Abstract
Egyptian cotton leafworm Spodoptera littoralis (Boisduval) is a dangerous pest of many field crops and vegetables in the world. The present study was conducted to evaluate the toxicity of Farnesol and its effects on the growth, development, and morphogenesis of this insect. The newly moulted larvae of 5th (penultimate) or 6th (last) instar larvae were fed on castor bean leaves previously treated with 7 concentrations of Farnesol (400, 200, 100, 50, 25, 12.5 & 6.25 ppm) for 24 hr. The most important results could be summarized as follows. After treatment of 5th or 6th instar larvae with Farnesol, various mortalities were recorded among larvae, pupae, and adults. Depending onLC50values, Farnesol exhibited stronger insecticidal activity after treatment of 6th instar larvae(LC50= 33.67 ppm) than after treatment of 5th instar larvae(LC50= 36.56 ppm). Farnesol caused a serious reduction of larval weight gain and deleterious regression of the growth rate. The larval and pupal durations had been remarkably prolonged, in a dose-dependent course. Disruption of the developmental program was recorded as a failure of ecdysis after treatment of 5th instar larvae and production of larval-pupal intermediates, regardless the treated larval instar. Farnesol exerted considerable suppressing action on the pupation. At higher concentrations, Farnesol interfered with the adult emergence, since eclosion was completely prevented at the highest concentration and partially blocked at other concentrations. Irrespective of the treated larval instar, some deformed pupae were developed only at higher two concentrations of Farnesol.
Highlights
Egyptian cotton leafworm Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) is a native pest to Africa (Shonouda and Osman, 2000; El-Khawas and Abd El-Gawad, 2002), it is distributed in many European countries (Pineda et al, 2007; Lanzoni et al, 2012; EPPO, 2019), Asia Minor (Brown and Dewhurst, 1975) and the Middle East countries (El-Aswad, 2007; El-Sabrout, 2013; Azzouz et al, 2014)
Insecticidal Activity of Farnesol Against S. littoralis: After treatment of newly moulted penultimate (5th) instar larvae of S. littoralis with 7 concentration levels of Farnesol, data assorted in Table (1) revealed that Farnesol exhibited toxicity on the treated larvae only at the higher three concentration levels (30.0, 10.0 & 10.0% larval mortalities, at 400, 200 & 100 ppm, respectively, compared to 0.0% mortality of control larvae)
Depending on data listed in the same table, Farnesol exhibited chronic toxicity on the successfully developed pupae only at the higher three concentration levels (100, 66.67 & 16.67% pupal mortality, at 200, 100 & 50 ppm, respectively, vs. 0.0% mortality of control pupae), because no pupae developed after treatment with the highest concentration level (400 ppm)
Summary
Egyptian cotton leafworm Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) is a native pest to Africa (Shonouda and Osman, 2000; El-Khawas and Abd El-Gawad, 2002), it is distributed in many European countries (Pineda et al, 2007; Lanzoni et al, 2012; EPPO, 2019), Asia Minor (Brown and Dewhurst, 1975) and the Middle East countries (El-Aswad, 2007; El-Sabrout, 2013; Azzouz et al, 2014). Some cultural and phytosanitary measures have been carried out, such as cultivating resistant plant varieties (Isman, 2002; Bavaresco et al, 2004; Khedr et al, 2015) and treatment with compost tea (Ibrahim et al, 2016) These measures have been applied, no satisfactory results can be achieved for controlling this pest, most farmers, prefer using chemically synthetic pesticides for obtaining fast results (Sallam et al, 2000; Temerak, 2002; Abd El-Mageed and Shalaby, 2011; Ghoneim et al, 2012; Fetoh et al, 2015), such as some organophosphates, carbamates, organochlorines and synthetic Pyrethroids (Abo-Elghar et al, 1980; Radwan et al, 1985; Abd-el-Aziz and Sayed, 2014)
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