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.
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. The crop is a rich source of important nutritional elements such as flavonoids, vitamin A, dietary fibres, potassium, folate, iron, magnesium, thiamine, riboflavin, copper, calcium, phosphorus, Omega-3 fatty acids and niacin [
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 [
Most of the smallholder farmers heavily rely on synthetic pesticides to manage the two spotted spider mite pest on French beans. Chemical pesticides used for spider mite suppression are usually weak acaricides and often do not perform well. Some of the active ingredients that have been used by smallholder farmers with little success include abamectin, spiromesifen, dicofol and chlorfenapyr [
Several studies have evaluated the potential of natural plant extracts to protect crops from insect and mite pest species such as whiteflies and spider mites [
Further studies by [
This study therefore sought to determine the miticidal activity of L. nepetifolia and O. gratissimum plants extracts against T. urticae and their subsequent influence on the quality and yield of French bean under field conditions.
Field experiments to evaluate the miticidal activity of L. nepetifolia and O. gratissimum methanolic crude plant extracts on the two spotted spider mite (T. urticae) were conducted at Horticulture Research and Teaching Farm of Egerton University situated in Nakuru county Kenya during the 2014 and 2015 growing seasons. The farm lies at a latitude of 0˚30'S, longitude 36˚30'E and an altitude of 2238m.The experimental site receives an annual rainfall of 1013 mm and the dominant soil type is mollic andosols.
Composite fresh leaves and tender stems of L. nepetifolia L. O. gratissimum were collected from fallow fields at Egerton University and the surrounding. The plant materials were dried in well ventilated room at 18˚C - 28˚C for two weeks. The dried leaves were ground into fine powder using an electric laboratory hammer mill, and subjected to methanol (100% AR) extraction at a rate of 200 gL−1. The extracts were kept in air tight containers refrigerated at 4˚C for use in the bioassays.
The two spotted mites were obtained from infested leaves of French bean plants which had not been sprayed with any acaricide. Rearing was done on 2 - 3 week old beans which were maintained in the greenhouse (25˚C ± 1˚C) and RH 65 ± 5 for use in the bioassays
The plant extracts were evaluated at different concentrations (1.5%, 3%, 6%, 12% w/v) in a Randomised Complete Block Design (RCBD) replicated three times. A synthetic acaricide (Abamectin 0.6 ml/L DW) was used as positive control and water as negative control. Experimental plots measuring 2 m × 2 m with a spacing of 0.5 m between and 0.2 within the rows were planted with certified French bean seeds (variety Teresa) according to recommendations for commercial French bean production. To avoid natural infestation, all plants in each plot were covered with nylon mesh size 0.4 by 0.5 mm and thread thickness of 0.1 mm at the primary leaf stage.
Twenty adult spider mites from greenhouse cultures were randomly introduced onto each bean plant at 21 days after planting using a fine hair brush. The plant extracts were applied as spray solutions using a hand held sprayer 14 days after mite infestation. A repeat treatment application was done after another 14 days. Six plants from the two middle rows in each plot were randomly selected and tagged for data collection.
The population of the mites was assessed three days before treatment application by counting the number of adult mites from the underside of leaves from the six tagged plants in each plot. A second mite population count was done at 72 h after the second treatment application. The corrected percent efficacy of the plant extracts was then calculated according to Sun-shepard formula [
%corrected efficacy = %change in treated − %change in control 100 − %change in control (1)
This was done by counting the number of leaves on the six tagged plants in each plot before treatment and after the second treatment application. The change in number of leaves constituted the damage by TSSM which was calculated as follows
%Leaf reduction = %change in treated − %change in control 100 − change in control (2)
This was done by hand plucking all immature green pods from the two middle rows. The number of pods was counted before measuring their pod length and diameter using a ruler and a veneer calliper respectively. The pods were then weighed when still fresh using an electric weighing balance.
