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Abstract
INTRODUCTION
The human desire to control insects has existed as long as humans themselves have. Insects annoy people by biting, stinging and generally getting in the way. However, this desire for insect control increased significantly with two events: the realization that insects can spread human diseases such as malaria and yellow fever, and the rise of agriculture. As the world’s population increases, the need to keep insects from destroying food crops becomes even more urgent; currently, insects destroy between 15-30% of crops intended for human consumption. This figure is even higher worldwide and varies between developed and developing countries.
There is a continuing need to increase food production, particularly in the developing countries. This increase has to come from increased yields of major crops grown on existing cultivable lands. One practical means of achieving greater yields is to minimize the pest associated losses. Insects are not the only cause of direct loss to the agricultural production, but also indirectly due to their role as vectors of various plant pathogens. In addition to direct losses caused by insects, there are additional costs in the form of pesticides applied for pest control. Human attempts at insect control have changed over time from natural methods to synthetic chemicals control, and now for environmental and health reasons, again, we look to natural methods. While today’s synthetic insecticides are presumably safer and less persistent than those used in the past, such as, DDT, they are still a cause for concern. Long term exposure to modern synthetic insecticides has been associated to low or high extents with cancer, liver damage, immunotoxicity, birth defects and reproductive problems in humans and other animals (Kegley and Wise, 1998). In addition, the massive application of pesticides results in adverse effects on the beneficial organisms. As a result, the chemical control of pests is under increasing pressure.
Pesticide use in the world is declining, largely due to major reduction in Europe as a result of regulatory mechanisms, environmental activism and public pressure. This has necessitated the use of target specific compounds with low persistence, and an increase in emphasis on integrated pest management. Although the benefits to agriculture from the pesticide use to prevent insect associated losses cannot be overlooked, but there is a greater need to develop alternative or additional technologies, which would allow a rational use of pesticides and provides adequate crop protection for sustainable food, feed and fiber production in the future. This is why development of biologically natural methods of insect control, or biopesticides, is preferential today.
Among the most promising alternative to conventional insecticides, is the avermectin insecticide group. Abamectin (avermectin B1) is currently the main avermectin compound used as a mitecide/ insecticide in a great variety of crops. Chemical modifications on its original structure with the aim of increasing its insecticidal spectrum resulted in the discovery of Emamectin benzoate (MK-244, 4” –deoxy-4”–epi-Nmethylamineavermectin B1), one of many 4 ” – substituted analogs that shows an increased potency against lepidoptera larvae (Mrozik, 1994). The mode of action of emamectin benzoate is similar to abamectin (a GABA and glutamate-gated chloride channel agonist) according to Dunbar et al. (1998). Emamectin benzoate is a novel semi-synthetic derivative of the natural product abamectin from the avermectin family of 16-membered macrocyclic lactones. This epi-methyl amino derivative is very effective against a broad spectrum of lepidopteran pests, with good photostability and translaminar movement, good field efficacy and lack of cross-resistance with other commercially-used pesticides (White et al.,1997). Therefore, the aim of this work could be summarized in the following items:
• Assessment of the insecticidal activity for emamectin benzoate against laboratory and field strains of different S. littoralis larval instars.
• Studying the effect of emamectin benzoate on the:-
- Aspartate aminotransferase (AST), (glutamic oxaloacetate transaminase) (GOT) for the laboratory strain 4th instar larvae of S. littoralis.
- Alanine aminotransferase (ALT), (glutamic pyruvate transaminase) (GPT) for the laboratory strain 4th instar larvae of S. littoralis.
- Alkaline phosphatases (ALP) for the laboratory strain 4th instar larvae of S. littoralis.
- Total proteases for the laboratory strain 4th instar larvae of S. littoralis.
- Acetylcholinesterase (AChE) for the laboratory and field strains 4th instars larvae of S. littoralis.
- γ-Aminobutyric acid (GABA) of laboratory and field strains for S. littoralis 4th instar larvae.
- Glutamic acid of laboratory and field strains for S. littoralis 4th instar larvae.
- Biochemical studies to elucidate more understanding of the compound mode of action.