Diafenthioron is a pesticide, specifically an insecticide. It is a chemical compound which belongs to the thiourea group. Diafenthiuron is sold under the brand names Derby, Diron, Pegasus, Polar and Polo.
History
Diafenthiuron was introduced to the market in 1990 by a pharmaceutical company called Ciba-Geigy in Switzerland and due to rising environmental concerns it is no longer approved in Switzerland and other countries of the European Union. However, until 2001, Switzerland still produced and exported the compound to other countries where its use as an agricultural pesticide was still legal.
Structure and reactivity
Diafenthiuron is an phenol ether of molecular weight 384.6 g/mole and containing two 6-carbon rings attached to an oxygen and nitrilo (tert-butylcarbamate) group substituting for a hydrogen atom at position 2. It is highly soluble in multiple organic solvents however dissolves poorly in aqueous solutions due to the hydrocarbon rich regions and oxygen atom which do not form H bonds with water molecules. Therefore diafenthiuron contamination of underground water storages by leaching is unlikely. Its lack of water solubility (partition coefficient logPow = 5.76) and strong sorption capacity, means it has the ability to accumulate persistently in aquatic and soil systems.
It is a broad spectrum, both systemic and contact as well as stomach activity synthetic pesticide impairing mitochondrial function in target pests (phytophagous mites). Due to progressive break-down of diafenthiuron to non-toxic metabolites, it poses no significant reactivity threat in the field of ecotoxicology (Hazard class III = slightly hazardous). However, its highly reactive metabolite carbodiimide (a result of S-oxidation → either non-enzymatic (light-catalyzed), P450 monooxygenases, or by FAD monooxygenases) binds to the mitochondrial ATPase and porin, inhibiting ATP production.
Synthesis
The synthesis of diafenthiuron starts with phenol reacting with 2,6-diisopropyl-4-chloroaniline, creating the intermediate product 2,6-diisopropyl-4-phenoxyaniline. 2,6-diisopropyl-4-phenoxyaniline will then undergo a reaction with thiophosgene and form N-(2,6-diisopropyl-4-phenoxyphenyl)isothiocyanate. This in turn will react with tert-butylamine and form diafenthurion.
Mechanism of action
Diafenthiuron is photochemically or metabolically transformed into a highly reactive metabolite, Diafenthiuron carbodiimide, which covalently and irreversibly binds to microsomal glucose-6-phosphate translocase, a part of the adenosine triphosphatase (ATPase) complex. The binding inhibits the function of the ATPase complex due to the modification of a single sulfhydryl/amino phosphate group of the glucose-6-phosphate translocase. Therefore, substrates such as inorganic phosphate, carbamoyl phosphate and pyrophosphate cross the microsomal membrane freely without the catalytic function of the translocase; ATP production is inhibited. Diafenthiuron carbodiimide also affects mitochondrial porins, but due to the fact that the function of these mitochondrial porins is not fully known yet, the effects of the carbodiimide on these porins is also not understood. In summary, diafenthiuron inhibits the process of mitochondrial respiration by binding to various mitochondrial components.
Efficacy and side effects
Efficacy
Diafenthiuron has shown high efficacy against many important agricultural pests, including mites, aphids, thrips, and whiteflies. Several studies have demonstrated the efficacy of diafenthiuron in controlling various pests affecting a variety of crops. For example, diafenthiuron has shown to be effective against cotton bollworm, cotton jassid, and spider mites. With citrus fruits, it has been shown to control citrus rust mites and aphids. With grapes, diafenthiuron has shown to be effective against grape leafhoppers and mites. The efficacy of diafenthiuron is dependent on several factors; the target pest species, the stage of the pest's life cycle, and environmental conditions such as temperature and humidity. Next to that, the repeated use of diafenthiuron can lead to the development of resistance in target pests, reducing the efficacy of the insecticide over time.
Side effects
Apart from enzymatic conversion, diafenthiuron readily dissolves in organic solvents or is broken down by sunlight into two major by-products; diafenthiuron carbodiimide and diafenthiuron urea. In aqueous systems, the carbodiimide was further observed to be photo-transformed into diafenthiuron urea in the presence of sunlight; a key photolysis pathway in diafenthiuron breakdown in the environment.