Moth Catching Techniques

Building a Moth-trap

Due to their largely nocturnal habits, we seldom come into contact with moths, except perhaps at the kitchen window or whilst gardening. The best way of learning more about these fascinating insects is through the use of a moth-trap. Such traps use a 125W MBF mercury vapour (MV) light bulb to attract insects from the immediate area. MV bulbs are particularly attractive to moths due to the ultraviolet light in their spectrum. The moths fall into the trap where they remain unharmed until the next morning.

There are different types of trap, but the one that is best suited to DIY construction is the ‘Skinner’ trap, which is named after its designer. This type of trap is easy to carry about, and both easy and cheap to make.

Above is an outline plan of a trap showing the standard measurements. As can be seen, it is basically a 45 cm square box with a bottom, which is 30cm high and made from 4mm plywood. The ‘lid’ is formed from two sloping sheets of 2mm Perspex with a gap between them at the bottom. The function of these Perspex sheets is to arrest the flight of the moth, funnelling it down into the trap.

Referring to the plan, two measurements are particularly important. The gap between the bottom of the two sheets of Perspex (marked A) must be 25mm to allow the moths to enter, but not too many to escape. The height of the top of the mounting blocks for the Perspex at A should be 15cm from the bottom of the trap. Small pieces of wood (2.5cm square) should be fixed to the centre of the top of the blocks (A) to keep the Perspex sheets 2.5cm apart. The Perspex must be cut (scored with a glass cutter or Stanley knife and then snapped off) to fit the particular trap. Lengthways the sheets must fit snugly between the sides of the trap (approximately 44cm) and the width of the sheets must equal the distance from the mounting block to the top edge of the trap, plus 3cm overhang (about 28cm).

There are 2cm square wooden lengths inside the trap running up each corner and along the bottom of the sides to hold the trap together. These are nailed (and glued) to the sides and bottom with tacks. Note that the vertical bits do not come quite to the top of the corners to allow for the slope of the Perspex. The plywood bottom of the trap holds the whole thing rigid and 6mm holes should be drilled in each corner to allow rainwater to escape.

The lamp mounting board (B) can be made from an odd bit of ply or other wood (7cm wide) and should be removable to facilitate emptying the trap.

An MV lamp must be operated with a ballast, which moderates the power when it is first switched on, allowing the bulb to warm up gradually; the ballast must be of the same wattage rating as the bulb. The bulb holder has to be earthenware because of the heat (plastic holders melt). Wiring the system is fairly simple, the ballast interrupting the positive wire between the mains and the bulb. Normally it is at the mains end of the circuit to save carrying it about; if you are not very experienced at wiring electrical appliances it is advisable to seek the help of a professional electrician. It is advisable to cover the bulb in the field with a glass jar as a drop of rain falling on the naked hot bulb can crack it. A replacement Pyrex coffee pot of an appropriate size is very suitable and easily obtainable from kitchen shops. Should the outer shell of the bulb crack, do not use it as this could result in damage to the eyes. In any case it is not advisable to look at the bulb for long periods due to its extreme brightness.

The electrics, ballast, bulb and earthenware bulb holder should not cost more than £40 from an electrical wholesaler; a retailer will charge a lot more.

Finally, one should obtain 3 or 4 square egg trays, usually available free from a corner shop, for the moths to sit on. These are normally about 30cm square and should be cut in half. They stand up, resting on the sides of the trap under the Perspex.

A Skinner Moth Trap (Photo: G.K. Smith) A Typical Catch (Photo: G.K. Smith)

By attracting moths to a trap you are interfering with their natural life cycle and you are then responsible for their safety. After examination of the catch, the moths should be scattered in long vegetation, hidden from the eyes of hungry birds. Do not put them all in one place, as if a predator finds one it will find them all. Some people prefer to keep the moths safe in a cool place during the day, releasing them at dusk. Bird predation can be a problem, particularly if the moth-trap is operated regularly in the same place. It is probably fairer on the moths if you do not trap every night; alternatively, if your garden is big enough, vary the trapping location.

As far as siting the trap goes, the ideal location is somewhere sheltered but not too enclosed. Some thought should also be given to the position of the sun in the morning as the heat of direct sunlight increases the moths’ activity and can also result in death through overheating.

Sugaring

Another way of attracting moths to your garden is by the use of sugar solution. This can be made by mixing stout, molasses and brown sugar in a pan, simmering for around half an hour and adding a dash of rum at the end of this time. The quantities will vary depending on how much solution you wish to make; in practice the exact proportions probably do not matter a huge amount so long as the liquid is not too runny. This sugar solution can be painted onto walls, trees, gateposts etc at dusk (note that it does stain so be careful where you put it!) and will hopefully attract moths to feed upon it, the sweet fragrance of the mixture acting as an attractant. Sugaring (the term used to describe this method of attracting moths) does vary in its effectiveness and will often result in negative results, so persistence is recommended. Weather conditions can have an influence upon the effectiveness of this method, the best nights being warm and humid with cloud cover and little or no wind. Late summer and autumn are often more productive times of year for sugaring, and some species are more easily attracted to sugar than to light. When it works, sugaring can produce some impressive results with moths jostling for space on the sugar patch. Even if it doesn't work, don't worry as the next day the remnants will prove attractive to butterflies and other insects.

Moth-attractant plants

The best plants for attracting moths to your garden are those that smell strongly at night. Sallow and ivy blossom, Red Campion, ice-plant Sedum, night-scented stock and honeysuckle Lonicera are favourites. One of the best species is tobacco plant nicotiana, which is particularly attractive to the Convolvulus Hawk-moth Agrius convolvuli, a rare migrant from southern Europe/North Africa that can turn up inland as well as on the coast. Other good attractants include Hebe, Red Valerian and Buddleia, which are also very attractive to butterflies.

Moth recording is a fascinating and addictive hobby. The bright colours and intricate patterns of some species come as a great surprise to many.