Tebufenozide is an insecticide used to control caterpillar pests in fruits, vegetables, and other crops. It is an agonist of an insect moulting hormone, which is selectively toxic to lepidoptera and a few diptera. It is registered in many countries around the world.
Tebufenozide is marketed in several formulations, the most important being two suspension concentrates and a wettable powder.
The compound was considered for the first time by the present Meeting.
The fate of tebufenozide has been studied in rats, goats, poultry, fish, apples, grapes, rice, sugar beet, soil and water, with the compound labelled with 14C (a) uniformly in the ethylbenzoyl ring (A-ring label), (b) uniformly in the dimethylbenzoyl ring (B-ring label) and (c) at the central carbon of the tert-butyl group (t-butyl label).
The principal metabolites and degradation products identified are as follows.
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RH-6595: |
N-tert-butyl-N'-(4-acetylbenzoyl)-3,5-dimethylbenzohydrazide |
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RH-1788: |
N-tert-butyl-N'-[4-(l-hydroxyethyl)benzoyl]-3,5-dimethylbenzohydrazide |
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RH-2703: |
N-tert-butyl-N'-(4-carboxymethylbenzyol)-3,5-dimethylbenzohydrazide |
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RH-9886: |
N-tert-butyl-N'-(4-ethylbenzoyl-3-hydroxymethyl-5-methylbenzohydrazide |
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RH-0282: |
N-tert-butyl-N'-[4-(l-hydroxyethyl)benzoyl]-3-hydroxymethyl-5-methylbenzohydrazide |
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RH-9871: |
N-tert-butyl-N'-(4-acetylbenzoyl)-3-hydroxymethyl-5-methylbenzohydra2ide |
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RH-2778: |
N-tert-butyl-N'- [4-(1 -hydroxyethyl)benzoyl]-3,5 -di(hydroxymethyl)benzohydrazide |
The metabolic degradation of tebufenozide has been found to proceed generally via oxidation at the alkyl groups located on the two rings, and sometimes conjugation of the oxidation products. Eventually the amide bonds are broken and the structural sub-units are oxidized to CO2. The parent compound has been a significant component in most metabolism studies together with its oxidation products such as monoalcohols, the RH-6595 ketone and the corresponding acid. More extensively oxidized products such as diacids, diols, and alcohol-acids have also been found in smaller quantities.
Male and female rats were dosed orally with two or three radiolabelled versions of tebufenozide at 3 mg/kg or 250 mg/kg bw and another group received a dose of 3 mg/kg of labelled tebufenozide after being dosed for two weeks with the unlabelled material.
Most of the radioactivity (94-104%) was excreted in the faeces, with low levels in the urine (0.5-8%) in the first 24 hours. The main routes of metabolism were via oxidation of the alkyl groups on the phenyl rings. The main component of the excreted activity was the parent compound, but seventeen metabolites were identified in the faeces. The extent of the metabolism was found to be highly dependent on the amount of the dose. A high dose produced a low proportion of metabolites, while about 50% of a low dose was metabolized.
In goats, most of the radioactivity was excreted in the faeces and urine, less than 0.3% was excreted in the milk and less than 0.4% of the dose remained in the tissues. Levels of 14C in the milk remained fairly constant through the dosing period. Tebufenozide was extensively metabolized by multiple oxidative transformation. The highest residues in the tissues were found in liver and the next highest in fat.
Milk contained only a low concentration of the parent compound (13.7% of the 14C) together with the alcohol metabolites RH-0282, RH-9871 and RH 9886 and its conjugates. In fat the parent compound and three fatty acid conjugates of RH-9886 were identified. Liver was unique in that no residue of the parent compound was found; the major component was the carboxylic acid RH-2703, together with an almost equal amount of 2-propanol. Tebufenozide was detected in the kidneys and muscle at low levels.
In poultry the highest total residues occurred in the liver, kidney and fat when laying hens were dosed with different radiolabelled versions of tebufenozide for 7 days at a level equivalent to 30 ppm in the feed. Residues in eggs increased during the feeding period and were still increasing on day 7, although they represented a low percentage of the dose. In eggs the principal residues were tebufenozide and RH-9871 (the alcohol-ketone). The parent compound and RH-0282 (dialcohol) were the major components in the fat, and acetaldehyde and 2-propanol were the major metabolites identified in liver. Excreta were the main source of metabolites in the study.
Fish. Bluegill sunfish were continuously exposed to a nominal concentration of 50 mg/l of [14C]tebufenozide for 29 days to study the kinetics of its uptake and elimination. Some bioconcentration of tebufenozide was observed in the fish, but depuration of the residue was rapid: its half-life was less than three days and by the last (15th) day of depuration at least 90% of the radioactivity had been eliminated from the fish.
