Toxic effect of Spirulina platensis and Sargassum vulgar as natural pesticides on survival and biological characteristics of cotton leaf worm Spodoptera littoralis

Abstract

In a trial to explore new alternatives to chemical pesticides whose negative side effects on humans and the environment are a serious concern, natural alternatives are being addressed to discover new, cost-effective and environmentally friendly compounds.; therefore, present work aimed to evaluate the toxicity of two algal species; Spirulina platensis (blue-green micro alga) and Sargassaum vulgar (brown marine macro alga) as natural pesticides against cotton leaf worm Spodoptera littoralis (Lepidoptera: Noctuidae). Water and ethanol extracts of tested algae prepared in three concentrations (3, 5 and 7 %). S. platensis extracts exhibited higher toxicity against 2nd and 4th larval instars, 2nd instar larvae were more susceptible to all treatments. Egg hatchability was slightly affected, larval and pupal duration prolonged with increasing extracts concentration. Ethanol extracts were more toxic especially at 7% concentration. Pupal malformation of 2nd larval stage was more affected than 4th one, similarly moth malformation of 2nd larval instar reached 59 and 43 % when treated with alcohol extract (7%) of S. platensis and S. vulgar respectively, while for 4th larval instar, adult malformation reached 33 and 29 % respectively. Female fecundity decreased with increasing concentration of algae extracts. Abnormal morphological changes observed during insect development. To evaluate insecticidal activity of examined algae, certain phytochemicals were screened in algae biomass including, total phenols, total tannins and total alkaloids; the highest percentages were recorded for total phenols followed by total tannins then total alkaloids in both algae, with more elevated levels in S. platensis biomass. Further investigations on phenolic constituents in S. platensis were done through HPLC analysis, indicating the occurrence of four phenolic compounds; quercetin (214.06 ppm), kaempferol (126.45 ppm), resorcinol (27.02 ppm) and Naphthaline (2.42 ppm). Study results revealed the efficacy of tested algae especially S. platensis as a potential source of natural pesticides to control mentioned pest.