Abstract: This study reports on the degradation of novaluron [(±)-1-[3-chloro-4-(1,1,2-trifluoro-2-trifluoromethoxyethoxy)phenyl]-3-(2,6-difluorobenzoyl)urea] in water at pH 4.0, 7.0 and 9.2. Degradation and the corresponding parameters were calculated from the data obtained by analyses of the residual parent compound. The half-lives of novaluron ranged within 94.1 to 1003.0 days irrespective of the pH of water. The order of novaluron degradation obtained was pH 9.2 > pH 7.0 > pH 4.0 indicating stability of the molecule in acidic and neutral pH.
Introduction
Novaluron (Fig. 1) is an Insect Growth Regulator (IGR) acts by inhibiting chitin formation, thereby causing abnormal endocuticular deposition and abortive moulting (Tomlin, 1997). Novaluron is active against larvae of various insects including Lepidoptera, Coleoptera, Homoptera and acts by both ingestion and contact (Su et al., 2003). This novel bonzophenyl urea IGR controls beet armyworm (Spodoptera exigua) and greenhouse whitefly (Trialeurodes vaporariorum) (Ishaaya et al., 1998). This compound is highly effective against Plutella xylostella, major pest of cabbage. American ballworm (H. armigera) a larvae attacking cotton crop (Murthy and Ram, 2002) and also against brinjal shoot and fruit borer (Leucinodes orbonalis). Novaluron affects only the larval stages, which actively synthesize chitin. Hence the adults of nontarget species viz. parasites and predators are seldom affected-an important criteria for including it in integrated pest management programmes for controlling important agricultural pests.
Several reports are available regarding the detailed studies on bioefficacy of novaluron. So far, no information is available about its degradation in soil and hydrolytic fate under different pH conditions. Therefore, the objective of this study was to evaluate the effect of pH on degradation of novaluron in water under laboratory simulated conditions. This information will be useful in determining the degradation of novaluron in soil solution and ultimately in soils of different agroclimatic conditions.
| Fig. 1: | Chemical structure of novaluron |
Materials and Methods
Novaluron was quantified using high performance liquid chromatography (HPLC 1050 Hewlett Packard equipped with UV detector and Chemito 5000 Data Processor). For recovery studies, triplicate samples were fortified with methanol solution of novaluron to obtain concentrations corresponding to different doses.
Buffer solutions of pH 4.0, 7.0 and 9.2 were prepared by dissolving the appropriate buffer capsules in 200 mL sterilized (by autoclaving at 121°C for 20 min) double distilled water. The calculated amount of sterile (passed through 0.2 μm membrane filter paper) solution of novaluron in 1 mL methanol was applied to the buffer solutions in individual amber coloured Erlenmeyer flasks (250 mL) plugged with cotton pad. Novaluron was spiked in triplicate to the buffer solutions at the rate of T1 = 0.25 and T2 = 0.50 μg mL-1. The flasks were incubated at 30°C under dark for periodic study. The control samples received only methanol and underwent the same procedure. Sampling was done after 0 (2 h after spiking), 1, 3, 7, 15, 30, 45, 60, 90 and 120 days.
The water samples (200 mL) were extracted with dichloromethane (100 + 50 +50 mL). The dichloromethane was evaporated to dryness, rinsed with HPLC grade methanol and filtered (0.2 μm) for direct HPLC analysis. Novaluron was separated on an Intersil 150x4.6 mm ODS 2, 5 μm (RP C18) using a mobile phase of methanol and water (80:20) at a flow rate of 1 mL min-1 and column temperature at 40°C. Quantification was performed against novaluron standard at a wavelength of 254 nm. Under this condition the retention time of novaluron was 3.7 min, the limit of detection was 0.01 mg and the sensitivity of the method was 0.05 mg kg-1. The average recovery was 94.0-96.2% for novaluron. Determination of novaluron residues in the treated samples was carried out as per the recovery study.
Results and Discussion
The initial deposits of novaluron in waters of pH 4.0, 7.0 and 9.2, percent degradation at different time intervals, regression equations and half-life values are presented in Table 1-3. The initial concentrations ranged between 0.226-0.238 and 0.455-0.462 mg L-1 in waters of different pH at novaluron spiking rate of 0.25 and 0.50 mg L-1, respectively.
| Table 1: | Degradation of novaluron in water at pH 4.0 under laboratory condition |
Novaluron (0.25 mg L-1) Regression equation: y = 1.3699-0.0003x Half life (t½) = 1003.0 days R2 = 0.88, Novaluron (0.50 mg L-1) Regression equation: y = 1.6583 - 0.0003x Half life (t½) = 1003 days R2 = 0.89 |
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| Table 2: | Degradation of novaluron in water at pH 7.0 under laboratory condition |
| Novaluron (0.25 mg L-1) Regression equation: y = 1.3649 - 0.0005x Half life (t½) = 602 days R2 = 0.93, Novaluron (0.50 mg L-1) Regression equation: y = 1.6509 - 0.0005x Half life (t½) = 602 days R2 = 0.92 | |
| Table 3: | Degradation of novaluron in water at pH 9.2 under laboratory condition |
Novaluron (0.25 mg L-1) Regression equation: y = 1.3386 - 0.0032x Half life (t½) = 94.1 days R2 = 0.99, Novaluron (0.50 mg L-1) Regression equation: y = 1.6405 - 0.0031x Half life (t½) = 97.1 days R2 = 0.99 |
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After 120 day, the respective residue remaining in water ranged between 0.095-0.219 and 0.196-0.425 mg L-1 for novaluron spiking at two application rates.
Novaluron is highly stable under acidic and neutral conditions. In pH 4.0 only 7.98-8.01% got degraded even after 120 day. In pH 7.0, the percent degradation varied between 14.88-15.61%. Only under alkaline condition, i.e., at pH 9.2 the compound showed significant degradation between 56.92-57.96% after 120 day.
The half-life values calculated from the best-fit lines of the logarithm of residual concentrations versus incubation period suggested first order reaction kinetics (R2>0.88) for novaluron degradation in water of different pH. The half-life values were 1003 days for pH 4.0, 602 days for pH 7.0 and 94.1-97.1 days for pH 9.2. The rate of novaluron degradation was found maximum in water of pH 9.2 followed by pH 7.0 and pH 4.0, thereby indicating the stability of the molecule in acidic and neutral pH.
Acknowledgments
The author is grateful to Department of Agricultural Chemicals, Bidhan Chandra Krishi Viswavidyalaya, India for excellent technical assistance and Indofil Chemical Company, Mumbai, India for sponsoring the present study.