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Significant contributions of synergists have been noted in improving the efficacy and especially when the resistance problem has arisen in insects against insecticides. These synergists have been used for about 50 years (Bernard and Philogene, 1993). Insecticide synergism can be used to control or study insecticide resistance. A recent publication on pesticide resistance in arthropods (Roush and Tabashnik, 1990) covers the detection, investigation and mechanisms and management of resistance, while the most efficient synergists are those which can interfere with the in vivo detoxification of the insecticides (Raffa and Priester, 1985). Synergists are presently considered, by a large number of authors as one of the most straightforward tools for overcoming metabolic resistance (Raffa and Priester, 1985; Scott, 1990), because they inhibit the detoxification processes. In resistant populations that have higher detoxification rates than susceptible ones, which is the usual case, synergists are more effective in resistant than in susceptible strain (Scott, 1990). In any case, synergists have a major role to play in the identification of the causes of resistance.
Raffa and Priester (1985) reviewed synergists as diagnostic tools in resistance and proposed a key to identify the resistance mechanisms potentially involved in resistance using synergists as specific enzyme inhibitor and this approach is very useful in resistance management (Roush and Tabashnik, 1990) and play a significant contribution to improve the efficacy of insecticides. Synergists are also an important research tool in laboratory to determine the mechanisms of resistance involved in a particular population (Lorini and Galley, 2000). This kind of investigation has generated valuable information in understanding pesticide resistance. Insecticide synergists have been used not only to monitor the insecticide resistance but also an admixture in these insecticides for the control of many insects including house fly (Ahmed and irfanullah, 2007).
Pyrethrins, whose insecticidal potential was, appreciated in ancient china and Persia were first developed as an insecticide from the extracts of the flower heads of Chrisanthenum cinerariaefolium. The pyrethroids were developed due to rapid decomposition of pyrethrins by light, synthetic products. Because of their high insecticidal potency, relatively low mammalian toxicity, lack of environmental persistence, and low tendency to induce insect resistance, pyrethroids have encountered great success in the past thirty years and now share for more than 25% of the global insecticide market (Soderlund et al., 2002). Pyrethroids are used widely as insecticides both in the house and in agriculture, in medicine for the topical treatment of scabies and head lice, and in tropical countries in soaked bed nets to prevent mosquito bites. Pyrethroids are known to alter the normal function of insect nerves by modifying the kinetics of voltage-sensitive sodium channels, which mediate the transient increase in the sodium permeability of the nerve membrane that underlies the nerve action potential (Soderlund et al., 2002).
Several insects have shown remarkable resistance. For example house fly has significant tendency to develop resistance to different types of insecticides and many strains of house flies are now cross-resistant to new classes of insecticides (Scott et al. 1986, Shen and Plapp 1990). To effectively control house fly pests, studies of mechanisms of resistance and cross-resistance to relatively new insecticides are fundamental. Similarly the cotton bollworm Helicoverpa armigera is a serious pest of cotton and it has a long history of insecticide resistance to DDT, pyrethroids (Gunning et al., 1994), carbamate (Gunning et al, 1992), and organophosphates endosulfon (Gunning and Easton, 1993, 1994). Pyrethroid resistance has gradually increased over recent years in Australia (Gunning et al, 1994). It was concluded that increasing resistance is accompanied by esterase activity while the conventional esterase inhibitor, such as profenofos have failed to synergize pyrethroids.
All pyrethroid insecticides contain an acid moiety, a central ester bond, and an alcohol moiety (Fig.1). The acid moiety contains two chiral carbons, thus pyrethroid typically exist as stereoisomeric compounds (Trans and cis). Additionally, some pyrethroids also have a chiral carbon on the alcohol moiety, allowing for a total of eight different stereoenantiomers. These chemical considerations are relevant, as pyrethroidsâ€™ effects on sodium channels, their insecticidal activity, and their mammalian toxicity, are stereospecific. The cis isomers are generally more toxic than the corresponding trans isomers (Casida et al., 1983). The acute oral mammalian toxicity of pyrethroids is generally low. Values of LD50 range, for example, from 100 mg/kg (deltamethrin) to 10,000 mg/kg (phenothrin). The low mammalian toxicity of pyrethroids is confirmed by the fact that despite their extensive worldwide use, there are relatively few reports of human poisonings, and only a dozen deaths (Bradberry et al., 2005). Most deaths occurred following accidental or intentional exposure to pyrethroids. For example, a 45-year-old man died three hours after eating beans and cheese prepared using a 10% cypermethrin solution instead of oil (Poulos et al., 1982).
In medical, veterinary and agriculture use, pyrethrins and its synthetic analogues are among the most widely used insecticides. This pertains to their low application rates; rapid knockdown and kill, range of duration of activity and good mammalian safety make them an active choice in most domestic applications. The major concern by the use of these compounds is of resistance in the target organisms (Hill, 2008).
Due to their low toxicity to mammals and rapid knock-down effect, natural pyrethrins have been widely used in aerosols or in mosquito coils. The development of allethrin by LaForage, initiated studies on synthetic pyrethroids (Nishizawa, 1971). However the stability of allethrin made it superior to natural pyrethrins in both kill and knockdown effects for use in mosquito coils. Some new synthetic pyrethroids are however superior to natural pyrethrins in both kill and knock-down effects. Around the years 1945-1950, pyrethrins and DDT, the most widely used insecticide of the time became less effective in the control of pest populations. The general consensus was however the synergists would solve problem rapidly. On the other hand resistance spread rapidly to most classes of insecticides. Resistance is currently affecting 500 species of pest insects and mites (Georgiou, 1990). This unsolved problem of resistance affects the agricultural production and public health worldwide. In United States, more than 50 of all insecticide applications for agricultural pests are the result of insects becoming more resistant to insecticides (Georgiou, 1980, 1990).
