Previous unpublished results from 2 open-field studies in South Florida evaluated the effectiveness of 4 ultra-low volume malathion formulations, applied by ground application against a USDA (Gainesville) laboratory-colonized strain of Aedes albopictus. Field results of the bioassays indicated that the species was highly resistant to malathion compared with other insecticide-susceptible mosquito species concurrently tested. Unfortunately, when the Ae. albopictus colony was initially started in 2003 from a field collection in Gainesville, FL, susceptibility to insecticides was not determined. Since that time, this species has been in constant production without exposure to pesticides by the Center for Medical, Agricultural, and Veterinary Entomology, Gainesville, FL. In order to determine its susceptibility level to malathion and compare it with 3 current field-collected Ae. albopictus populations, 3 organophosphates (malathion, chlorpyrifos, naled) and 2 pyrethroids (permethrin, deltamethrin) were tested in the laboratory, using the Centers for Disease Control and Prevention bottle bioassays. Results of the study confirmed that the USDA-Gainesville Ae. albopictus was still highly resistant to malathion even after being maintained in the laboratory for 16 years without exposure to pesticides. The 3 field-collected populations were also resistant to malathion but slightly less so than the USDA-Gainesville population. Rotation of pyrethroid products 16 years ago did not result in any significant reduction of malathion resistance in the field populations. Neither the USDA-Gainesville nor the field-collected Ae. albopictus populations showed cross-resistance to chlorpyrifos and naled. Lower mortalities were observed in the 10-min diagnostic time from the 3 field-collected populations against permethrin but rose significantly at 15-min exposure. Resistance was not detected for deltamethrin in any of the 4 Gainesville Ae. albopictus populations.
Keywords: Chlorpyrifos; cross-resistance; deltamethrin; insecticide resistance; naled; permethrin; rotation.
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