By Gerry Haggett

Modern light trapping of moths arose from studies made by H.S. and P.J.M. Robinson, and their discoveries, together with an originally designed trap, which were published in the Entomologist's Gazette 1950, 1:3-20 and 1: 104-107. This was updated and some further work added in a paper published in Proceedings of the Royal Entomological Society 1952, 27:13-27. Both papers were supported by clear and detailed text figures.

The Robinson brother's explanation of moths attending light, of accounts of electric forms of illumination, designs of traps and effects upon different groups of moths and between sexes has remained unchallenged to the present day. Ford repeated their findings in his book Moths 1955, New Naturalist, 13-17.

As some of these references may not easily be available today I list below the most significant principles of those early papers.

1. The most efficient source of light and one that has the greatest number of species in attendance is a lamp that has the smallest light-source but the highest surface brilliance together with the maximum output in the spectrum seen by the insect eye. The most readily available lamp is the mercury-vapour discharge.

2. Moths do not see a light and head towards it, or are at all attracted to it from any distance. Instead they simply find themselves flying into a source and will attempt, and often succeed, in avoiding it, but once near the source they become dazzled or unbalanced and they remain close to it.

3. The hemispherical volume of light that emits from one ground source has at its outer radius an annulus of repulsion. This is a band into which a moth first flies and from which it will attempt to turn away and so from the light source. Different types of lamp produce bands of varying width and again the mercury-vapour lamp has the advantage of the narrowest band. Stronger flying and heavier bodied moths will be more likely to break through this band than weaker and lighter ones.

4. The range of the commonly used MV lamps is no more than 100 yards. Lamps should not be used closer than this because they would then produce the "street lighting effect" whereby the repulsion bands link and fields of dazzle intersect, so their combined effect is of one gigantic source into which few moths will penetrate.

5. Any part of light that is in shadow offers potential escape for moths wheeling from the dazzle source, and the larger the area of shadow the greater the number of escapes. The most efficient source on flat open ground is at or near ground level and placed on a white sheet.

6. But traps are most usually operated in situations such as gardens where adjoining buildings, shrubs and trees may be fully illuminated. Moths freely settle on to them, and crawl into hiding or sit undetected. They may be taken by early birds. The Robinson's found that a trap with a disc below its lamp would catch more moths than one without and they concluded that moths that had overshot the illumination into this limited area of shadow lost their dazzled condition and took flight back into the area of dazzle and eventually into the trap.

The Robinson trap incorporates these principles in as small a device as is reasonably portable, and it provides for drainage of rainwater. As hot MV lamps become fragile and liable to destruction in pelting rain, it is common practice to place an inverted Pyrex basin over the lamp, resting on the vanes.

For a more static trap it is possible to use a larger container, that is big enough to provide for an area of shadow where moths pass in and out until they hit the dazzle source and so access the trap. But surrounding obstacles remain out of the illumination by discreet siting of the lamp that is neither too highly set as to provide for no shadow, nor too lowly placed and so achieve too much.

I have achieved some sort of compromise by employing a galvanised dustbin fitted with a shallow metal cone beneath which a mercury-vapour lamp is slung so that it stands upright. This throws a shaft of light upwards but with a zone of shadow into which moths pass and re-pass until they gain access through the mouth of the cone. To give a snug fit for the cone to sit within the dustbin, some form of felt can be glued to the dustbin lip. The insulation where electric supply enters the mounted lamp socket must - as in all outdoor electrical appliances - be absolutely sound and waterproof.

One disadvantage of this trap is that moths within the egg-trays of the metal bin remain in darkness and will be attempting to escape should the lamp be switched off before dawn. So either the light must remain on or the catch must be examined very early at daybreak. Another problem is that it can be so successful that a large catch is ruined by being constantly disturbed not only by its own members but also especially by wasps, burying beetles and water beetles, but I have not had a bird inside as not infrequently happens with the Robinson trap!

Gerry Haggett.