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Abstract
The insecticidal activity biological, histological and biochemical effects of a bacterial bioagent spinosad and an insect growth regulator tebufenozide were evaluated on larvae of Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae).
Second instar larvae were more sensitive to spinosad than 4th instar larvae as LC50 was 5.73 and 7.83 ppm, respectively. Meanwhile, LC90 was 37.15 and 43.49 ppm for the respective mentioned instars. Tebufenozide also had a greater larvicidal activity on 2nd instars as LC50 and LC90 were 0.316 and 1.6 ppm. Meanwhile, these values were 0.35 and 2.06 ppm for 4th instar larvae.
The duration of the subsequent instars, from the initial treated one was lengthened by approximately two days when either 2nd or 4th instars S. littoralis larvae were treated with either spinosad or tebufenozide. Percentage of larvae entering the pupal stage was nearly half the value of the control (i.e. approximately 50 %) when either chemical was tested. Furthermore, pupal stage duration was shorter by 31 and 22.2 % in pupae treated as 2nd or 4th instar larvae with spinosad. Meanwhile, these percentages were 30 and 31.5 % for the respective mentioned instars treated with tebufenozide. Percentage of adult emergence was insignificantly affected. Several morphological malformations were induced in the subsequent larval instars as well as formed pupae and to a lesser extent in eclosed moths following treatment with either spinosad or tebufenozide. The percentage of their occurance was higher when the latter chemical was used.
Moths reproductive potential was impaired whether in moths both emerging from the treatment of larvae or those mated with untreated moths of the opposite sex. Egg fecundity was lowest when untreated females were mated with males eclosing from the treatment of either 2nd or 4th instar larvae with LC50 of spinosad, which was 28.8 and 43.1% less than their control. Meanwhile, treatment with tebufenozide showed that egg fecundity was lowest in the pairing of both female and male moths emerging from the treatment of 2nd instar larvae. Also, these deposited eggs exhibited the lowest egg viability i.e. 35.7%.
The histological structure in the integument of larvae surviving treatment of 4th instar larvae with LC50 tebufenozide showed disruption, mainly in the endocuticle. Tissues of the midgut in larvae treated as 4th instar larvae with either spinosad or tebufenozide 6 days following treatment exhibited several alternations. Vaculation in the epithelial layer, disruption of the peritrophic membrane as well as in the microvilli were observed. Ultrastructure studies showed that treatment by spinosad caused aberrations in some cell organelles mainly mitochondria, goblet cells, microvilli and nucleolus.
Six days following treatment of 4th instar larvae with LC50 of spinosad caused the activity of α-esterase, β-esterase, acetylcholineterase and total adenosine triphosphatases to be significantly reduced by 63.23, 66.99, 49.58 and 75.41 %, respectively, than their value in untreated insects. Meanwhile, when tebufenozide was tested, the specific activities of the previously mentioned selected enzymes were insignificantly affected. The activity of the glycoside hydrolases, amylase, invertase and trehalase were markedly significantly increased by109.54, 61.88 and 84.2 % more than their value in control insects when spinosad was treated. The percentage of the respective mentioned enzymes were significantly much higher when tebufenozide was tested as they increase by 215.91, 112.37 and 171.35 % respectively, than in untreated larvae.