field).  This classic lamp, or modern variations on it, is probably still by far the best choice for attracting insects but it comes with disadvantages. Discharge lamps use a lot of power and will generally need to be run with a portable generator, they would also use a lot more power if not run through a choke which limits and smoothes the current, so a good power source and some control gear will need to be taken into the field, in domestic gardens the mains electricity can be used but the control gear will still need to be attached. Running a discharge lamp anywhere near dwellings might also cause complaints because they can be intrusively powerful. On the other hand the results are superb as the powerful light produces a large catchment zone in all directions, and while the bulbs tend to be expensive (this has been partly negated by other, more friendly, types of lamps becoming available and so older discharge lamps are often available at small cost at entomological exhibitions etc.) they will work for many years if used properly. An interesting variant of the 125W discharge lamp had a black outer bulb which filtered out most of the visible light but allowed much of the UV through, these are more sociable in an urban garden but they run very hot indeed and are otherwise difficult to work with, The proper use really only includes attaching the control gear, plugging the bulb in and turning it on, then turning it off when finished and leaving it to cool before packing it away; they can be used upright or dangled from a wire but if used at an angle the inside mountings can become fragile and fall apart, never been a problem! Filament bulbs have never been good in insect traps although they can produce some results, depending on the spectrum, they are easy to use and inexpensive, and the fact that they work is obvious as so many insects are attracted to illuminated rooms and gardens, but given the opportunity to trap for beetles in specific and interesting habitats these should not be considered too seriously. More powerful types of filament bulb, which have produced mixed but often good results for us, are those sold for photography, here we mean the older type of photoflood bulb which are powerful and produce a white light but run very hot and tend not to last too long. Much better than filament bulbs are the various types of modern plasma bulbs, these are basically fluorescent tubes or light emitting diodes. Both are good for attracting insects because they can be designed to produce a spectrum loaded with short wavelength light, the LED by passing a specific current through a specific gas, and the fluorescent tube by exciting various internal coatings as the hot plasma conducts, neither type runs very hot (in the sense of mercury discharge lamps) and both are reasonably economical to run and so a generator is not needed (more of this below.), the disadvantage of both is that the light, while perfectly adequate for attracting insects, usually falls short in intensity. Lights are frequently marketed under the name ‘Actinic’, these are simply fluorescent lights with a significant output at short wavelengths; actinic originally referred to light that could initiate or cause chemical reactions or change, and a very wide range of actinic tubes are available both for moth traps and many other purposes. Lots of claims are made about how good various bulbs are at attracting insects and this is fair enough as actinic bulbs usually work that way but it should be remembered that these lights have been chosen from what is commercially available and that they have not been specifically designed and manufactured with insect traps in mind (although traps designed to electrocute insects might be an exception here), and so it might be worth trying some of the actinic tubes widely available at larger hardware stores, pet shops also stock UV tubes for aquariums, and nowadays even torches are readily available with UV LEDs or tubes. Fluorescent lights may be long and straight or twisted into various designs to make them compact, and both types are available in three basic forms; broad-spectrum ‘white light’ which usually includes a decent UV level, actinic, and ‘black tubes’ that are loaded with UV but emit little visible light and so may be more sociable in a domestic setting. It is not possible to say which are best without testing them over time as UV content alone may not be sufficient to make them good for attracting insects, this is rather a function of both the light spectrum and the spectrum over which the insects are sensitive, there is also the factor of reach because without a decent sphere of attraction a light will attract very little. LED lights are still very much in the R&D phase so far as insect traps are concerned; they have stimulated a lot of interest among nocturnal lepidopterists and there are even commercial traps available that use them, usually as arrays of multiple tiny lights, such traps have earned good reviews and the sample sizes are often said to be similar to those obtained with fluorescent tubes, which is to say much less than that obtained with MV discharge bulbs, but they are easy to run and handle, and in any case it might be worth keeping an eye on these lights as this is (sadly) almost certainly where the future of light trapping lies. Finally it should be noted that the spectrum may change as a bulb gets older, this will happen very gradually and will almost certainly not be noticed until the sample size becomes greatly reduced, it is very difficult to detect but if bulbs are obtained in pairs then the most frequently-used can occasionally be compared with the fresh bulb.

