Recent Work on Mercury as an Insecticide against Insect Pests of Stored Grain

Abstract

Mercury is not a new addition to the list of preservatives for stored grain. It has insecticidal properties that are well known to the peasants who insert a drop of mercury in an excavated soap-nut {Sapindus detergens) or cowdung ball which is then placed in the stored grain to protect it against the ravages of insect pests. This method of application was considered dangerous, in the case of grain meant for consumption, because the cowdung balls are porous and with a slight jerk mercury droplets get mixed with the grain. A little mishandling in the case of soap-nuts also causes the mercury to escape into the grain heap, thus affecting the health of the consumer adversely. These facts were first recorded by Kunhi Kannan (1920) who initiated the study of the insecticidal properties of mercury on scientific lines by experimenting with Bruchid beetles and silkworms and found that the eggs of these insects, when exposed to mercury vapour, did not hatch but turned black. Larson in the United States (1922) tested mercury vapour against Bruchus quadrimaculatus, F., a pest of cowpeas, and confirmed these observations. Dutt and Puri (1929) carried out investigations at Pusa and recommended the use of mercury in the form of mercury-tin amalgam because it was easy to handle and its vapour pressure being low did not render the grain unfit for seed purposes or human consumption (60 maunds* of grains treated in this manner were used by a family without any untoward effect). The amalgam was prepared by rubbing together 2 parts of tin and 3 parts of mercury into a homogenous mass of paste-like consistency. This mass was then squeezed through a piece of linen, thus removing the free droplets of mercury and eliminating the possibility of mercury becoming mixed with the grain. The amalgam was flattened to expose a greater effective surface and half an ounce was sufficient to keep 14 seers* of wheat free from the attack of Sitophilus oryzae, L. Rahman (1942) recommended mercury at the rate of 3-4 tolas* per maund of wheat. He suggested that mercury carried in twill or long-cloth bags measuring 2x2 ins. at the rate of 1 tola of mercury per bag, should be distributed evenly in layers throughout the bin, with a depth of 8 to 12 ins. of wheat between two successive layers. These bags come out easily when the grain is removed through the exit hole and he was of the opinion that this type of application had no deleterious effect on the germination of the seed or its eating quality. The author on the other hand feels that bags are not safe to use because a little mishandling will cause the mercury to escape and render the grain unsafe for consumption.The Government Entomologist, Punjab, in his Annual Report for the year 1938-39 stated that the use of mercury in bins to control Khapra, Trogoderma granarium, Everts, completely checked the attack when it was uniformly distributed in the grain, but, if only the top and bottom portions were treated, the attack continued although at a slower rate than in the control bins. In view of these differe...