lethal effects of pyriproxyfen, spinosad, and indoxacarb and sublethal effects of pyriproxyfen on the 1st instars larvae of beet armyworm, spodoptera exigua hübner (lepidoptera: noctuidae) in the laboratory

the beet armyworm (spodoptera exigua hübner) is an important pest of many agricultural crops all over the world. most of the sugar beet growing regions in iran are infested. in this study, the acute lethal effects of pyriproxyfen, spinosad, and indoxacarb as well as sublethal effects of pyriproxyfen on the 1st instar s. exigua were assessed by leaf dip bioassay method. mortality was recorded 48 hours after treatment. lc50 and lc90 values for spinosad were 0.096 and 0.252 mg ai l-1, respectively, and for indoxacarb, they were 2.510 and 38.828 mg ai l-1, respectively. the lc50 value for spinosad was 26 times lower than that of indoxacarb. preliminary experiments revealed that pyriproxyfen did not cause acutely lethal effects on the beet armyworm larvae even following exposure at recommended doses. pyriproxyfen, however, did show considerable delayed effects against this pest. significant differences in biological, reproductive, and population growth parameters were found in pyriproxyfen treated insects in comparison with the control insects. population growth parameters including net reproduction rate (r0), gross reproduction rate (grr), intrinsic rate of population increase (rm), and finite rate of population increase (λ) were reduced by 14.7-, 6.63-, 2.33-, and 1.09-fold, respectively, following pyriproxyfen exposure. additionally, mean generation time (t) and doubling time (dt) of the population were increased by 1.12- and 2.3-fold, respectively.