A huge range of devices are available to attract insects to light and trap them without harming them, these generally consist of a light source and one or more adjacent transparent vanes above a container, the idea being that the attracted insects collide with the vanes, become stupefied, and fall into a container which is designed to prevent them flying away and is packed with some material such as egg-trays in which they can escape from the light and rest until it goes away. Traps are thus designed to run remotely, without supervision, and to collect a sample over a given time period, this last aspect can easily be facilitated by incorporating a timer into the circuit. Some traps have panels that can be removed while they are running and so the catch can be investigated, while others are more-or-less sealed until the light is turned off and the trap partly dismantled, and most traps have transparent panels so that light enters and encourages the occupants to shelter in the dark among the packing material – it is curious that most insects that are strongly attracted to light try very hard to avoid it after they arrive. In all their various forms these are very simple types of sampling traps that are ideal for general survey work but much more serious designs are available that can partition the sample into time slots through the night to monitor insect movements. The type of trap chosen will depend on many factors but the two most important practical ones are power source and carriage, whether the trap can be opened or otherwise monitored while in use is also important, but often the most important overall factor is the cost of the trap and its associated power source, and of course in the modern market they can be very expensive indeed. The market has recently become so overwhelmingly stuffed with new and ever more complex and expensive traps that we can give no advice because we have not used them or even seen them in use. Some are designed to fold down so that they can be transported much like a brief case, but most traps can be dismantled and assembled quickly and as one soon becomes very adept at this kind of thing it is as well to be aware that a certain amount of gimmickry might be entering the market. Our experience, since the 1970s, has been with Robinson, Heath and Skinner traps, the former best left alone while they work and the latter easily accessed and rummaged through while they are glowing, and all are excellent, but many modern designs also look very good and so opinions should be sought as they are not inexpensive. Probably the best way to find out about traps is to attend a few moth trapping meetings and offer to help set up the equipment. This is all we have to say about trap choice but most coleopterists will attend the light while it is running and in such cases there is no better way of monitoring the beetles arriving than to use white sheets, either laid on the ground or, better, suspended with the light close by, ideally vertical and horizontal sheets should be used together and good results can be had by running a powerful light inside a small white tent. Searching online will provide a huge range of traps, lights, accessories and, of course, prices.

Light trap.jpg

A brief discussion about power sources might also be useful as, other than in domestic settings, a mobile device will certainly be needed. Many fluorescent and LED lights will run for long periods on batteries, either dry cells or small lead-acid types more generally designed for motorbikes etc. These have the advantage of being small and light and so can be used next to the trap while it is running, indeed some traps incorporate batteries into the design. As ever it is very advisable to shop around, you will basically be paying for amp-hours but prices vary hugely so beware. The ideal power source is a petrol inverter generator; these are small, virtually silent and produce plenty of power, commonly between 1.5 and 2.5 KW which is ideal for discharge lamps and the miles of cable needed if several traps are run at the same time. They are also very expensive and if used regularly for long periods prone to breaking down. Much less expensive but much heavier and noisier are standard 4-Stroke single-cylinder generators, these may produce much more power but need to be used sympathetically. The market is daunting. On the other hand there are some very attractive lithium battery power suppliers that produce 240V and have impressive capacities, easily enough to run a discharge lamp or several fluorescent tubes etc for a few hours. In terms of expense these battery packs are on a par with the more expensive smaller generators but they require no petrol and there are no mechanics to go wrong, they are reasonably light and portable and most have very good electronics so that the charge and output can be managed, some even come with dedicated solar panel arrays, not much good for light trapping but maybe useful when camping etc. These devices are in their infancy and the market is already flooded so some serious research should be undertaken before buying.

Traps can be essential for research, they can be used to monitor sites remotely over long periods and the data extracted can be quantitative as well as qualitative and so all sorts of manipulations can be applied and results obtained. For amateur entomologists they usually provide qualitative data in the form of lists of species that change over time, they are also an ideal way to obtain specimens for collections and this is probably the main use to which they are put.

The message from all the above is to be very careful when buying this stuff, shop around and do plenty of research because the potential for getting ripped-off is very high. Occasional use of light traps by coleopterists may not justify buying a trap, especially as the light is likely to be monitored, and here the best way to sample is to use white sheets and monitor what’s going on.

Tim Hodge Hydrophilus.jpg

Hydrophilus piceus attracted to light in one night.

