Abstract

An experiment to evaluate the bio-control potential of Leonotis nepetifolia and Ocimum gratissimum plant extracts against two-spotted spider mites on French beans was conducted in the field. Five plant extract concentrations (1.5%, 3.0%, 6.0% and 12.0% w/v) were applied with water and Abamectin 0.6 ml/L as controls. Mite counts were done before and after treatment application and expressed as corrected percent efficacy. The impact of the mites on the French beans was evaluated by recording percent leaf reduction and quality and quantity by number of pods, pod length, diameter and yield. There was a dose dependent response in percent mite and leaf reduction, number of pods and yield. Treatments applied at 12% w/v indicated higher mite reduction (82.75%) for L. nepetifolia and 69.06% for O. gratissimum compared to abamectin (65.76%). The lowest percent leaf reduction of 1.71% for L. nepetifolia 0.39% for O. gratissimum and abamectin (20.46%) was also at 12% w/v. Similarly, the highest number of pod (61.00) for L. nepetifolia, 48.67 for O. gratissimum compared to 28.33 abamectin and yield (0.88 kg) for L. nepetifolia and 0.90 kg for O. gratissimum was also recorded at 12% w/v compared to 0.36 kg for abamectin. There were no significant differences in pod diameter and pod length between the extracts concentrations and abamectin. The study demonstrated the efficacy of L. nepetifolia and O. gratissimum in managing two-spotted spider mite and subsequent increase in French bean yield under field conditions.

Highlights

  • French bean (Phaseolus vulgaris L.) is among the most important horticultural crops grown and consumed worldwide

  • Treatments applied at 12% w/v indicated higher mite reduction (82.75%) for L. nepetifolia and 69.06% for O. gratissimum compared to abamectin (65.76%)

  • The highest number of pod (61.00) for L. nepetifolia, 48.67 for O. gratissimum compared to 28.33 abamectin and yield (0.88 kg) for L. nepetifolia and 0.90 kg for O. gratissimum was recorded at 12% w/v compared to 0.36 kg for abamectin

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Summary

Introduction

French bean (Phaseolus vulgaris L.) is among the most important horticultural crops grown and consumed worldwide. It is grown for its tender pods and shelled green or dry beans. French bean production in Kenya is mainly undertaken by smallholder farmers who constitute approximately 80% of all growers. Production of this crop is often undermined by major pests like spider mites, bean fly, white flies and aphids [2]. The two-spotted spider mite, Tetranychus urticae Koch (Acari; Tetranychidae), is one of the most important pests that attack French beans and other crops worldwide [3]. The rapid developmental rate, short generation time and high net reproductive rate of T. urticae allow them to achieve damaging population levels very quickly when growth conditions are good, resulting in an rapid decline of host plant quality [8]

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