All data collected was subjected to analysis of variance (ANOVA) at P ≤ 0.05 using SAS statistical package [
Results of the field bioassays indicated a dose dependent percent mite population reduction expressed as corrected percent efficacy during the two seasons (
The plant extracts concentrations 12% w/v, 6% w/v and 3% w/v showed significantly lower percent leaf reduction 1.71, 4.88% and 5.92% respectively for L. nepetifolia and 0.39%, 3.5% and 19.86% for O. gratissimum compared to abamectin (20.46%) in 2014 (
A higher number of pods was recorded in the plots that were treated with plant extracts compared to the synthetic acaricide (abamectin). L. nepetifolia plant extracted recorded 61.00, 48.33 pods at concentrations levels of 12% and 6% w/v followed by O. gratissimum plant extract which recorded 48.67 and 35.00 pods while abamectin recorded 28.33 pods during 2014 growing season (
Corrected percent efficacy | ||||
---|---|---|---|---|
Season 1 | Season 2 | |||
Rate (%w/v) | L. nepetifolia | O. gratissimum | L nepetifolia | O. gratissimum |
Control | −14.16 ± 1.35* | −1.42 ± 2.37* | −20.00 ± 2.35* | −2.34 ± 1.47* |
1.5 | 30.98 ± 0.00 | 40.50 ± 8.92 | 31.63 ± 9.12 | 44.17 ± 6.13 |
3.0 | 62.22 ± 7.90 | 45.19 ± 2.00 | 45.72 ± 7.30 | 46.34 ± 1.45 |
6.0 | 53.82 ± 5.77 | 57.57 ± 5.75 | 69.61 ± 6.06 | 57.85 ± 7.51 |
12.0 | 82.75 ± 6.13 | 69.06 ± 4.03 | 78.63 ± 10.38 | 77.56 ± 1.85 |
Abamectin (0.6 ml/L) | 65.76 ± 3.47 | 65.76 ± 3.47 | 69.05 ± 9.30 | 69.05 ± 9.29 |
*Means in a column whose standard error values do not overlap are significantly different at P ≤ 0.05.
Percent Leaf Reduction | ||||
---|---|---|---|---|
Season 1 | Season 2 | |||
Rate (%w/v) | L. nepetifolia | O. gratissimum | L nepetifolia | O. gratissimum |
Control | 48.49 ± 8.88* | 48.49 ± 8.88* | 53.63 ± 5.47* | 53.63 ± 5.47* |
1.5 | 14.69 ± 1.94 | 37.39 ± 9.49 | 30.31 ± 6.79 | 22.69 ± 5.27 |
3.0 | 5.92 ± 3.71 | 19.86 ± 5.51 | 21.72 ± 2.23 | 33.58 ± 2.58 |
6.0 | 4.88 ± 6.36 | 3.50 ± 4.89 | 5.09 ± 1.11 | 13.86 ± 7.77 |
12.0 | 1.71 ± 2.16 | 0.39 ± 0.67 | 0.58 ± 0.19 | 12.49 ± 1.02 |
Abamectin (0.6 ml/L) | 20.46 ± 8.09 | 20.46 ± 8.09 | 23.94 ± 7.56 | 23.94 ± 7.56 |
*Means in a column whose standard error values do not overlap are significantly different at P ≤ 0.05.
Treatment (% w/v) | Number of pods | Pod diameter | Pod length | Pod yield | ||||
---|---|---|---|---|---|---|---|---|
SEASON 1 | LN | OG | LN | OG | LN | OG | LN | OG |
Control | 12.67 ± 1.15* | 12.67 ± 1.15* | 0.50 ± 0.10* | 0.50 ± 0.10* | 7.83 ± 1.86* | 7.83 ± 1.86* | 0.16 ± 0.04* | 0.16 ± 0.04* |
1.5 | 17.33 ± 4.16 | 19.33 ± 1.53 | 0.63 ± 0.11 | 0.66 ± 0.10 | 10.83 ± 1.0 | 11.03 ± 0.42 | 0.39 ± 0.06 | 0.37 ± 0.03 |
3.0 | 37.00 ± 6.08 | 22.33 ± 1.15 | 0.67 ± 0.06 | 0.67 ± 0.31 | 11.57 ± 0.0 | 11.07 ± 1.83 | 0.52 ± 0.03 | 0.48 ± 0.02 |
6.0 | 48.33 ± 4.66 | 35.00 ± 3.23 | 0.73 ± 0.06 | 0.67 ± 0.12 | 11.57 ± 0.7 | 12.17 ± 0.42 | 0.65 ± 0.02 | 0.53 ± 0.05 |
12.0 | 61.00 ± 8.19 | 48.67 ± 5.51 | 0.80 ± 0.10 | 0.77 ± 0.06 | 11.50 ± 0.2 | 12.60 ± 0.17 | 0.88 ± 0.02 | 0.90 ± 0.03 |
Abamectin | 28.33 ± 2.58 | 28.33 ± 2.58 | 0.70 ± 0.17 | 0.70 ± 0.17 | 9.27 ± 1.44 | 9.27 ± 1.44 | 0.36 ± 0.02 | 0.36 ± 0.02 |
SEASON 2 | ||||||||
Control | 11.33 ± 3.79* | 11.33 ± 3.79* | 0.50 ± 0.10* | 0.53 ± 0.06* | 11.33 ± 1.3* | 0.16 ± 0.04* | 0.13 ± 0.11* | 0.13 ± 0.11* |
1.5 | 17.00 ± 1.00 | 12.33 ± 4.73 | 0.57 ± 0.06 | 0.60 ± 0.10 | 13.70 ± 1.5 | 0.37 ± 0.03 | 0.16 ± 0.12 | 0.37 ± 0.20 |
3.0 | 22.33 ± 2.08 | 19.33 ± 4.16 | 0.60 ± 0.10 | 0.60 ± 0.00 | 13.63 ± 1.7 | 12.10 ± 1.23 | 0.35 ± 0.18 | 0.19 ± 0.02 |
6.0 | 27.67 ± 1.53 | 31.00 ± 1.00 | 0.60 ± 0.00 | 0.60 ± 0.00 | 14.97 ± 1.0 | 15.00 ± 0.53 | 0.35 ± 0.05 | 0.37 ± 0.13 |