© Tim Hodge

Beetles at Light

Before any discussion it is worth quoting from the Coleopterists Handbook: ‘It is not worth investing in a light trap for collecting in Britain’, and further ‘Those recorded as coming to light in Britain are usually common species but some quite rare beetles have been captured regularly at light...’ These are surprising statements for a number of reasons. The majority of specimens found by almost any technique will always represent common species and so light traps should be no exception. Powerful light traps have been used for many decades but it may be that coleopterists have ignored them or considered them the tools of the lepidopterists, whatever the reason it is astounding that light traps have not gained a more general currency among beetlers. This can be said for two very good reasons, firstly because our own results from light-trapping have been so very good, and secondly the results regularly posted on Facebook have often been surprising and impressive. Running a 125W MV lamp in our Watford garden has provided a few species not otherwise recorded locally and even leaving a window open in the summer usually produces a good number of beetles. So running a decent light in any situation might be rewarding but, as lepidopterists know all too well, some situations are better than others. A valuable consideration, and one soon learned with experience, is that of contrast. Bright moonlit nights are not as productive as dark nights, a little moonlight can be compensated for by placing the light in a dark rather than an open situation e.g. in woodland or among crops near arable borders, but the idea should be to produce as much contrast as possible and this often involves choosing the right nights according to what the cloud cover and moon look like. There will be plenty of nights when not very much happens and this can be disparaging but there will also be plenty of nights providing large samples or small samples that include unusual specimens. Plenty of beetles are photophobic and so will never appear at light, in our experience this applies to many species commonly found at sap or under bark, but conversely there are many saproxylic species that will appear regularly at lights placed in woodland or near trees in gardens or parkland. Other good sites include calcareous grassland, dung pasture, coastal dunes, moorland and wetland margins, and lights placed within or on the margins of reed beds can be very productive indeed. Lights will attract insects in most situations but to obtain good results some careful consideration should be given to placing them e.g. a survey of the vegetation and the species and condition of any trees present should be made to optimize the chance of a varied sample. Mature and decaying trees can be excellent, especially in orchards and woodlands, mixed woodland might produce more species than deciduous or conifer woodland, and moorland or even open grassland can provide large and varied samples. The chances of a good sample might also be increased by placing lights near to flowering trees and shrubs e.g. near hawthorn in the spring or ivy in the autumn. Luck does play a part but the greater part of a successful night is down to experience, it can be annoying and baffling when lights placed only twenty metres apart produce very different results but when this sort of thing happens each light should be considered carefully e.g. what vegetation is actually illuminated by each light and is one light intercepting insects and so reducing the sample from adjacent lights? In general lights should be placed far enough apart so that their spheres of attraction do not overlap and they do not interfere with each other. Such things soon become intuitive.

Having placed a light in a good situation it remains only for the beetles to arrive and here coleopterists need to behave differently from other entomologists. Moths and a whole range of things like wasps and flies that flutter and buzz around lights and illuminated sheets are a constant nuisance when looking for beetles, especially when hornets turn up in numbers. These are readily spotted and collected or ignored as the case may be by other entomologists but beetles are different. There are exceptions such as various chafers but the majority of beetles coming to light will be small and they will become inactive soon after they settle; many will be content to sit on the sheet and remain inactive until the light is turned off and this is why it is best to erect a vertical sheet over a large sheet placed on the ground. Furthermore most of the specimens will arrive unnoticed and so spotting them becomes an active and constant process. The best way to do this is to use a large and powerful magnifying glass and work around each sheet in turn, it helps to use a pooter but most specimens are easily taken from sheets by using small tubes. In this way the small cryptophagids and latrids etc, will soon be found and many can be identified in situ with a more powerful X20 lens. Larger weevils and ground beetles also tend to remain still but these will be spotted with ease, occasionally larger beetles such as geotrupids, dytiscids and hydrophilids will hit the sheet and these may soon escape if a constant eye is not kept on proceedings. Species of ground beetles etc which do not fly may also be found on illuminated ground sheets and it is often productive to search among adjacent grass or substrate for nearby specimens that do not make it onto the sheet. It might be thought that using traps might negate the need to constantly search for specimens and there is an element of truth in this, but many smaller beetles will settle on the outside of the traps and leave as it begins to get light and so many of the more interesting specimens will be missed. Beyond these few snippets of advice it is obvious that most coleopterists are quite fanatical and obsessed with their hobby and so each will develop their own very good methods of sampling at light, the only advice we can give is that by looking at beetles included within traps after running through the night it is very likely that many interesting specimens will be missed because after being attracted to light they either failed to enter the trap or did so and then managed to crawl out again. The best times to trap, very obviously, are warm spring and summer evenings and nights, the lights should be running as it is getting dark, with little or no moon and good cloud-cover but lights can be used through the year on any but the coldest nights. A very large number of beetle species may be attracted to light and any list will be incomplete as more are recorded and so what follows is a more general list with a few examples of genera or species likely to be sampled, it is largely based on our own experience and so groups not included such as haliplids or gyrinids may well occur as well.

