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Agriculture and Sustainable Development:
The losses caused by various pests to crops generally vary from 30-50% depending on the region and the crop. In order to satisfy the alimentary needs of an ever increasing population, it is imperative to reduce these losses caused by various pests to crops and stored products. Several methods were used for controlling pests, in which the conventional insecticides such as organophosphorous, carbamates and pyrethroids play an important role. Improving pest control means is a global challenge to preserve human and animal health, secure food reserves and reduce losses while respecting the environment and the rules of sustainable development. It is therefore essential to develop new products and / or new approaches that combine different ways of pest and vector control.
The negative effects of conventional neurotoxins on the environment has resulted in the implementation of integrated pest management approaches applying environmentally-friendly and sustainable methods to control major insect pests of crops and veterinary and human importance. Among these approaches the development of novel insecticides with selective properties acting on biochemical sites or on physiological processes. Based on the advances in insect physiology selective agents such as the insect growth disruptors (IGDs) have been developed. These IGDs affect the hormonal regulation of molting and developmental processes. Since their discovery, these new insecticides have been the subject of several detailed reviews. The following classes of IGDs have been successively developed; the juvenile hormone analogs, the inhibitors of chitin biosynthesis and the molting hormone agonists.
Larval growth and development in insects are regulated by the polyhydroxylated steroid 20-hydroxyecdysone and the sesquiterpenoid juvenile hormone (JH). In the adult stage, these hormones are also involved in the regulation of reproductive maturation. In 1967, C. Williams suggested that compounds that mimic the concept of JH action can be used as third generation insecticides with an eco-toxicological benign profile, and since then, numerous JH analogs (JHAs) have been discovered. Such compounds are toxic during the embryonic, last larval, and reproductive stages of insects, and some have been commercialized as a new class of insect growth regulators (IGRs). Most of the early analogs are based on the terpenoid structure of JH. The most active ones are methoprene and hydropene. More recently, several highly active compounds, such as fenoxycarb, pyriproxyfen and kinoprene have been synthesized. These products have not the expected success, because of their rapid photo-degradation (methoprene) and high toxicity to aquatic organisms (pyriproxyfen).
The insect cuticle is secreted by the epidermal cells and consists mainly of chitin and protein. Chitin is a polysaccharide (N-acetyl-D-glucosamine polymer). It is present in fungi, diatoms and certain groups of invertebrates such as arthropods and nematodes. The fact that chitin has only a limited distribution among certain organisms makes it an attractive target for the development of specific active agents. The benzophenylureas (BPUs) prevent typically the molting process. They were found to disturb the cuticle secretion (thickness and fine structure) and to inhibit the incorporation of radio-labeled precursors into chitin confirming their primary mode of action on chitin biosynthesis. In addition, these chemical agents affect reproduction in several insect orders, primarily by causing a reduction in egg hatch. Diflubenzuron (Dimilin 25WP) is the most investigated BPU derivative widely used in the control of insect pest in agriculture and forestry. Since its introduction, a number of other BPU derivatives have been developed such as flucycloxuron, triflumuron, hexaflumuron or novaluron.
Besides these IGDs, the non-steroidal ecdysteroid agonists, also called moulting-accelerating compounds (MACs), have been discovered. They mimic the action of the steroid insect moulting hormone, 20-hydroxyecdysone (20E), and manifest their activity by binding to the ecdysteroid receptor complex in a manner competitive with ecdysteroids, inducing a precocious and incomplete lethal moult in several insect orders . Four products have been commercialized. Halofenozide was found to be more toxic to coleopteran pests while methoxyfenozide was more potent against lepidopteran species. The ecdysteroid agonists affect reproduction in different insect orders, mainly by reducing fecundity and egg viability, or sexual behavior.
Conclusively, the use of pesticides is expected to shrink more and more in coming years due to environmental imperatives. Integrated pest management is necessary with a reasoned chemical control based on the appreciation of a threshold of harmfulness or economic tolerance and using selective insecticides such as the IGDs. These new selective insecticides have potential for use in integrated pest management regarding their specific mode of action on insects and their low eco-toxicological risks. They constitute a good alternative to conventional insecticides. For environmental risk assessment, the potential side-effects of these alternative chemical agents on ecosystems particularly on non-target organism like the auxiliary agents, must be investigated. The valorization of plant extracts is a promising alternative. However, the further research must focus on their mode of action and their potential hazards on ecosystems.
Prof. Noureddine Soltani