Bio-efficacy of promising medicinal and aromatic plants against diamondback moth, Plutella xylostella (L.) on cabbage under field conditions

Introduction

Cabbage is an important cruciferous vegetable crop; however, its productivity is severely constrained by the diamondback moth (Plutella xylostella L.), one of the most destructive pests of cruciferous vegetables worldwide. The pest is capable of causing up to 90% loss in marketable yield of cabbage (Gashawbeza, 2006) [4] . At the farmer level, management of this pest relies predominantly on chemical insecticides. Unfortunately, the effectiveness of these chemicals has declined drastically due to the pest's rapid development of resistance to almost all major groups of insecticides, including biopesticides such as Bacillus thuringiensis and spinosyns (Tabashnik et al., 2003) [12] . As a result, satisfactory control has become increasingly difficult even with newer insecticide molecules. The continuous and indiscriminate use of synthetic insecticides has also led to several serious problems such as insecticide resistance, pest resurgence, environmental contamination and health hazards to farmers and consumers. These challenges have created an urgent need for pest management strategies that are both environmentally safe and economically feasible. Extensive literature indicates that many medicinal and aromatic plants possess a rich array of insecticidal active ingredients and bioactive chemical compounds. These plants offer several advantages for pest management, including easy availability, long-standing medicinal use and low or negligible toxicity to humans. Moreover, they exhibit a wide range of biological properties such as insecticidal, repellent, anti-feedant, growth inhibitory, ovipositiondeterrent, ovicidal and growth-disrupting effects against various insect pests due to the presence of diverse insecticidal principles (Liu et al., 2007) [8] . Unlike synthetic insecticides, botanical products are biodegradable, environmentally safe and generally harmless to nontarget organisms and natural enemies. Although many medicinal and aromatic plants have been reported to possess insecticidal properties, their field performance against diamondback moth on cabbage has not been adequately evaluated.

Therefore, the present investigation was undertaken to assess the bio-efficacy of selected promising medicinal and aromatic plant extracts against diamondback moth on cabbage under field conditions, with the aim of identifying effective and eco-friendly alternatives for sustainable pest management.

Materials and Methods

Preparation of plant extracts:

The plant extracts such as neem, pongamia, nagadali, tulasi, hebbevu, bael, datura and calatropis were selected for field evaluation against DBM on cabbage.

Preparation of Neem seed kernel extract (NSKE) and

Pongamia seed extracts: The kernels were separated from freshly collected neem seeds by pounding and in the case of pongamia, the seeds were separated. Neem seed kernels and pongamia seeds were ground in to a fine powder using an electrical grinder. 250 g of the fine powder was then mixed in a 750 ml of water. After mixing, the solution was stirred carefully until all the powder was mixed completely with the water. This solution was left overnight. Then the suspension was filtered through a muslin cloth and volume was made up to five litres by adding water. The concentration of the extract so prepared was five percent (Begna, 2017) [2] .

Preparation of leaf extracts of Nagadali, Tulasi, Hebbevu, Bael, Datura and Calatropis: Fresh leaves of Fresh leaves of these plants were collected a day before spraying. The extracts of these six plants were prepared by weighing 500 g of each plant separately and crushing into small pieces using a knife, then the chopped leaf was ground using an electrical grinder with little amount of water to make paste. The paste was soaked in 1000 ml of water and left overnight. The next day morning the solution was filtered through muslin cloth. At the time of application, the aqueous extract was diluted in 4 litres of water and used for field spray. The concentration of the extract so prepared was ten percent. Whereas in the case of datura and calotropis, the concentration maintained was five percent by adopting similar procedure (Oke et al., 2010) [9] .

Experimental details

The field experiment was carried out during summer season of 2019 (April-June) at a farmer's field, Nellakunte, Doddaballapur taluk, Bengaluru Rural district. The experiment was laid out in a randomized block design with ten treatments and three replications. The treatment plot size was 2 m × 4 m. The cabbage crop (Unnathi) was raised as per the recommended package of practices of the University of Horticultural Sciences, Bagalkot. The botanical extracts and insecticides were applied by using a high-volume knapsack sprayer at the rate of 400 to 500 liters of spray solution per ha depending on the stage of the crop. Soap solution was added to the spray solution as a sticker and spreader. Totally, three sprays were given to the crop at an interval of seven days depending on the level of DBM infestation. First spray was given when the population crossed ETL.

Treatment details of promising medicinal and aromatic plants evaluated against Plutella xylostella on cabbage under field condition

The experiment consisted of ten treatments, which included eight promising different medicinal and aromatic plant extracts along with a standard insecticidal check and an untreated control. The treatments comprised neem seed kernel extract (5%; 50 g l⁻¹), pongamia seed extract (5%; 50 g l⁻¹), nagadali leaf extract (10%; 100 g l⁻¹), tulasi leaf extract (10%; 100 g l⁻¹), hebbevu leaf extract (10%; 100 g l⁻¹), bael leaf extract (10%; 100 g l⁻¹), datura leaf extract (5%; 50 g l⁻¹) and calotropis leaf extract (5%; 50 g l⁻¹). Spinosad at 0.3 ml l⁻¹ was included as the standard chemical check, while an untreated control was maintained for comparison.

