Microclimate Effects on Toxaphene and DDT Volatilization from Cotton Plants1

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Abstract
Although the majority of chemicals used for insect control have low vapor pressures, the ability of these chemicals to vaporize is important in relation to possible efficacy reduction and their potential hazard as contaminants of nontarget areas. Micrometeorological methods were used to determine volatile losses of toxaphene (chlorinated Camphene) and DDT [1,1,1‐trichloro‐2,2‐bis(p‐chlorophenyl)ethane] from cotton (Gossypium hirsutum L.) plants under field conditions. Volatilization of the two insecticides from foliage was correlated best with air temperature and windspeed. Diurnal curves were developed for insecticide flux density, air temperature, windspeed, and net radiation. Peak air temperatures and windspeed coincided with peak flux densities in midafternoon whereas peak net radiation occurred 1 to 2 hours earlier. About 60 to 70 % of the mean daily flux occurred between 0600 and 1800 hours. Insecticide flux densities and the insecticide loads on the cotton plants decreased exponentially with time. The daily flux densities were about 3 to 5 % of the insecticide loads on the plants. The 50 %‐disappearance times (DT50) for toxaphene during mid‐growing season was 4.7 and 10.8 days later in the season. The DT50 for DDT was 10.3 days. The increase in surface soil pesticide loading during the study probably resulted from plant wash‐off, vapor‐phase movement downward from the plant foliage, and possibly mass movement in the evaporation stream from lower soil depths.