Urea Pesticides

Urea herbicides form, together with phenoxy derivatives and triazines, the most important agricultural herbicide group. The urea-derivatives are typical pre-emergence herbicides applied usually as aqueous emulsions to the surface of soil. Almost all of the urea compounds with good herbicidal action are trisubstituted ureas, containing a free iminohydrogen. According to the receptor theory, this hydrogen plays a role in the formation of the hydrogen bond being significant in the mode of action of ureas. Chemically, the urea type herbicides contain a urea bridge substituted by triazine, benzothiazole, sulfonyl, phenyl, alkyl or other moieties. Besides herbicidal activity, some analogous structures have other biological activity (Lányi&Dinya, 2005). The general structure of a phenylurea herbicide (PU) is (substituted) phenyl–NH–C(O)–NR2. The phenyl ring is often substituted with chlorine or bromine atoms, but methoxy, methyl, trifluoromethyl, or 2-propyl substitution is also possible. Most PU are N-dimethyl PU, but a combination of a methyl substituent and another group also occurs (Niessen, 2010). PU are used as selective and non-selective herbicides in substantial amounts, including the use as systemic herbicides to control broadleaf and grassy weeds in cereals and other crops, as total herbicides in urban areas, and as algicides in paints and coatings. Sulphonylureas (SU) form a group of selective herbicides with R1–NH–C(O)–NH–SO2–R2 as general structure. R1 and R2 generally are substituted heterocyclic rings such as 4,6dimethylpyrimidin-2-yl and 2-(benzoic acid methyl ester) (Niessen, 2010). The mode of action of these herbicides consists of inhibiting acetolactate synthase (ALS) which is a key enzyme in the biosynthesis of branched amino acids (valine, leucine, and isoleucine). SU are low dose herbicides (10 – 40 g a.i. ha-1) used to control broad leaved weeds in cereals exhibiting very low acute and chronic mammalian toxicities (Wang Y. S. et al., 2010). Benzoylureas (BU), which were introduced in the early ‘70s, represent a class of insect growth regulators (IGRs) which act on the larval stages of most insects by inhibiting or blocking the synthesis of chitin, a vital and almost indestructible part of the insect exoskeleton during the molting stage; therefore, the failure to successfully cast off the old exoskeleton leads to the eventual death of the larvae. Diflubenzuron is the prototype of all benzoylurea chitin synthesis inhibitor insecticides (Shim et al., 2007). The specificity of benzoylureas to species whose structural integrity depends upon chitin, their low acute toxicity to mammals along with their high biological activity, make them suitable for inclusion in integrated pest management programs for fruit and vegetables