Pyrethrins and DDT, the most widely used insecticides became less effective during 1945 to 1950. Resistance spread rapidly to other classes of insecticides and the insects population were not controlled by the same treatments. More than 500 species of pest insects and mites has developed resistance to different types of insecticides. (Georghiou, 1980). The problem of insecticide resistance arises when insects become tolerant to one or more than one insecticides after a period of exposure to these insecticides. This is due to evolutionary and heritable changes in pest population. The rate of development of resistance depends on numerous factors including frequency and combination of insecticides with synergists and intensity of application (Brown, et al 1998). Cross resistance also adds complexity to this scenario. Cross resistance arises when an insect develops resistance to an insecticide when applied but it also develops resistance to some other group of insecticides which have never been applied to that insect population. Anopheles mosquitoes have developed resistance to all major insecticides groups (Georghiou, 1990).
Mixture of PBO with pyrethroid or carbamate insecticides are often more effective against insect strains resistant to these compounds where oxidative metabolism is responsible for the decreased effect. From a general perspective the effectiveness and lethality of the insecticide can be increased by the addition of non toxic chemicals called synergists in insect pests (Metcalf, 1967). The increased insecticidal activity of pyrethrum was observed when Sesamin was added to it (Haller et al., 1942). Thus the definition of synergist is based on its activity. Casida (1970) classified synergists according to their structure like methylenedioxyphenyl synergists including compounds of plant origin such as Sesamin, Safrol, Sesamolin and derivatives like piperonyl butoxide, sulfoxide, sesame and tropital. Initial trials in United Kingdom showed that extracts of sesame gave better results in overcoming the insecticide resistance in pests of green house. In Australia and South Africa trials were successful in cotton against B biotype of Bemesia tabaci, that is not easily controlled by common insecticides and as well as against cotton aphid (Ahmed and irfanullah, 2007).
The mechanism of resistance occurs when an insect population becomes less sensitive to one or several insecticides after a period of exposure to compounds. It involves hereditable, evolutionary changes in pest population. The food and agricultural organization recommended that word resistance should be used to indicate only a hereditary decrease in sensitivity. The rate of development of resistance over time and for a given insect species depends upon the type of combination of insecticide plus synergist applied, the frequency and intensity of the applications and probably a number of factors yet to be identified (Brown, et al. 1998). However, the repetitive and inappropriate use of compounds in all these classes has led to resistance worldwide (Shen and Plapp, 1990). Insecticides are chemicals selected to kill or repel or adversely affect insects of different categories like phytophagous and hematophagous. The insects escape and avoid toxicity by different means including accelerated elimination or detoxification of target sites, erection of chemicals barriers and by modification of target sites. Detoxification affects all xenobiotics to some degree, including insecticides and synergists. Transformation of the compounds is achieved in two steps, primary and secondary referred as phase I and phase II (Hodgson, 1985). Pyrethroid resistance in some insects has been associated with changes in the levels of expression of certain P450s (Liu and Scott, 1998). Cytochrome P450 monooxygenase in insects are important in the metabolism of endogenous substances as well as the catabolism of xenobiotics such as plant toxins, drugs and insecticides (Romero, et al. 2009). Enhanced oxidative metabolism of xenobiotics by P450s is one mechanism by which insects become resistant to insecticides (Casida, 1970). Insecticide synergists have been used not only to monitor the insecticide resistance mechanisms but also as an admixture in these insecticides for the control of many insects including house fly.
To cope with the problem of insecticide resistance, role of synergists have been studied by various scientists and which depicts the high efficacy of insecticides in combination with synergists in controlling the different insects which has developed resistance to insecticides (Farnham, 1973). Piperonyl butoxide (PBO) is primarily used as synergist in combination with natural pyrethrins or pyrethroids in spray, residual methods, and admixture in controlling of insects in or around domestic and commercial areas, especially food preparation areas.
There have been done certain efforts to delay the start of resistance with the use of synergists. Wilkinson (1983) described that carbaryl and PBO selected houseflies exhibited only a 3 fold resistance over 20 generations, as compared to a 25 folds increases with carbaryl alone. Resistance developed more quickly under strong selection pressure of insecticide even if the ratio of insecticide to the synergist remained constant. As most insecticides are detoxified through more than one metabolic route, e.g. in methyl parathion (Brattsen, 1988), the combination of the insecticide and a synergist blocking one metabolic pathway will most probably select for resistance by another route. The long term effect may be a rapid buildup of resistance to the synergized mixture and ultimately an alarming for efficient control agents for resistant populations. Several strategies have been proposed to minimize this problem, including the use of variety of synergists acting at different locations of insectâ€™s physiology (Roush and Tabashnik, 1990).
The role and importance of synergists have changed over the last 50 years. Biologists, agronomists have become aware of the dangers of increased load of chemical toxicants on both insect pests and environment and of the rising cost of crop production. Simultaneously the world economy cannot afford a decrease in food production. Moreover under strong selection pressure, the number of resistant pests has increased with populations of arthropods that no longer are contained by the type and amount of insecticides that previously control them. These synergists which are a group of relatively non toxic compounds can enhance the toxicity to the target pest of already effective compounds and keep the use of toxic insecticides to minimum level.
Synergists of increasing specificity remain a major tool in the management of resistant pests. These agents keep the selection pressure to a minimum level, particularly when act on a unique detoxification route of an insecticide. The role of synergists in metabolic research in insects has become more important. The need to control an increasing number of resistant insects, and other pest species, requires the identification of compounds with novel mode of action.