A Culex quinquefasciatus Say colony was selected for 45 generations at LC70–90 levels using Natular XRG, a granular formulation of 2.5% spinosad for induction of spinosad resistance. Resistance to spinosad was noticed in early generations (F1–F9). Resistance levels increased gradually from generations F11–F35, and elevated significantly from generation F37 through F47. when resistance ratios reached 2,845–2,907-fold at LC50 and 11,948–22,928-fold at LC90 The spinosad-resistant Cx. quinquefasciatus colony was found not to be cross-resistant to Bacillus thuringiensis israelensis (Bti), a combination of Bti and Bacillus sphaericus, methoprene, pyriproxyfen, diflubenzuron, novaluron, temephos, or imidacloprid. However, it showed various levels of cross-resistance to B. sphaericus, spinetoram, abamectin, and fipronil. Conversely, a laboratory colony of Cx. quinquefasciatus that is highly resistant to B. sphaericus did not show cross-resistance to spinosad and spinetoram. Field-collected and laboratory-selected Cx. quinquefasciatus that showed low to moderate resistance to methoprene did not show cross-resistance to spinosad and spinetoram. Mechanisms of cross-resistance among several biorational pesticides were discussed according to their modes of actions.
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Vol. 51 • No. 2