Resistance of Aedes aegypti to temephos and adaptive disadvantages

Abstract

OBJECTIVE To evaluate the resistance of Aedes aegypti to temephos Fersol 1G (temephos 1% w/w) associated with the adaptive disadvantage of insect populations in the absence of selection pressure.METHODS A diagnostic dose of 0.28 mg a.i./L and doses between 0.28 mg a.i./L and 1.40 mg a.i./L were used. Vector populations collected between 2007 and 2008 in the city of Campina Grande, state of Paraíba, were evaluated. To evaluate competition in the absence of selection pressure, insect populations with initial frequencies of 20.0%, 40.0%, 60.0%, and 80.0% resistant individuals were produced and subjected to the diagnostic dose for two months. Evaluation of the development of aquatic and adult stages allowed comparison of the life cycles in susceptible and resistant populations and construction of fertility life tables.RESULTS No mortality was observed in Ae. aegypti populations subjected to the diagnostic dose of 0.28 mg a.i./L. The decreased mortality observed in populations containing 20.0%, 40.0%, 60.0%, and 80.0% resistant insects indicates that temephos resistance is unstable in the absence of selection pressure. A comparison of the life cycles indicated differences in the duration and viability of the larval phase, but no differences were observed in embryo development, sex ratio, adult longevity, and number of eggs per female.CONCLUSIONS The fertility life table results indicated that some populations had reproductive disadvantages compared with the susceptible population in the absence of selection pressure, indicating the presence of a fitness cost in populations resistant to temephos.

Variation in temephos resistance in field populations of Aedes aegypti (Diptera: Culicidae) in the State of Sergipe, Northeast Brazil

“…aegypti populations in Sergipe, as well as in other areas in the country [5][6][7][8] , may be related to how temephos is used in each locality. It may be used at different intensities and frequencies, and the mosquito populations could be under different selection pressures.…”

“…Many natural products have been assessed with the goal of discovering new insecticide products with lower costs, lower toxicity, and better cross-acceptance between the population and public health workers 6 . Taking into account that the mechanisms of action for these products are not yet known, the possibility of cross-resistance should be considered.…”

“…Due to the continued use of temephos throughout the world, many populations of Ae. aegypti were reported to be resistant not only in Brazil [4][5][6][7][8][9][10][11] , but in several other countries [12][13][14][15] , as well. In 1999 and 2000, populations of Ae.…”

“…As a consequence of the high degree of resistance, the Brazilian Ministry of Health initially promoted its replacement by products based on Bacillus thuringiensis (Bti) in regions with reported resistance 16 ; these were subsequently replaced by growth inhibitory products, such as inhibitors of chitin synthesis and juvenile hormone analogs 22 . However, products used to replace temephos have the disadvantage of short residual effects 6 , which necessitates periodic reapplication, and they do not cause immediate mortality in larvae, but rather mortality occurs at the pupal stage or during the emergence of an adult 9 . This latter fact makes it necessary to modify the method of estimating mosquito infestation, as the House Index may be overestimated in treated areas 23 .…”

“…RR 50 and RR 99 were obtained by dividing the results for each population by the equivalent Rockefeller values. RR values between 3 and 5 were classified as low, 5 to 10 as moderate, between 10 and 20 as medium, and above 20 as high resistance 6 . The ratio of LC 99 to LC 50 for each population was calculated in order to verify heterogeneity of resistance.…”