Mechanisms of pyrethroid resistance in Haematobia irritans (Muscidae) from Mato Grosso do Sul state, Brazil Mecanismos de resistência da Haematobia irritans (Muscidae) a piretróides em Mato Grosso do Sul, Brasil

Horn fly resistance to pyrethroid insecticides occurs throughout Brazil, but knowledge about the involved mechanisms is still in an incipient stage. This survey was aimed to identify the mechanisms of horn fly resistance to cypermethrin in Mato Grosso do Sul state, Brazil. Impregnated filter paper bioassays using cypermethrin, synergized or not with piperonyl butoxide (PBO) and triphenyl phosphate (TPP), were conducted from March 2004 to June 2005 in horn fly populations (n = 33) from all over the state. All populations were highly resistant to cypermethrin, with resistance factors (RF) ranging from 89.4 to 1,020.6. Polymerase chain reaction (PCR) assays to detect the knockdown resistance (kdr) mutation also were performed in 16 samples. The kdr mutation was found in 75% of the tested populations, mostly with relatively low frequencies (<20%), and was absent in some highly resistant populations. Addition of TPP did not significantly reduce the LC50 in any population. However, PBO reduced LC50s above 40-fold in all tested populations, resulting in RFs &#8804; 10 in most cases. Horn fly resistance to cypermethrin is widespread in the state, being primarily caused by an enhanced activity of P450 mono-oxygenases and secondarily by reduced target site sensitivity.<br>Resist&#234;ncia da mosca-dos-chifres a inseticidas piretr&#243;ides ocorre em todo o pa&#237;s, entretanto, o conhecimento sobre os mecanismos envolvidos &#233; ainda incipiente. Este estudo objetivou identificar os mecanismos de resist&#234;ncia desta mosca &#224; cipermetrina em Mato Grosso do Sul. Bioensaios utilizando pap&#233;is impregnados com cipermetrina, isoladamente ou sinergizada por but&#243;xido de piperonila (PBO) ou trifenil fosfato (TPP), foram realizados de mar&#231;o&#8725;2004 a junho&#8725;2005 em 33 popula&#231;&#245;es. Todas as popula&#231;&#245;es apresentaram elevada resist&#234;ncia &#224; cipermetrina, com fatores de resist&#234;ncia (FR) variando de 89,4 a 1.020,6. Ensaios de rea&#231;&#227;o em cadeia da polimerase (PCR) visando a detec&#231;&#227;o de kdr (&#8220;knockdown resistance&#8221;) foram realizados em 16 amostras. A muta&#231;&#227;o kdr foi detectada em 75% das popula&#231;&#245;es, geralmente em baixas frequ&#234;ncias (<20%) e ausente em algumas popula&#231;&#245;es resistentes. A adi&#231;&#227;o de TPP n&#227;o reduziu significativamente a CL50 em nenhuma popula&#231;&#227;o. Entretanto, o PBO reduziu em mais de 40 vezes a CL50 de todas as popula&#231;&#245;es testadas, resultando em FR &#8804; 10 na maioria dos casos. Resist&#234;ncia da mosca-dos-chifres &#224; cipermetrina encontra-se disseminada no estado, sendo causada primariamente por um aumento da atividade de P450 mono-oxigenases e secundariamente pela redu&#231;&#227;o da sensibilidade do s&#237;tio de a&#231;&#227;o do inseticida.