Feeding Deterrence and Insecticidal Activity of Selected Plant Essential Oils Against the Larger Grain Borer Prostephanus truncatus in Stored Maize Grains

Abstract

In our screening program for botanical pesticides from local plants, the powders and essential oils from Plectranthus marrubioides (Hochst ex Benth), Tetradenia riparia (Hochst Codd), Ocimum suave (Wild), Lippia javanica (Burm, Spreng), and Ocimum lamiifollium (Hochst ex Benth) were found to possess insecticidal activity against the maize weevil, Prostephanus truncatus. The essential oils from aerial parts of the plants were obtained by hydro-distillation and tested under laboratory conditions (25 ± 1ºC, 70–75% R.H.) for their ability to protect the grains from damage by P. truncatus. The insects were reared and tested on whole maize grain (variety DK 8031). Bioassays were conducted using five dilution levels of essential oils in hexane (10, 5, 2.5, 1.25, and 0.625% wt/wt), each tested on ten unsexed adult large grain borers per replicate, with four replicates per treatment. The mortality data of the insects at each level of dilution were collected, and the mean values were computed and subjected to Student-Newman-Keuls (SNK) t-test. Essential oils were also tested as antifeedants of the insect in dried maize. It was observed that within the 0.625% level of dilution, the oil extracts showed a significant difference (p<0.05) in the antifeedant test done using the different plant essential oils. Of the five plant oils, P. marrubioides and T. riparia showed the highest insecticidal activity with the lowest mean of 0.55 ± 0.15 and 0.40 ± 0.11, respectively, as compared to O. lamiifolium (2.4 ± 0.11), O. suave (2.55 ± 0.11), and L. javanica (2.5 ± 0.15) after a period of six months. The chemical constituents of the two most potent oils were analyzed by GC-MS. The main compounds in P. marrubioides essential oil were δ-2-carene (10.84%), o-cymene (8.72%), 1,8-cineole (7.89%), and camphor (5.65%). T. riparia essential oil was found to be rich in fenchone (24.74%), β-pinene (5.23%), o-cymene (5.16%), and 1,8-cineole (10.89%). These results show that these plant oils, and particularly P. marrubioides and T. riparia, have the potential to protect dry stored grain products from damage by the larger grain borer. Consequently, the plant oils can be developed into post-harvest control agents of the larger grain borer in stored grain products.