moth trap

For years, entomologists have used mercury vapour bulbs to attract moths, as MV bulbs emit a high proportion of their light as ultra violet making them especially attractive to moths. An MV light is simply, the best way to attract numbers of moths within a relatively short period of time and is suitable for conducting random sample surveys, to determine what species are present in a given site/area or to aid the assessment of resident populations with regular trapping. A single night's trapping in woodland habitat during the Summer months, can result in well over 1000 moths being caught on favourable nights, but moth traps run from urban and suburban gardens will also yield excellent results.
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Although traps are available to buy from entomological suppliers, building a trap yourself is far cheaper. Constructing a trap is a simple enough procedure and the equipment is usually readily available from most electrical wholesalers and large DIY stores, though selected parts (the choke and bulb) may have to be ordered.
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To construct a trap similar to the design we use, the following items are required. One 125W Mercury Vapour light bulb, a Choke (we use a Parmar Q 125-24.2-4) one porcelain lamp holder, a suitable length of three-core electrical cable and one or two sheets of exterior plywood measuring 606mm x 1220mm x 12mm. It is best to always carry at least one spare bulb, as these inevitably pack up when you switch the trap on after a 20 mile trip on a particularly favourable night. Our original trap shown on the right, was a basic box measuring 45cm square and 35cm high. 12mm thick plywood was used for strength and durability. The two top panels were fitted and angled inwards, leaving a gap of around an inch for the moths to enter the trap. These were hinged for ease of opening at the end of the trapping session. Similar commercially built traps are usually fitted with perspex panels, whilst we used plywood. A wooden spar was then fixed across the top of the box and the porcelain lamp holder screwed to it's centre and the bulb fitted.
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		Electrics and wiring The choke regulates the amount of power going to the bulb as it fires up, allowing the bulb to warm-up gradually. It should be wired as shown in the photograph on the left. The wiring is fairly simple. Standard three-core cable should be stripped back and the wires cut to appropriate lengths for connection as instructed below. The choke and all the electrics, were encased on the side of the original trap design, to avoid any contact with the ground and dampness. Live (Brown) is connected directly to the choke. Neutral (Blue) should be connected to one side of the porcelain bulb holder. An additional length of the same wire is needed to run back to the choke from the bulb holder. Earth (Yellow/green) is connected directly to the choke via one of the fixing screws, on the side of the box.
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Care should be taken to protect all electrics from water. In many cases where people trap moths in their own gardens, traps are left on all night and checked in the morning. When trapping moths at sites away from home, sessions are often much shorter, but dew can start to form at dusk and cause a problem. Mercury vapour bulbs run at very hot temperatures and rain can be a hazard and an unprotected bulb can explode if it is particularly cold or heavy. As such, it is wise to cover the bulb with a heat-resistant glass cover, such as the glass from a cafetiere. Portable power supplies In order to provide electricity away from home, a portable generator is needed. Top quality generators can be up to £500-£800 and are an unrealistic proposition to many. There are cheaper generators on the market and although these are berated by many in favour of the more expensive models, are much better value for money in my mind. In 2003, the first generator I purchased was a Powercraft 720 portable generator from Aldi. It cost much less than £50, but performed excellently for a number of years until 2008, when it eventually failed. A slightly more powerful Omega Wolf generator was then purchased for £69, but the company went into administration in 2010.
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This original trap design was later modified into a simple, smaller square box with the light sat on top and the choke and all electrics housed within the box. This made the whole trap and generator easier to get into the boot of the car and if trapping in the field and attending the trap for a few hours at a time, then this design certainly works best. We have never run a trap all night yet and when trapping away from home, find it is not really practical. Our trapping sessions can last from an hour to four or five hours, depending on the weather or work commitments etc.

Moths will alight on the box itself, or on the sheet and can be potted for later identification as and when required. Many moths will stay at the light for a while, but often become active and fly off later, so the potting up of any specimens is best done immediately after they arrive at the trap. An added advantage of a simple box trap is that Hornets and other predators cannot get into the trap and eat most of your catch. Two or three Hornets are attracted most sessions during the late Summer and Autumn and can be safely potted and released after the trapping session, or just left to settle down on their own.

Right: The modified light trap in operation at Sherwood Forest NNR during June 2009.