Temephos-resistant larvae of Simulium sanctipauli associated with a distinctive new chromosome inversion in untreated rivers of south-western Ghana

Larvae of the Simulium damnosum Theobald complex (Diptera: Simuliidae) were sampled in June 1996 from two sites in south-west Ghana where larviciding has not been applied: Sutri Rapids on the Tano river (05 degrees 23'N 02 de grees 38'W) and Sekyere-Heman on the Pra river (05 degrees 11'N 01 degrees 35'W). All specimens were identified as Simulium sanctipauli Vajime & Dunba r sensu stricto (Diptera: Simuliidae). Bioassays with temephos (organophosp horus larvicide employed by the Onchocerciasis Programme for systematic tre atment of most rivers across West Africa since the 1970s) showed about five -fold resistance in the Tano population (LC95 2.37-3.14 mg/L) and slight to lerance to temephos in the Pra population (LC95 0.67-0.76 mg/L), vs. the di agnostic concentration of 0.625 mg/L. Larval salivary polytene chromosomes of S. sanctipauli showed fixed inversions 1S-24/24, standard IIL-6 and a ne w inversion IL/36 polymorphism at Sutri on the Tano. These karyotype charac teristics differ from those of temephos-resistant S. sanctipauli in rivers of Cote d'Ivoire and other sites on the Tano in Ghana. Thus, temephos resis tance in S, sanctipauli at Sutri is associated with distinct chromosomal co nfigurations, showing that immigration was unlikely. This resistance could have been locally selected by exposure of S. sanctipauli larval populations to agrochemicals run-off from cocoa, coffee and oil plantations flanking t he rivers.