The Effect of Controlled Temperature and Humidity on the Residual Life of Certain Insecticides1

Abstract

The residual life of commercial emulsifiable formulations of heptachlor, aldrin, Diazinon® (O, O-diethyl O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate), parathion, malathion, and an acetone solution of heptachlor epoxied was determined under controlled conditions of temperature and humidity, using three- to four-week-old adults of the red flour beetle, Tribolium castaneum (Herbst). At 90° F and a R.H. (relative humidity) of 55%±5%, the residual life in days of aldrin, heptachlor, heptachlor epoxide, and Diazinon was 8, 10, 28, and 99, respectively, whereas at the same R.H. but at 110° F, the residual life was 4, 4, 21, and 28 days, respectively. Parathion lost its effectiveness in 41 days at 90° F and 55%±5% R.H. but still gave 44% mortality on the 18th day at 110. and 55%±5% RH. At 90° F and 55%±5% R.H. malathion gave 78% mortality on the 21st day, whereas, at the same R.H. but at 110° F, it gave 32% mortality on the 28th day. Relative humidity during the time of exposure of the test insects markedly affected mortality. When assayed at a R.H. of 5.5%±5%, Diazinon showed a complete loss of toxicity on the 28th day when held at 110° F and 5,5%±5% R.H., but it gave 100% mortality when held at similar temperature conditions but assayed at 90%± 5% R.H. Likewise, parathion, aldrin, heptachlor, heptachlor epoxide, and malathion also gave higher mortality of the insects when assayed at 90%± 5% R.H. Investigation of the possible causes of increased toxicity at higher R.H. revealed that all these insecticides possessed greater fumigant activity at 90%±5% RH. Diazinon, malathion, and parathion showed no detectable fumigation effect at 55%±5% R.H., but produced mortalities of 100%, 68%, and 55%, respectively, at 90%±5% R.H. with similar temperature conditions prevailing. Heptachlor, heptachlor epoxied, and aldrin residues produced mortalities of 76%, 73%, and 72%, respectively, when assayed at 55%±5% R.H., but when assayed at 90%±5% R.H. the mortalities were 96%, 95%, and 93%, respectively. It may be postulated that the higher mortality obtained at the higher R.H. is caused largely by increased volatility as shown by the increased fumigant action. This view is also supported by a comparison of the loss in weight of heptachlor epoxide residues held at 90%±5% R.H. and at 55%±5% R.H., under similar temperature conditions. It is concluded that atmospheric humidity greatly affects toxicity of chemicals and for the most efficient use of insecticides they should be applied when the R.H. is high.