Degradation of diflubenzuron

Schaefer, C.H., A.E. Colwell, and E.F. Dupras, Jr. 1980. The occurrence of p-chlorophenylurea from the degradation of diflubenzuron in water and fish. Proc. Califor. Mosquito Contr. Assoc. 48 84-89. 

Diflubenzuron (Dimilin , TH-6040) is an IGR which inhibits the normal deposition of chitin. The metabolic fate of diflubenzuron has been studied in sheep U, 2), cattle (1 ), rats (1, 3), house flies (4, 5), stable flies (5), chickens (6), swine (6), boll weevils (7), plants (8 9), and soil (2, 8, 10). Since good reviews of diflubenzuron metabolism have been given by Ivie (11) and Verloop and Ferrell (9), we will present only a tabular summary of the degradation of diflubenzuron in nonaguatic systems for comparative purposes (Table I). The remaining discussion will focus on diflubenzuron degradation in the aquatic environment. 

Plectonema boryanum, were voracious degraders of diflubenzuron (Table III). Just 5 mg of algae cells could metabolize almost 80% of the applied diflubenzuron in just 1 hr. Curiously, this pace was not sustained since 45 mg of algae could degrade only 95% of the applied dose after four days. 

Table IV. Degradation of Diflubenzuron in the Metcalf et al. (2) Model Ecosystem.

Table 1. Influence of particle size on apparent rate of degradation of diflubenzuron (28, 29).

With the related compound PH 60-38, (III), a comparable but smaller effect of the particle size on the apparent rate of degradation in soils was observed as is illustrated in Table 1 The influence of the type of soil on the rate of degradation of diflubenzuron is much less important, because with five agricultural soils and three hydrosoils, including the soil types recommended by the EPA and the German BBA, the variation in the halflife was only a factor of approximately two. 

Figure 4. Proposed pathways of the degradation of diflubenzuron in agricultural soils and hydrosoils. Abbreviations rid = reversed isotope-dilution ms = mass spectrometry tic = thin layer chromatography.

Plants, In contrast to the fast and complicated degradation of diflubenzuron in soils, the fate of the insecticide in plants after leaf application is rather simple. 

Diflubenzuron adsorbs to and remains relatively immobile in soil. Degradation of diflubenzuron is largely through microbial hydrolysis and photolysis with a half-life range of 0.5-1 week. Uptake of diflubenzuron through leaves does not occur. Residual diflubenzuron in soil may be absorbed by plants. 

Cunningham, P.A. 1986. A review of toxicity testing and degradation studies used to predict the effects of diflubenzuron (dimilin) on estuarine crustaceans. Environ. Pollut. 40A 63-86. 

Gattavecchia, E., A.M. Di Pietra, D. Tonelli, and A. Borgatti. 1981. Effect of diflubenzuron and its major degradation products on the growth of Euglena gracilis Z. and incorporation of glycine-U-14C in protein. Jour. Environ. Sci. Health B16 159-166. 

Hydrolysis. Schaefer and Dupras (12) investigated the hydrolytic stability of diflubenzuron as a 0.1 ppm aqueous solution. At pH 7.7 diflubenzuron is stable at 10-24°, but gradually decomposes at 38°. At pH 10 it is stable at 10°, but degrades slowly at temperatures greater than 24°. 

Photodegradation. The photochemical degradation products of diflubenzuron in strictly aqueous solution are unreported, perhaps because of the compound s refractory solubility. Exposure of thin films on glass or a 0.1 ppm aqueous solution to sunlight... 

More detailed studies of the metabolic pathways of diflubenzuron in soils have been carried out with radioactive preparations labeled in four different positions of the molecule for a study of the ultimate fate of the primary degradation products. 

The main degradation pathway of diflubenzuron in soils would lead to the formation also of 2,6-difluorobenzoic acid as a primary metabolite. 

From the results it is clear that no residues were found up to the sensitivity limit of the analytical methods used. Furthermore untreated leaves were found to contain practically no radioactive material. It can be concluded that diflubenzuron after application on plants is very persistent and has no systemic properties. In other studies it was found that diflubenzuron does not permeate through the cuticular barrier into the leaves of broad bean. These studies on the fate of diflubenzuron on and in plants will be published more extensively elsewhere ( J ) Essentially the same results were obtained by Still in a study on the metabolic fate of diflubenzuron on cotton plants ( 2). In addition to metabolism, other factors might influence the fate of diflubenzuron on plants, i.e. washing off and photochemical degradation. Ruzo et al. (jQ) and Metcalf et al. (30)... 

However, under more natural conditions, e.g. according to the methods recommended in the EPA Guidelines the rate of the photochemical degradation was found to be very low h.) In other experiments washing off of diflubenzuron from plant leaves with high amounts of simulated rainfall was found to be negligible All these results point to a high... 

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