Date Published: August 23, 2018
Publisher: Public Library of Science
Author(s): Souvic Sarker, Un Taek Lim, Miguel Lopez-Ferber.
Oriental fruit moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae), is an important pest of stone and pome fruits. Growers usually depend on chemical insecticides to control this pest, but demand for more environmentally-friendly means of controlling pests is increasing. At least 91 plant extracts have been reported to be effective against other lepidopterans, but their acute toxicity against G. molesta has rarely been studied. Among these 91 materials, we assessed the residual toxicity of 32 extracts against first instar larvae (< 5 h old) of G. molesta in the laboratory. Nicotiana tabacum L., used at the concentration of 2 mg/ml, showed the highest corrected mortality (92.0%) with a lethal time (LT50) value of 12.9 h. The extract was followed in its efficacy by Allium sativum L. (88.0%), Zanthoxylum piperitum (L.) De Candolle (70.0%), and Sapindus mukorossi Gaertner (65.0%), when mortality was assessed at 20 h after exposure. Against adult fruit moths (< 5 d old), N. tabacum also showed the highest corrected mortality among tested extracts, being 85 and 100% in adult females and males, respectively, at 168 h after exposure. However, there was no synergistic effect of the combined application of any of the top four extracts in either laboratory or greenhouse assays. Oviposition by G. molesta on peach twigs was reduced 85–90% when N. tabacum was applied at 4 ml/ twig compared to control (methanol), demonstrating that N. tabacum may have potential for use as a botanical insecticide against G. molesta.
Oriental fruit moth, Grapholita molesta (Busck) (Lepidoptera: Tortricidae), is a serious pest of fruit trees in the temperate regions, worldwide [1–4]. Its host range encompasses species within the family Rosaceae, mostly those from the genera Prunus and Pyrus . Stone fruit peach [Prunus persica L. (Rosales: Rosaceae)] is considered the primary host of G. molesta whereas the pome fruits pear [Pyrus communis L. (Rosales: Rosaceae)] and apple [Malus domestica L. (Rosales: Rosaceae)] are considered secondary hosts .
The synthetic pesticide λ-cyhalothrin was more toxic than any of plant extracts to first instar larvae. Based on the comparison of plant extract LT50 values to that of λ-cyhalothrin, we selected N. tabacum, A. sativum, Z. piperitum, and S. mukorossi as the most effective botanical extracts for control of first instar larvae of G. molesta. Although the highest mortality was observed in larval stage of G. molesta from N. tabacum treatment, for both adult males and females N. tabacum and A. sativum were equally effective in a subsequent assay. Nicotiana tabacum has several modes of action. It can be a nerve poison [27, 28], stomach poison, or repellent . Baskaran and Narayanasamy  found N. tabacum to be effective against aphids, thrips, psyllids, tingids, beetles, sawflies, and lepidopterans. Evaluation of N. tabacum against G. molesta has been made here for the first time. In addition, N. tabacum is easy to apply in the field. Amoabeng et al.  ground N. tabacum leaves in tap water containing 0.1% Sunlight® detergent solution and sieved them through fine linen for immediate application to a cabbage field. This preparation resulted in 93.0% reduction of Plutella xylostella larvae, while λ-cyhalothrin reduced the same population by only 51.0%. The best efficacy was recorded with the extract of N. tabacum against Cydia molesta Busch. (98.3%) and Anarsia lineatella Zell. (99.0%) . Vandenborre et al.  found that a jasmonate-inducible lectin named NICTABA present in tobacco leaf is responsible for the larval mortality of lepidopteran insects. Nevertheless, a major active compound of N. tabacum was nicotine, which mimics acetylcholine and activates the nicotinic acetylcholine receptor causing an influx of sodium ions to flood the receptor . Methanolic extracts of A. sativum have also caused mortality of 81.0% against Spodoptera litura . A constituent of the A. sativum extract, alliin (derived from the amino acid cysteine) is converted by an enzyme to allicin, which is believed to act as an antifeedant, repellent, and insecticide .