Spodoptera littoralis (Boisd.), is one of the most dangerous pests in Egypt, targeting many economically crucial crops. In addition, population growth is major challenge to agricultural production. The aim of the current study is evaluation of insecticidal properties of chloroform/ methanol extract prepared from Conyza dioscoridis aerial part as well as study the biochemical changes on the 4thlarval instars of the cotton leaf worm, S. littoralis under laboratory conditions. Moreover, the bioactive secondary metabolites were isolated from the tested extract by chromatographic techniques and identified using spectroscopic analysis. Also, the volatile constituents distilled from Conyza dioscoridis aerial parts were characterized by GC/MS analysis.
Column chromatography of bioactive chloroform/ methanol extract led to the identification of five major compounds including three triterpenes; β-amyrin-3-acetate, β-lupeol-3-acetate, β-amyrenone along with dotriacontane hydrocarbon as well as one methoxyflavone; 5,4ʹ-dihydroxy-6,7-dimethoxyflavone.
Insight on insecticidal efficacy, the percentage of cumulative mortalities during pupal and adult stages reached to 76.6% and 83.3%, respectively after feeding 4thlarval instars of S. littoralis on tested extract with insecticidal activity in a dose-dependent manner. Whereas, β-amyrenone, lupeol acetate and 5,4ʹ-dihydroxy-6,7-dimethoxyflavone suppressed 50, 60 and 73.3% of 4th instar larvae of S. littoralis at concentration 0.3, 0.5 and 0.5%, respectively. Also, the extract showed marked decreasing in acetyl cholinesterase activity, total lipids and protein contents.
The GC/MS analysis of volatile oil of C. dioscoridis aerial part led to the identification of 63 sensory metabolites that represent 93.68% of total volatile constituents. The oil was characterized by a high percentage of oxygenated sesquiterpenes (36.00%) and sesquiterpene hydrocarbons (21.09%), in addition phenylated and aliphatic hydrocarbons (15.43% and 14.58%, respectively). α-Cadinol was the major component (8.47%) accompanied by a high percentage of sesquiterpene alcohols.
C. dioscoridis aerial part is a promising botanical pesticide with different mechanisms of action targeting 4th instars of larvae of S. littoralis. It had adverse effects survival, fecundity, oviposition, pupae and adults development and also acts as metabolic and acetylcholine esterase inhibitors against S. littoralis. So, the present findings may encourage more applied researches to evaluate crude extract and/or tested compounds in semi-field and field trial for controlling S. littoralis.