The main residue in the fish was the parent compound, but eight metabolites were also isolated and identified. Metabolism occurs via oxidation of the ethyl group to an alcohol or ketone and of the two equivalent methyl groups successively to monoalcohol, dialcohol and acid-alcohol.
The fate of residues in plants was studied in apples, grapes, rice and sugar beet using the three radiolabelled versions of tebufenozide. The compound was metabolized by oxidation of the alkyl substituents of the aromatic rings to combinations of alcohols, the ketone and acids. Only grapes were not metabolized.
The parent compound was the main component of the residue in all four crops, constituting essentially 100% of the 14C in grapes, 77% in apples, 72% in rice grain, 67% in sugar beet root and 41% in sugar beet tops. No single metabolite occurred at more than 10% of the total residue or more than 15-20% of the tebufenozide residue in any studied crop.
The extent of metabolism appeared to be related to the length of time in the plant. In grapes where the exposure was 30 days, the residue was 100% parent compound. In rice after 64 days 72-76% of the residue was tebufenozide, and in sugar beet after 120 days 41-67% was the parent compound.
The degree of oxidation also appears to be a function of exposure. In rice two monoalcohols were produced with further oxidation to form the corresponding ketone and aldehyde. In apples the same monoalcohols were again the first metabolites, followed by oxidation of substituents on the second ring to produce the diol RH-0282 and triol RH-2778. In sugar beet, with the longest exposure, there was further oxidation to carboxylic acids. Sugar beet after foliar treatment with tebufenozide showed a moderate translocation of the parent compound and its metabolites from the foliage to the roots. All the metabolites isolated in plants were also observed as metabolites in the rat.
Metabolites were identified by HPLC, TLC, LSC, and MS.
In summary, the plant metabolism studies indicated that the major residual compound in crops is unchanged tebufenozide. Metabolites were not observed in grapes at a 30-day PHI, showing that metabolism of tebufenozide in plants is slow. The principal metabolites found in other crops were RH-6595, RH-1788 and RH-9886. The proportion of metabolites in the residue increased with time.
Studies of the fate of tebufenozide in soil (high organic sand, low organic sand, loamy sand, and sandy loam) under aerobic condition showed that it is degraded by oxidation of the ethyl group to produce the ketone and two carboxylic acids, followed by mineralization to CO2. The half-life of tebufenozide ranged from 7 to 105 days. Carbon dioxide and unextractable material associated with humic and fulvic acid fractions were the major terminal residues.
A study of the adsorption and desorption of [14C]tebufenozide in five different soils showed that it could be classified as having low mobility. The study suggested that some of the tebufenozide may be irreversibly bound to soil.
The leaching of tebufenozide and its aged residues was studied in different soils. At the end of the leaching period (48 h) only small amounts of tebufenozide had been leached. The study with aged residues, in clay loam, sandy loam, loam and sand, showed that while tebufenozide had some mobility in the sand, the two carboxylic acid products were mobile in all four soils. In another study with aged residues applied to a sandy soil with low organic matter, tebufenozide and its ketone derivative remained in the soil whereas both carboxylic acids were found in the leachates.
Tebufenozide in sterile buffered solutions at pH 5, 7 and 9 at 25°C showed no hydrolytic degradation.
The photolysis of tebufenozide was studied in a treated soil irradiated with a xenon lamp for 30 days with a 12-hour light/dark cycle. The parent compound was degraded slowly to yield a total of 7 detected products of which the ketone and aldehyde were the major compounds identified. The half-life under these conditions was 98 days. Photolysis was also studied in natural pond water, again with irradiation by a xenon lamp. Tebufenozide was degraded with a half-life of 67 days. The major product was the ketone RH-6595 (5.3% of the radioactivity). Eight other compounds were also detected but none exceeded 3.5% of applied activity. Almost no degradation occurred in the dark control.
Field dissipation studies conducted at different sites in various countries according to the worst-case scenario (a high rate applied directly to bare ground) showed that tebufenozide and its soil degradation products decline at a moderate to a fairly fast rate. When the ground was covered with grass tebufenozide was translocated to the soil, reaching a maximum concentration in soil on day 122. These studies also demonstrated that neither tebufenozide nor its ketone degradation product showed any downward mobility. Residues of tebufenozide were principally found in the top (7.5-10 cm depth) of the soils. It was also shown that the parent compound is degraded more slowly in soils with a low pH (3.9-5.1). Degradation products of tebufenozide were only occasionally observed above the limit of determination (0.01-0.02 mg/kg).
The Meeting was informed that a study on the uptake of tebufenozide from soil by crops was in progress and the full report would be available to a future Meeting.
