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Influence of Humidity on the Volatilization of Insecticide Residues as Affected by Formulation and Substrate

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Adult Tribolium beetles were suspended on nylon organdy cloth 6 mm above the residues of certain volatile insecticides at a temperature of 32.2°C (90°F) and at humidities ranging from lows of <2 or 20% relative humidity (RH) and highs of 80 or >98%. Based on bioassay, residues of 2 organophosphorous insecticides (diazinon and naled) deposited by dilute emulsions, volatilized more rapidly at the high humidities when resting on filter paper, but volatilized more rapidly at the low humidities when resting on lipophilic surfaces. Lindane was not affected in this way by differences in substrate and humidity. The volatilization of diazinon residues deposited from solutions of toxicant in acetone or xylene was not greatly affected by differences in substrate and humidity unless surfactants were added.

Volatilization of insecticides from nonsorptive diluents was much greater than from sorptive diluents at 20% RH but at 80% RH differences tended to be less and, in fact, with 0.5% aldrin and 1% naled emulsions there was significantly greater volatilization from the sorptive diluents.

When naled was formulated in a porous and highly sorptive montmorillonite clay (Olancha Clay), its vapors resulted in an 84% mortality of Tribolium beetles at 80% RH and no mortality at 20% RH. On the other hand, when the toxicant was formulated in a nonporous and relatively nonsorptive pyrophyllite clay its vapors resulted in a mortality of 71% at both 80% and 20% RH. Since the 2 clays are chemically similar, it appears that the factor causing the difference in the relative volatility of the insecticides at the 2 humidities was the great difference in the porosity (specific surface) and/or sorptiveness of the two clays.

The displacement of insecticide molecules from a substrate, by water, has been offered by some investigators as an explanation for increased volatility at high humidity. The present investigation shows that this could take place only on a hydrophilic substrate. With some insecticide formulations an increase in humidity can increase the rate of volatilization of toxicant even when the substrate has been eliminated as an influencing factor. In such cases some other explanation must be found for increased volatility at high humidities.

Document Type: Research Article

Publication date: April 1, 1965

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  • Journal of Economic Entomology is published bimonthly in February, April, June, August, October, and December. The journal publishes articles on the economic significance of insects and is divided into the following sections: apiculture & social insects; arthropods in relation to plant disease; forum; insecticide resistance and resistance management; ecotoxicology; biological and microbial control; ecology and behavior; sampling and biostatistics; household and structural insects; medical entomology; molecular entomology; veterinary entomology; forest entomology; horticultural entomology; field and forage crops, and small grains; stored-product; commodity treatment and quarantine entomology; and plant resistance. In addition to research papers, Journal of Economic Entomology publishes Letters to the Editor, interpretive articles in a Forum section, Short Communications, Rapid Communications, and Book Reviews.
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