Mosquitoes pyrethroid resistance

The issue of pyrethroid resistance in houseflies and mosquitoes and the countermeasures are described below. 

Until recently, the resistance of mosquitoes to pyrethroids has not been taken as a serious issue. In Japan, C. p. pallens and Aedes albopictus (Skuse) are the main species living around houses. Although mosquito coils have utilized natural pyrethrins as insecticidal ingredients for about 50 years and then allethrin for about 50 years, there has been no report on resistance development. The reason for this is considered to be the short active time of 4-5 months per year for C. p. pallens. Yasutomi et al. [50] reported in 1989 the presence of pyrethroid-resistant Culex tritaeniorhynchus in Okinawa, but Japanese encephalitis transmitted by C. tritaeniorhynchus decreased markedly after 1992 and disappeared. 

After World War II, the production of pyrethrum in Japan fell markedly and declined to only 1,000 tons in terms of dried flowers in 1965. At present, pyrethrum is not cultivated in Japan and the main producers are Kenya, Tanzania, Tasmania, and China, with worldwide production in 2010 amounting to around 10,000 tons of dried flowers. Dried flowers are extracted and purified at pyrethrum-extracting factories on the spot, producing 25-50% pyrethrin extracts. While pyrethrum extracts have been replaced with various synthetic pyrethroids, they are still used in houses, food factories, gardens, and organic farms, all of which emphasize the importance of safety. Katsuda [1] reported that natural pyrethrins showed a low development of resistance by flies and mosquitoes compared with many synthetic pyrethroids, against which a high development of cross-resistance was observed. 

While the mechanism of resistance to various synthetic pyrethroids in flies has been elucidated in terms of physiology, biochemistry, and genetics, it seems that the resistance mechanism is mostly common to mosquitoes. 

Mainly against the Group III mosquitoes, Katsuda et al. investigated the efficacy of mosquito coils containing various pyrethroids. In the 25-m3 semi-field test shown in Table 9, mosquito coils with d.d-Y-prallethrin at concentrations of 0.1-0.15% plus a synergist, /V-(2-ethylhexyl)bicyclo 2.2.1 -hept-5-ene-2,3-dicarboxyimide (39, Fig. 11), were effective even for the allethrin-resistant A. aegypti. 

A. aegypti colonies were found to have developed cross-resistance to even polyfluoro benzylalcohol ester pyrethroids with potent insecticidal activity. Mosquito coils of these compounds were effective against allethrin-susceptible A. aegypti colonies at ultra-low concentration, but needed several times higher concentrations for A. aegypti colonies in Group III in Table 8 (unpublished). 

Cross-resistance to pyrethroids for outdoor use has developed markedly in M. domestica, mosquitoes, cockroaches, and so on however, it has also been found that natural pyrethrins as well as d-allethrin and prallethrin (ETOC ), which have very similar chemical structures and the same configuration as natural pyrethrins, show an extremely low degree of cross-resistance development by these highly-resis-tant sanitary pests compared to photostable pyrethroids. Many novel synthetic pyrethroids recently developed as household insecticides have tended to pursue efficacy improvements in terms of rapid knock-down effects, residual efficacy or volatility. 

Only countries with political clout and political sense about DDT, notably South Africa, China, and India, still use it. South Africa had stopped using DDT in 1996 under pressure to join the world s Green community and switched to the next best alternative, the synthetic pyrethroids, which are three times the price and are effective over a shorter time span. Four years later, in 2000, South Africa decided to resume DDT spraying after malaria cases jumped by 1,000 percent because of mosquito resistance to the synthetic pyrethroids. 

Carboxylesterases are involved in resistance to ester-containing insecticides such as organophosphate, carbamate, and pyrethroid insecticides. Resistance to organophosphate insecticides caused by enhanced carboxylesterase activity has been demonstrated in numerous insects and mites, including the mosquito (Cidex tarsalis, Culex pipiens, and... 

Esterasss. Esterases are a major mechanism of resistance to organophosphates and in certain cases may also contribute resistance toward carbamates and certain pyrethroids. The role of esterases in resistance in mosquitoes and aphids has been the subject of especially fruitful research during the past few years. In collaborative studies between our laboratory and French laboratories in Montpellier, Antibes and Pau, several electrophoretic forms of esterases A and B have been identified in mosquitoes (Figure 5). Esterases Ai, A2, A4, Bi, B2, and B4 are found in the Culex pipiens complex, i.e. C. pipiens and C. quinquefasciatus. Two other forms, esterases A3 and B3, are present in C. tarsalis, and still others (not yet named) in Aedes aegypti and Ae. nigromaculis from California, C. tritaeniorhynchus from Japan (22) and various Culex species from Mexico. 

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