The plastics and synthetic fibre materials used in insect traps may acquire and hold an electrical charge that could affect the efficiency of the traps. This became apparent while the Belleville rotary trap (Nicholls 1960) was being used to determine the influence of weather on insect numbers.

Over a period of a few weeks my colleague, Dr. J. A. Juillet, found that one net of the trap consistently caught more parasitic Hymenoptera (sometimes twice the number) than the other. Tests with an electrometer showed that the net of the trap that caught the lesser number of insects carried a negative electrical charge that created a potential difference with respect to ground of 40 volts at one centimetre from the net. Further observation revealed chat the charged net was set slightly higher than the other and that it struck a leaf of a nearby sapling as it rotated. The repeated brief contact was apparently sufficient to cause the net to be electrified. When the leaf was removed the charge on the net gradually dissipated and the numbers of insects caught in the two nets of the trap tended to be equal.