Observations

Observations were made on the number of DBM larvae per plant on ten randomly selected plants in each plot by adopting visual counting method. The pretreatment count was made a day before treatment imposition and post treatment counts were made at third and seventh days after each spray. Observation recorded on seventh day after each spray served as a pretreatment count for the next spray. Observations on the occurrence of natural enemies viz., Cotesia plutella and spiders were also made from ten randomly selected plants in each treatment at one day before and third and seventh day after each spray. Further, yield was recorded in each treatment plots and then converted in to tonnes per hectare basis.

Benefit cost ratio (BCR)

The yield of cabbage from each plot was recorded and converted to tonnes per hectare. The price of inputs that prevailing at the time of their use was considered for working out the cost of cultivation. Benefit-cost analysis was expressed in terms of cost-benefit ratio by using the following formula: Net return (Rs./ha) Cost-benefit ratio = Cost of treatment (Rs./ha) Statistical analysis: Data on the DBM population on the cabbage plant and percent population reduction over control were subjected to square root transformation before analyzing the data with one-way ANOVA. The treatment means were separated by using DMRT (Duncan's Multiple Range Test).

Results

The field experiment was carried out during summer season of 2019 at farmer's field, Nellakunte, Doddaballapur taluk, Bengaluru rural district to assess the efficacy of aqueous extracts of promising medicinal and aromatic plants against diamondback moth on cabbage with fairly good level of infestation. During the study period, the DBM larvae per plant in the untreated control ranged from 1.17 to 2.03. The crop received three sprays at an interval of seven days. Pretreatment count on number of larvae per plant in all the plots ranged from 1.13 to 1.90 per plant, indicating uniform distribution of pest population throughout the experimental field and there was no significant difference between the treatments. Then onwards, the tested plant extracts were found statistically significant in reducing the DBM population compared to control and results obtained are mentioned in the (Table 1). The percent reduction of DBM larval population over control worked out after three rounds of spray revealed that, the NSKE at 5 percent registered superior efficacy against DBM on cabbage as that of other plant extracts evaluated by recording 0.43 larvae per plant and 74.04 percent reduction in the larval population over control. The mean number of larvae per plant and percent reduction over control recorded in other plant extracts ranged from 0.49 to 0.70 and 59.06 to 70.28, respectively (Table 1). While, the lowest mean of 0.14 larvae per plant and maximum percent reduction of larval population on cabbage plant was registered with the plots treated with spinosad @ 0.3 ml/l. The mean larva population prevailed in the untreated plots during the experimental period was 1.70 per plant.

Activity of natural enemies prevailing in cabbage ecosystem after application of different plant extracts

The activity of Cotesia plutella a promising parasitoid on Plutella xylostella and spiders was recorded in the plots where different medicinal and aromatic plant extracts were treated during the experimental period. The results obtained are mentioned in Table 2. A day before spray, distribution of Cotesia plutella and spiders was uniform throughout the experimental field and it was ranged from 0.03 to 0.17 per plant. However, the significant difference among the treatments was observed at subsequent sprays made on the crop.

On the whole, it is evident from the data recorded on both C. plutellae and spider after three rounds sprays of plant extracts that, the maximum activity of both the natural enemies was observed in the plots treated with leaf extract of datura and NSKE both at 5 percent with 0.47. population per plant, followed by nagadali leaf extract at 10 percent, pongamia seed extract at 5 percent and tulasi leaf extract at 10 percent recorded with 0.41, 0.40 and 0.36 per plant, respectively and found on for with each other. Whereas, chemical treated plots registered comparatively lowest population of 0.17 per plant.

Yield and cost economics

Yield and cost economics of different plant extracts evaluated against Plutella xylostella on cabbage are presented in Table 3 percent, pongemia oil at 3 percent, NSKE at 5 percent and neem leaf extract 50 ml/lit. Thus, these plant products proved their safety to spiders and coccinellids by recording 0.46 to 0.63 per plant, and they were on par with untreated check.

The present findings are also close agreement with that of Jitendra et al. (2017) [6] who reported that the plot treated with spinosad showed yield of 275.15 q/ha, followed by NSKE at 2 percent (138.15 q/ha), neem leaf extract at 10 percent (130.15 q/ha) and untreated control (70.32 q/ha).

The present findings in respect of cost-benefit ratio are in close agreement with Devi and Tayde, (2017) [3] , who reported that, the highest cost benefit ratio was obtained with NSKE (1:6.9) followed by Neem oil (1:6.8) and varied from 1:3.1 to 1:6.9 in different insecticidal treatments against Plutella xylostella on cabbage. The present findings on yield and benefit cost ratio obtained with spinosad are in line with Suresh Singh and Tayde (2017) [11] , who also obtained highest yield and benefit cost ratio in spinosad 45 SC (195.22 q/ha and 1:7.06, respectively). On the whole, it is evident from field studies that the NSKE, leaf extract of datura, bael, tulasi, nagadali, hebbevu and calotropis, and seed extracts of pongamia were found promising medicinal and aromatic plants by exhibiting their superior insecticidal activity against Diamondback moth and can be effectively utilized in sustainable cabbage pest management practices.

Acknowledgement

AcknowledgementThe authors gratefully acknowledge the Department of Entomology, COH, Bengaluru, for providing the facilities and support for this study. Sincere thanks are also extended to the farmers of Nellakunte village, Doddaballapur taluk, Bengaluru Rural district, for their cooperation and for providing the experimental field.

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