2.4. Studies on Larvicidal Activity of Temephos in Veracruz State

The temephos treatment of used tires did not control Aedes spp. larvae and pupae, suggesting that the granular temephos preparation obtained from the Secretaria de Salud had lost insecticidal activity or that the mosquito populations were no longer susceptible to this compound. To address this issue, a sample of the temephos preparation that this study had used was tested against a susceptible strain of Ae. aegypti (Rockefeller strain) using a colony maintained at the Centro de Investigación en Salud Pública (INSP) in Tapachula, Chiapas, Mexico, and reared following the methods described elsewhere [35]. For this, groups of 100 third instars were placed in plastic trays 35 × 25 × 5 cm that contained 1 L dechlorinated water and the quantity of temephos granules recommended for use as a larvicide (100 mg/L, equivalent to 1 mg a.i./L) [34]. The insects in the control trays were treated identically but without the presence of temephos granules. The experiment was performed with 16 trays of temephos-treated larvae and 4 trays of control larvae. After 24 h, the mortality of larvae in the treated and control trays was noted.

A brief field study was then performed in the state of Veracruz. For this, 2 L of dechlorinated water in which a small quantity of dried grass stalks had been soaked for one week was placed in each of six plastic 4-L water containers painted black on the exterior surface. The containers were placed in shaded sites in the back yard of houses of persons known to us at 11 sites: Three in the North, three in the South and five in the central region of the State, at altitudes between 5 and 1200 m above sea level (Supplementary Material, Table S1). After two weeks, the traps were examined for the presence of larvae and pupae. Three of the six containers at each site were selected at random and treated with 100 mg/L of temephos granules (1 mg a.i./L) while the other containers remained as untreated controls. Two weeks after treatment, the traps were re-examined and larvae and pupae were counted and recorded.