The influence of humidity on the residual action of benzene hexachloride (BHC)

Abstract

In controlled hum idity chambers in the laboratory differences in the absorption velocity o f B H C were observed depending on the substrate sprayed. While B H C is no longer used in C hagas' disease control this data could have relevance to spraying houses in a controlprogram m e with other insecticides. Key-words: Benzene Hexachloride (BHC). R esidual activity on different natural substrates.The application of BHC on the wall surfaces inside of houses was the primary method to control triatomine bugs which are vectors of Chagas' disease in Brazil. Residual activity of the insecticide BHC was studied on building materials taken from Mambai, Goias-Brazil4. Results of experiments in the field and laboratory suggested that higher mortalities coincide with higher ambient relative humidities. Data presen ted! and Nocerino^ demonstrated that old insecticide deposits of BHC had greater killing power at higher atmospheric humidities. The objective of this study is to demonstrate the residual insecticidal activity of BHC on different building materials in controlled hu midity chambers in the laboratory. MATERIAL AND METHODSTwo experiments were used to show the effect of humidity on the insecticidal activity of BHC, 30% gamma isomer, on laboratory reared, first instar nymphs of Dipetalogaster maximus. Nymphs were exposed to the vapor of BHC which were being released from the treated surface (0.5mg of gamma BHC/m^) of building materials taken from Mambai, Goias.In the first experiment, six humidity chambers were prepared by using covered 30cm glass dessicators with relative humidities (RH) regulated in each chamber by concentrations of glyceral solutions in the bottom. The humidities were measured by a miniature thermocouple psychrometer. The test material was fired roof tile sprayed with BHC 20 months earlier. Treated tiles were placed on porcelain trays in faour chamber, three of which were calibrated at 50%, 60% and 80%, RH respectively, and one chamber was left open to ambient conditions which ranged from 64% to 84% RH. Four control chamber without treated tiles were set up in the same way. Mean temperature during the experiment was 22°C (range 21.5°C to 26°C).In the second experiment, six covered dessicator chambers were prepared for testing five different house construction materials which had been treated with BHC 24 months earlier. Fired roof tile, unfired mud brick, plaster with lime, plaster without lime and wood were placed on the porcelain trays in five cham bers, only one substrate for each chamber. The sixth chamber used as the control had no substrate. All chambers were regulated to 90% RH.The LT50 activity of the insecticide was mea sured by observing and recording the mortality of the nymphs in terms of lethal time in days at which 50% of any given population of bugs had died. Each chamber had three replicates. A replicate consisted of 10 unfed nymphs placed in a flat round tin (5.4 cm in diameter and 8mm deep) covered with fine nylon mesh netting. Three tins with bugs were spaced 5 mm from the netting to...