Chlorpyrifos degradation in soil at termiticidal application rates

Abstract

Chlorpyrifos [O,O‐diethyl O‐(3,5,6‐trichloro‐2‐pyridyl) phosphorothioate] is an organophosphorus insecticide applied to soil to control pests both in agricultural and in urban developments. Typical agricultural soil applications (0.56 to 5.6 kg ha−1) result in initial soil surface residues of 0.3 to 32 μg g−1. In contrast, termiticidal soil barrier treatments, a common urban use pattern, often result in initial soil residues of 1000 μg g−1 or greater. The purpose of the present investigation was to understand better the degradation of chlorpyrifos in soil at termiticidal application rates and factors affecting its behaviour. Therefore, studies with [14C]chlorpyrifos were conducted under a variety of conditions in the laboratory. Initially, the degradation of chlorpyrifos at 1000 μg g−1 initial concentration was examined in five different soils from termite‐infested regions (Arizona, Florida, Hawaii, Texas) under standard conditions (25°C, field moisture capacity, darkness). Degradation half‐lives in these soils ranged from 175 to 1576 days. The major metabolite formed in chlorpyrifos‐treated soils was 3,5,6‐trichloro‐2‐pyrid‐inol, which represented up to 61% of applied radiocarbon after 13 months of incubation. Minor quantities of [14C]carbon dioxide (< 5%) and soil‐bound residues (⩽ 12%) were also present at that time. Subsequently, a factorial experiment examining chlorpyrifos degradation as affected by initial concentration (10, 100, 1000 μg g−1), soil moisture (field moisture capacity, 1.5 MPa, air dry), and temperature 15, 25, 35°C) was conducted in the two soils which had displayed the most (Texas) and least (Florida) rapid rates of degradation. Chlorpyrifos degradation was significantly retarded at the 1000 μg g−1 rate as compared to the 10 μg g−1 rate. Temperature also had a dramatic effect on degradation rate, which approximately doubled with each 10°C increase in temperature. Results suggest that the extended (3–24 + years) termiticidal efficacy of chlorpyrifos observed in the field may be due both to the high initial concentrations employed (termite LC 50 = 0.2– 2 μg g−1) and the extended persistence which results from employment of these rates. The study also highlights the importance of investigating the behaviour of a pesticide under the diversity of agricultural and urban use scenarios in which it is employed.

Determination of selected pesticides in water samples adjacent to agricultural fields and removal of organophosphorus insecticide chlorpyrifos using soil bacterial isolates

“…Earlier attempts to degrade chlorpyrifos by soil bacterial isolates have not been successful before 2000 (Racke et al 1994;Mallick et al 1999). In the present study, biodegradation of chlorpyrifos by three soil bacterial isolates was performed in comparison with bacterial cell growth in the MSM.…”

Concentration-Dependent Degradation of Three Termiticides in Soil Under Laboratory Conditions and Their Bioavailability to Eastern Subterranean Termites (Isoptera: Rhinotermitidae)

“…Also, the initial concentration at which the termiticides were applied affected degradation rate. Chlorpyrifos exhibited lower degradation rate when applied at Ϸ1,000 g/g soil than when applied at typical agricultural levels of 0.3Ð32 g/g soil (Racke et al 1994). Currently registered termiticides have soil organic partition coefÞcients values (K oc ), which place them in the immobile classiÞcation, implying they do not readily leach through the soil proÞle (Helling andTurner 1970, McCall et al 1979).…”

Factors Affecting Performance of Soil Termiticides

“…Chlorpyrifos exhibited a lower degradation rate when applied at ≈1,000 μg/g soil than when applied at typical agricultural levels of 0.3-32 μg/g (Racke et al, 1994). When fipronil was applied at the labeled rate for locust control (8g AI per ha), 75% degraded within 3 d (Bobé et al, 1998).…”