Among the larger and more spectacular beetles regularly recorded at light are Chafers (Melolontha and others), Dung beetles (Geotrupidae), Water Beetles (Dytiscus and Hydrophilus), carrion beetles (Silphidae) and the Stag Beetle (Lucanus cervus L.) and Lesser Stag Beetle (Dorcus parallelipipedus L.), and running lights in any situation will sooner or later produce at least some of these. More generally members of many families are likely to be regular visitors; these include Carabidae Latreille, 1802, Helophoridae Leach, 1815, Hydrophilidae Latreille, 1802, Staphylinidae Latreille, 1802.

Some species seem to be photophobic over the entire spectrum and so never come to light, others like Sphindus dubius (Gyll.) will flee and hide from torchlight but readily fly to UV, and many species are not regularly recorded from light but this may be because lights tend to be operated in ‘safe’ situations and so the same sorts of habitats e.g. gardens, woodland and grassland, tend to get well-worked while others e.g. marshland and carr, tend to be avoided, and there is always scope for running lights in novel situations e.g. we have often thought that running light-traps in tree canopies might produce unusual results but we have yet to try this. Running a light in an area of flooded woodland in our local park produced two carabids, Stenolophus skrimshiranus St. and Badister sodalis (Duft.) that we had not previously recorded locally, and all our records of the anthicid Stricticollis tobias Marseul, a species only rarely recorded, have been from light. Likewise the nocturnal oedemerid, Oedemera femoralis Ol. is frequent at light but otherwise only rarely seen, and the carabid Polistichus connexus (Geoff.), which has historically been considered very rare (and might actually have been so until recently, we do not know) has proved to be widespread and even locally common following a recent increase in popularity in light trapping and posting pictures of the results on facebook. Lots of things that do not fly, or do so only rarely, like various carabids and staphs, will walk some considerable distance to visit the right sort of light, these will remain cryptic unless a sheet is laid beneath a light suspended above the ground, and the mix of species that turn up will depend on the habitat e.g. Panagaeus bipustulatus (Fab.) or various Lebia might be expected from calcareous grassland. Lepidopterists have often been very accommodating in taking beetles and passing them on to coleopterists and this has no doubt produced some valuable records, but these specimens, understandably, tend to be larger things that are both visible and obvious in a trap, useful no doubt but they could hardly be expected to find and collect the large range of small stuff that is really interesting like Cercyon, Atomaria, Cryptophagus, Stilbus, Megasternum and various latrids, the list would be a long one but the point is that to coleopterists such microscopic stuff is by far the most valuable, at least after the larger stuff becomes banal. The point being that sampling beetles at light requires imagination and constant work, it is not like running traps for moths where the equipment can be set up and left to run unattended. As with sampling moths, a light run in the same place throughout the year will produce a succession of species because many are have a fairly-well defined season of activity and here the cockchafer and stag beetle are good examples, but beetles (and this is a very broad statement) are on the whole less seasonal than many other insects and so in a sense it is wasteful to constantly run a light in the same place, and sampling a particular site once a week or once a month is much more sensible for the coleopterist as it frees-up time to sample a wider diversity of habitats. Running the right kind of lights in a range of habitats will provide a constant supply of new species and it is precisely because new species are constantly recorded at light that the activity is so entertaining for the coleopterist, it also has the great benefit of being non-intrusive or destructive as it requires no sweeping or bark-stripping and no samples need be taken for extraction, in this sense it is also a rather lazy way of getting good results as things will fly down from tree canopies or up from reed-litter while the observer carries on searching with the magnifying-glass and tubes. What could be more satisfying? It should now be obvious that providing a list of species attracted to light is pointless, but for the sake of providing some guidance for the beginner at least some of the following beetles should be expected from a wide range of situations.

Polistichus connexus.jpg
Nicrophorus vespilloides 1.jpg
Typhaeus typhoeus 2.jpg
Melolontha melolontha 4.jpg
Stenagostus rhombeus 1.jpg
Tenebrio molitor 4.jpg
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