It is a well-known fact that moths are attracted to light - be it a small flame or a bright bulb - and we can employ this attraction to great effect. Almost any source of light will do but there are two considerations.
Firstly, the ultra-violet (UV) content of the light source is important as moths are particularly sensitive to this region of the spectrum. Secondly, the attraction to the light is stronger if it is the sole source of illumination i.e. there is a good contrast between the light source and its background. For example, a solitary white street lamp (the orange ones are no good) in a dark country lane will be more attractive than one in a well-lit urban area.
However, it is worth noting that not all species are equally attracted to light and, with some, there is even a difference in attraction between the sexes too. It is important to realise, therefore, that a moth trap will not necessarily give a complete list of the species inhabiting a given area so one needs to employ a range of survey techniques. That said, let us briefly consider some of the ways in which we can use light to attract moths.
The simplest source of light is an illuminated window. Moths will often sit on the window or on the adjacent wall. Alternatively, if you have a security light that stays on then it is worth checking the surrounding area at night and first thing in the morning. Campsite toilet blocks with their outside lights and white-washed walls are renowned for attracting moths!
Before the advent of modern moth traps, the traditional way of attracting moths, especially in the field, was to use a spirit lamp such as a Tilley Lamp. Today, a butane gas lamp would do just as well. This is still a good method and to make it more effective, the lamp can be suspended in front of a vertical white sheet. The moths will then rest on the sheet where they can be examined.
By far the most effective method is to use a moth trap. The basic principal is to mount a light source within a funnel situated on top of a closed box lined with egg cartons. The moths are attracted to the light and fall through the funnel into the box. Here they hide amongst the egg cartons quite happily until the morning when the catch is examined and the moths released - well away from any waiting birds!
| The best light source is a mercury vapour (MV) lamp - as used in the white street lights mentioned earlier. The problem with these is that they require a 240 volt supply and the bulbs become very hot and need to be protected from the rain. However, these lamps can, in some circumstances, attract vast numbers of moths in a night. They also emit UV so it is important to take sensible precautions, and not to look at the bulb directly. The Skinner trap, illustrated opposite, is a common moth trap of this type. It can be used in the field but requires a portable generator or access to a mains supply. |  |
| An alternative is to use the blue fluorescent lamps that are incorporated into the insect killers that you see in butcher's shops and bakeries. It's not the bulb that kills the insects but a wire grill in front of it. These tubes, known as Actinic tubes, emit quite a lot of UV but they are not as bright as the MV lamps. The advantage of them is that they will run off a normal 12 volt car or motorbike battery as well as off the mains. This means that they are portable! They also will not disturb the neighbours! On any one night they will not attract the numbers of moths that a MV lamp would but, over a period, they will eventually attract the same species. Such a trap - known as a Heath trap - is illustrated opposite. |  |
Note, however, that not all the moths attracted to the light will end up in the trap. It is common for moths to dive for the shadows or to land on the ground around the trap rather than fly straight to the bulb. To overcome this, the trap can be placed on a white sheet and the moths examined as they land. From personal experience I have also found that the Skinner-type trap attracts fewer geometer or "carpet-type" moths than the Heath trap.
Be warned; moth trapping can become addictive! There is always a sense of excitement when emptying the trap for you never know what is going to be inside. Most of the time the moths are fairly predictable but now and again there is always a surprise, especially when trapping in a new area. Moth trapping provides hours of harmless fun while at the same time providing valuable data.
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