A baseline of organophosphate susceptibility/resistance in Trinidad and Tobago strains of the dengue fever vector Aedes aegypti (Linnaeus)

OBJECTIVE: To establish a baseline of organophosphate susceptibility/resistance in Trinidad and Tobago strains of 4th instar Aedes aegypti. DESIGN AND METHODS: Twenty-one strains of Aedes aegypti (Linneaus) were collected throughout Trinidad and Tobago and 4th instar larvae were bioassayed for sensitivity to five organophosphate insecticides, temephos, malathion, fenthion, fenitrothion and bromophos. The data were computer probit analyzed, resistance ratios [RR] were calculated and statistical analysis was done to determine any incidence of cross-resistance. RESULTS: Low to moderate levels of larval resistance to tenephos [RR 0.8-6.3], malathion [RR 0.8-3.8], fenthion [RR 0.7-6.7], fenitrothion [RR 0.7-3.] and bromophos [RR 0.9-6.4] were observed. Significant levels of cross-resistance were found between pairs of insecticides as follows: temephos/fenthion [rsq = 0.6219]; temephos/fenitrothion [rsq = 0.6117]; fenthion/malathion [rsq = 0.5751]; fenthion/bromophos. [rsq = 0.7255] and fenthion/fenitrothion [rsq = 0.6111]. CONCLUSIONS: The generally low levels of organophosphate insecticide resistance shown in this study indicate that these chemicals can continue to be used in the fight against Ae aegypti and dengue, while we continue to monitor trends in changing resistance patterns.(AU)

Insecticide resistance in some Caribbean strains of the dengue vector aedes aegypti and its possible impact on chemical control of the mosquito - abstract

When 35 strains of Aedes aegypti from 17 different Caribbean countries bioassayed for their sensitivity to temephos, malathion, fenitrothion and fenthion, larval populations showed varying levels of resistance to the insecticides. There were fairly high levels of resistance in Tortola (18 - 20-fold resistance) and Antigua (5 - 13-fold resistance) strains to temephos and to fenthion (Tortola, 21 - 24-fold; Antigua; 10 - 17-fold resistance). Most other strains showed some resistance, but only moderate levels to malathion and fenitrothion. All of these chemicals can still be used as larvicides in vector control, but as an adjunct to the environmental modification in source reduction. Larvicides against which there were highest levels of resistance for a particular strain should be changed. Adult populations of Ae.aegypti- Aruba, Jamaica, Trinidad, Puerto Rico and Antigua strains - also only showed moderate resistance to malathion. Practical studies in mosquito control in the field supported the laboratory findings. Doubling the diagnostic dosage of temephos for Ae.aegypti was only partially effective against a more resistant strain and, even so, the chemical lost its limited efficacy over a very short period of time (AU)