Water turnover limits the duration of residual effects of the larvicide temephos for Aedes aegypti (Diptera: Culicidae) control in Northern Argentina.

GARELLI FM; WEINBERG D; ESPINOZA M; TRIVELLI MA; GURTLER RE

PLOS NEGLECTED TROPICAL DISEASES

PUBLIC LIBRARY SCIENCE

Lugar: San Francisco; Año: 2011 vol. 5 p. 991 - 991

1935-2735

Background: A five-year citywide control program based on regular application of temephos significantly reduced Aedesaegypti larval indices but failed to maintain them below target levels in Clorinda, northern Argentina. Incompletesurveillance coverage and reduced residuality of temephos were held as the main putative causes limiting effectiveness ofcontrol actions.Methodology: The duration of temephos residual effects in household-owned water-holding tanks (the most productivecontainer type and main target for control) was estimated prospectively in two trials. Temephos was applied using spoonsor inside perforated small zip-lock bags. Water samples from the study tanks (including positive and negative controls) werecollected weekly and subjected to larval mortality bioassays. Water turnover was estimated quantitatively by adding sodiumchloride to the study tanks and measuring its dilution 48 hs later.Principal Findings: The median duration of residual effects of temephos applied using spoons (2.4 weeks) was significantlylower than with zip-lock bags (3.4 weeks), and widely heterogeneous between tanks. Generalized estimating equationsmodels showed that bioassay larval mortality was strongly affected by water type and type of temephos applicationdepending on water type. Water type and water turnover were highly significantly associated. Tanks filled with piped waterhad high turnover rates and short-lasting residual effects, whereas tanks filled with rain water showed the opposite pattern.On average, larval infestations reappeared nine weeks post-treatment and seven weeks after estimated loss of residuality.Conclusions: Temephos residuality in the field was much shorter and more variable than expected. The main factor limitingtemephos residuality was fast water turnover, caused by householders’ practice of refilling tanks overnight to counteractthe intermittence of the local water supply. Limited field residuality of temephos accounts in part for the inability of thelarval control program to further reduce infestation levels with a treatment cycle period of 3 or 4 months.