Degradation of bifenthrin, chlorpyrifos and imidacloprid in soil and bedding materials at termiticidal application rates

Organophosphorus, pyrethroid and chloronicotinyl insecticides have been use d to control termites in building structures in recent years. We investigat ed the degradation behaviour of three insecticides (bifenthrin, chlorpyrifo s and imidacloprid) at termiticidal application rates under standard labora tory conditions (25 degrees C, 60% field moisture capacity and darkness) fo r 24 months. The study was carried out on one soil and two bedding material s (sand-dolomite and quarry sand), which are commonly used under housing in Australia. Experiments were also conducted to examine the effect of soil m oisture on the degradation of these insecticides. Insecticide residues in t he samples collected at different days after application were measured by h igh performance liquid chromatography (HPLC). The rate of degradation of bi fenthrin and imidacloprid insecticides was adequately described by a first- order kinetic model (r(2) = 0.93-0.97). However, chlorpyrifos degradation w as biphasic, showing an initial faster degradation followed by a slower rat e. Therefore, the degradation data during the slower phase only (after a tw o-month period) followed the first-order law (r(2) = 0.95). Soil moisture h ad little effect on degradation of imidacloprid and bifenthrin. Among the t hree insecticides, bifenthrin and imidacloprid were most stable and chlorpy rifos the least. Chlorpyrifos showed a major loss (75-90%) of residue durin g the 24 months incubation period. In the bedding materials, simultaneous a ccumulation of the primary metabolite of chlorpyrifos, TCP (3,5,6-trichloro -2-pyridinol) was observed. Hydrolysis appeared to have caused the observed rapid loss of chlorpyrifos, especially in the highly alkaline bedding mate rials (sand-dolomite and quarry sand). (C) 1999 Society of Chemical Industr y.