12.0 | 41.00 ± 3.11 | 40.67 ± 2.52 | 0.63 ± 0.06 | 0.67 ± 0.06 | 14.23 ± 1.6 | 14.77 ± 1.42 | 0.76 ± 0.06 | 0.86 ± 0.20 |
Abamectin (0.6 ml/L) | 18.00 ± 5.20 | 18.00 ± 5.20 | 0.63 ± 0.06 | 0.63 ± 0.06 | 12.60 ± 0.2 | 12.60 ± 0.72 | 0.25 ± 0.08 | 0.25 ± 0.08 |
*Means in a column whose standard error values do not overlap are significantly different at P ≤ 0.05.
and 27.67 pods at concentrations levels of 12% and 6% w/v followed by O. gratissimum which recorded 40.67 and 31.00 pods while abamectin recorded 18.00 pods.
Plant extracts applied at 12% concentration level also indicated the widest pod diameter of 0.80 cm and 0.77 cm for L. nepetifolia and O. gratissimum respectively in 2014. This was however not significantly different from abamectin which recorded a pod diameter of 0.70 cm. No significant differences in pod diameter was observed between the plant extracts and abamectin in 2015.
Although L. nepetifolia plant extracts caused a significant increase in in pod length compared to abamectin and the untreated control, there were no significant differences in pod length across the plant extracts applied either at low or a high concentrations in 2014 and 2015. O. gratissimum however recorded significantly longer pods at plant extract concentration levels of 6% and 12% w/v in 2014.
Results also showed significant differences in pod yields at different plant extract concentrations. The highest pod yield (0.88) was recorded at 12% w/v for L. nepetifolia and 0.90 for O. gratissimum in 2014 and 0.76 L. nepetifolia and 0.86 for O. gratissimum compared to 0.36 kg and 0.25 for abamectin in 2014 and 2015 respectively.
The plant extracts from L. nepetifolia and O. gratissimum demonstrated biological potency against the two spotted spider mites. Leaves, stems and roots extract of O. gratissimum have been found to contain potent bioactive components (essential oils) made up of eugenol and other compounds such as diterpenes, coumarins, iridoids, saponins, condensed tannins, flavonoids, alkaloids and steroids that have antioxidant and insecticidal properties [
The highest efficacy of the plant extracts in reducing TSSM populations was demonstrated at the highest plant extract concentrations of 12% w/v. This is consistent with studies by [
This study has thus demonstrated the efficacy of L. nepetifolia and O. gratissimum in managing two-spotted spider mite and subsequent increase French bean yield under field conditions.
This study illustrates the potential biological potency of L. nepetifolia and O. gratissimum plant extracts against the two-spotted spider mite on French beans. Yields obtained from the fields where the plant extracts had been applied were comparable to or even better than yields from the fields where the Abamectin acaride had been applied. This research has however shown that although French bean pod yield increased with increase in plant extract concentrations, pod quality parameters (diameter and length) do not appear to significantly improve with increase in plant extract concentrations. The study also suggested that L. nepetifolia and O. gratissimum may contain similar compounds with similar bioactivities in their plant extracts.
The authors declare no conflicts of interest regarding the publication of this paper.
Ogayo, K., Nyaanga, J., Ogweno, J. and Ogendo, J. (2019) The Effect of Lion’s Ear (Leonotis nepetifolia) and African Basil (Ocimum gratissimum) Plant Extracts on Two-Spotted Spider Mites (Tetranychus urticae) for Improved Yield and Quality of French Beans. Advances in Entomology, 7, 21-31. https://doi.org/10.4236/ae.2019.71003