Scientific validation of plant extracts used by farmers in the management of bean flower thrips on French beans in Kenya

Abstract

Bean flower thrips (Megalurothrips usitatus) is a major French bean (Phaseolus vulgaris L.) pest. Small‐scale farmers manage the pest using mixed plant extracts although their efficacy has not been scientifically validated. We evaluated the efficacy of mixed plant extracts comprising; Capsicum frutescens, Allium sativum, Lantana camara, Tagetes minuta and Azadirachta indica, against M. usitatus under laboratory and screenhouse. We identified and quantified the secondary metabolites associated with insecticidal activity using spectrophotometry and liquid chromatography–mass spectrometry (LC–MS). The plant combinations included PE1 (C. frutescens + A. sativum + L. camara + T. minuta extracts infused for 14 days), PE2 (same as PE1 but infused for 24 h) and PE + N (the five plant extracts infused for 24 h) in distilled water. We used an organic commercial botanical (Pyneem) as a positive control and distilled water as a negative control. Pyneem and PE + N induced the highest mortality at 88% and 77%, respectively, in the laboratory, and 68% and 71%, respectively, in the screenhouse. Phenolics, terpenoids and organosulfur compounds were identified in PE + N and individual plant extracts in varied quantities. These compounds were significantly higher (p < 0.001) in PE + N compared to individual plant extracts. The study showed that PE + N efficiently manages bean flower thrips, and mixing different plant extracts amplifies the secondary metabolites' abundance. The use of mixed plant extracts could be incorporated into integrated pest management strategies for thrips management in legumes. The specific compounds identified in PE + N should be investigated further to understand their modes of action against the pest.