Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

A field microcosm study was conducted to determine persistence of tebufenozide, an insect growth regulator, in sandy litter and soil. Litter and soil plots (c. 4.5 m2 each) were sprayed with an aqueous suspension concentrate formulation of tebufenozide at rates of 35, 70 and 140 g Al ha-1. Samples were collected at intervals up to 408 days after spraying and analyzed for tebufenozide residues. The data were subjected to regression analysis and half-life (DT50, the time required for 50% of the initial residues to disappear) values were computed. The DT50 was c. 62 days for both substrates treated with the two lower dosage rates. At the highest dosage rate, the DT50 was 115 days for the litter and c. 52 days for the soil, indicating irregular variations in persistence. Downward movement in soil occurred only in trace amounts, suggesting strong adsorption. Laboratory microcosm studies were conducted to investigate the relative importance of rainfall, exposure to light and volatilization on persistence. Vertical movement occurred in litter and soil (both sandy and clay types) during rainfall. The amount moved increased with the amount of rainfall, but decreased with the rain-free period. The larger the rain droplets, the greater the downward movement. When the rainwater could move laterally along the surface of the substrate (as would occur on a slope), more lateral movement than vertical movement of tebufenozide occurred. The photolysis study indicated that disappearance of tebufenozide was directly related to the duration of exposure to radiation and radiation intensity. Volatilization of tebufenozide depended upon the ambient temperature and the duration of air passing through the substrates. Nonetheless, the amount lost by volatilization was much lower than the amount lost after rainfall or exposure to radiation, thus indicating the greater influence of rainfall and sunlight on persistence. In the laboratory microcosm studies, more tebufenozide was lost from the sandy substrates than from the clay substrates. This behaviour was attributed to the greater adsorptive capacity of the clay substrates. thus providing a greater protection against downward mobility and loss due to radiation.