Abstract
Knowledge of how mosquitoes respond to insecticides is of paramount importance in understanding how an insecticide functions to prevent disease transmission. A suite of laboratory assays was used to quantitatively characterize mosquito responses to toxic, contact irritant, and non-contact spatial repellent actions of standard insecticides. Highly replicated tests of these compounds over a range of concentrations proved that all were toxic, some were contact irritants, and even fewer were non-contact repellents. Of many chemicals tested, three were selected for testing in experimental huts to confirm that chemical actions documented in laboratory tests are also expressed in the field. The laboratory tests showed the primary action of DDT is repellent, alphacypermethrin is irritant, and dieldrin is only toxic. These tests were followed with hut studies in Thailand against marked-released populations. DDT exhibited a highly protective level of repellency that kept mosquitoes outside of huts. Alphacypermethrin did not keep mosquitoes out, but its strong irritant action caused them to prematurely exit the treated house. Dieldrin was highly toxic but showed no irritant or repellent action. Based on the combination of laboratory and confirmatory field data, we propose a new paradigm for classifying chemicals used for vector control according to how the chemicals actually function to prevent disease transmission inside houses. The new classification scheme will characterize chemicals on the basis of spatial repellent, contact irritant and toxic actions.
Highlights
Science and society label almost any chemical used against insects as an ‘‘insecticide.’’ By definition, an insecticide is a chemical that kills insects
Our findings from laboratory tests for contact irritancy showed that the percent of Ae. aegypti females escaping from treatment chambers was proportional to the dose of insecticide used
This study clearly showed that the primary indicators of chemical actions in huts were proportions repelled, proportions stimulated to prematurely exit, and proportions that died
Summary
Science and society label almost any chemical used against insects as an ‘‘insecticide.’’ By definition, an insecticide (insect-icide or insect-icidal) is a chemical that kills insects. This single term is not adequate for meaningful discourse about chemicals, chemical actions, insect responses to chemicals, and the different ways in which chemicals are used. This single response is the foundation for the old paradigm that classifies chemicals sprayed on house walls for malaria control based solely on their killing action. As will be shown by research presented here, this assumption is wrong
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