Bioefficacy of lion’s ear (leonotis nepetifolia) and african basil (ocimum gratissimum) extracts against two-spotted spider mite (tetranychus urticae) on french beans (phaseolus vulgaris l)

Abstract

French bean is a major export vegetable crop in Kenya that is currently facing a steady decline in production. Two-spotted spider mite, (Tetranychus urticae Koch) is one of the major pests that has contributed to the fluctuations in French beans production. The conventional management of this pest using miticides has caused serious environmental and safety concerns particulary in the export market. Plant-based products provide safe alternatives that can be included in integrated pest management systems. The objectives of this study were to determine; (i) miticidal, oviposition and repellent effects of Leonotis nepetifolia and Ocimum gratissimum plant extracts on spider mites (ii) effect of L. nepetifolia and O. gratissimum plant extracts on quality and yield of French beans. Field and laboratory bioassays were conducted at Egerton University using four concentrations (1.5, 3.0, 6.0 and 12.0 % w/v) of each plant extract in a randomized complete block design and completely randomized design, respectively. Field experiments were replicated three times and laboratory experiments four times. Abamectin (0.6 ml/L) was used as negative while methanol and water were used as positive controls in the laboratory and the field respectively. Laboratory data on mortality, repellence and oviposition were collected 24, 48 and 72 hours after treatment. Field data were collected on spider mite population which was expressed as percent corrected efficacy, percent leaf damage, pod number, pod length, pod diameter and yield. The data was subjected to analysis of variance using SAS. Data on counts were normalized using arcsine data transformation before analysis. Means were separated using Tukey’s HSD test (P < 0.05). Laboratory bioassay results showed concentration- and exposure time-dependent with increase in efficacy of L. nepetifolia and O. gratissimum extracts against adult mite. At 12% w/v and 72h, L. nepetifolia and O. gratissimum extracts produced 93.8 and 96.4% mortality of adult two spotted spider mites, respectively. Both extracts produced 100% reduction in the number of eggs laid. The plant extracts showed 93.8% and 100% repellence respectively at 72 hours exposure time. Abamectin had 43.5 and 45.3 % mortality, and 34.4 and 62.9 % reduction in oviposition, respectively. Results from the field experiments also showed a concentration dependent effect of the plant extracts on mite population, leaf damage, and yield of French beans. Abamectin activity on mite populations was however higher than L. nepetifolia and O. gratissimum plant extracts at all concentration levels except at 12.0% w/v (82.75 and 69.06 %) mite reduction repectively during the first season. The plant extracts indicated lower leaf reduction and higher pod yield, compared to abamectin at all concentrations. These findings demonstrate the potential use of L. nepetifolia and O. gratissimum extracts in the management of two-spotted spider mite on French beans.