Determination of insecticidal and seed protective properties of pawpaw extracts against bean weevils (acanthoscelides obtectus) in storage

Abstract

The study was an experiment carried out at the Nutrition and Bio-analytical laboratory at NaCRRI, Uganda from November 2016 to October 2017. The aim of the study was to investigate the efficacy of pawpaw leaf powder and liquid extracts in seed protection against bean weevils in storage over a time period. The specific objectives were; to determine the concentration of the secondary metabolites in pawpaw liquid extracts and pawpaw leaf powder; determine the effectiveness of pawpaw leaf powder, water and ethanol extracts in the control of bean weevils and to establish the effect of pawpaw leaf powder on seed germinability. Methods used in the study were; phyto-chemical screening of the secondary metabolites such as total tannins, total phenolics, total flavonoids, total alkaloids and total anti-oxidant activity. Pawpaw leaf extracts were prepared in water and ethanol at concentrations of 5%, 10%, 15% and 0% (control) where no treatment was applied. Pawpaw leaf powder was also applied at varying rates of 5%, 10% and 15% (w/w of sample) to bean seeds of NABE 6 cultivar. Pyto-chemical screening results revealed that the highest concentration of secondary metabolites in extracts was for total anti oxidants at 5.5% in the pawpaw leaf accession M-5 which was significantly higher (P<0.05) than in other accessions. Powder treatment had the highest concentration of total anti-oxidants at 11.6% in M-2 followed by total tannins at 5.2%. Generally, pawpaw leaf powder had a higher concentration of secondary metabolites in the different pawpaw leaf accessions as compared to the liquid extracts. Pawpaw leaf powder provided the most effective control measure of bean weevils in bean storage. Pawpaw leaf powder significantly (P<0.05) reduced on the number of damaged seeds, the percentage weight loss of bean seeds, insect mortality, F1 progeny emergence. The same trends were observed from week one up to week 12 of the experimental set up. Pawpaw water extracts had the least effect in the control of weevils. Pawpaw leaf ethanol extract also had a significantly higher (P<0.05) mortality rate at 15% concentration as compared to other treatment levels. Pawpaw leaf powder did not significantly reduce on the germination percentage of the different bean varieties (P<0.05). It was discovered that the effect of pawpaw leaf powder water and ethanol extract in the control of bean weevil were doze dependent with 10% being the most ideal for powder extracts. The effectiveness of pawpaw leaf extracts in the control of bean weevils is attributed to the higher concentration of the different secondary metabolites in the leaves. The study demonstrated that pawpaw leaves could act as cost effective bio pesticides with potential to substitute synthetic insecticides for the control of bean weevils in stored beans