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

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.


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. 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 [1]. French bean production in Kenya is mainly undertaken by smallholder farmers who constitute approximately 80% of all growers. Production of this crop is however 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 pest feeds on the plants through piercing and sucking cell contents. The resulting symptoms include tiny yellow or white speckles and bronzing of leaves [4] [5]. Severe mite infestations cause premature defoliation leading to reduced sugar content and drastic reduction in the crop yield [6] [7] and quality due to increased risk of pod damage from ultraviolet rays. 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 equally rapid decline of host plant quality [8].
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 [9] [10]. Synthetic acaricides have also caused serious problems such as pesticide resistance, environmental contamination, unacceptable pesticide residues in food and lethal effects on non-target organisms [11] [12]. Results from a study by [13] demonstrated variable levels of abamectin resistance in T. urticae populations.
These negative effects have resulted in the increasing interest for natural plant-based pesticides which might be safer, biodegradable and have shown low pest resistance [14].
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 [15].
Plant species have been found to contain natural deterrents which are toxic to various insect and mite pests [16] but safe to mammals. The extracts of Satoreja hortensis L. (Lamiaceae) were found to be toxic to TSSM [17]. Similarly the extracts from neem (Meliaceae), some species of solanaceae, Capparis aegyptia (Capparaceae), Nerium orleander L. (Apocynaceae) and Alianthus altissima L. 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.

Preparation of t Plant Extracts
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.

Mass Rearing of Two-Spotted Spider Mites
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 DOI: 10.4236/ae.2019. 71003 24 Advances in Entomology

Field Bioassay
The plant extracts were evaluated at different concentrations (1.5%, 3%, 6%, 12% 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.

TSSM Population Reduction
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 [29] %change in treated %change in control % corrected efficacy 100 %change in control

Percent Leaf Reduction
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 % change in treated % change in control % Leaf reduction 100 change in control

Number of Pods, Pod Length, Diameter and Yield
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.

Data Analysis
All data collected was subjected to analysis of variance (ANOVA) at P ≤ 0.05 using SAS statistical package [30].

Two Spotted Spider Mite Population Reduction
Results of the field bioassays indicated a dose dependent percent mite population reduction expressed as corrected percent efficacy during the two seasons (

Percent Leaf Reduction
The plant extracts concentrations 12% w/v, 6% w/v and 3% w/v showed signifi-

Advances in Entomology
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.

Conclusion
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.