Comparative efficacy of plant extracts with synthetic against insect pests of okra ( Abelmoschus esculentus L. Monech)

A field study was carried out during the two cropping seasons of okra to investigate the efficacy of plant extracts and synthetic insecticides applied singly and synergistic effect of botanicals with the synthetic insecticide on some insect pests of okra. Plots were arranged in a Randomized Complete Block Design (RCBD), with three replications. Three insect pests of okra ( Podagrica uniformis , Podagrica sjostedti and Zenocerous variegatus ) were observed on the field . Cypermethrin applied singly and Cypermethrin combined with plant extracts effectively controlled observed insect pests, followed by lone application of plant extracts ( L camara, T. vogelii and Z. officinales plant extracts at 1 st and 2 nd season. Okra yield revealed that both plant extract and Cypermethrin combined with plant extract were not significantly different from one another at first season and at second season Cypermethrin combined with plant extracts performed better than Cypermethrin applied singly. This study indicates that the studied plant extracts can be good synergist for Cypermethrin in controlling field pest of okra.


INTRODUCTION
Vegetables constitute an important food item, supplying vitamins, carbohydrates and minerals needed for a balanced diet. Their value is important especially in developing countries like Nigeria, where malnutrition abounds (Randhawa, 1974; Masood Khan et al., 2001). Among the species grown in Nigeria is okra (Abelmoschus esculentus L. Moench), also known as lady's finger or bhendi, which belongs to family Malvaceae and is an important crop grown throughout the year. Besides Nigeria, it is a popular vegetable grown in many tropical and subtropical parts of the world grown for its pod (Folorunso and Ojeniyi, 2003). However, the yield of okra has been reported to be very low in Nigeria, hardly up to 7t/ha (Schippers, 2000).
The major problems of okra production in Nigeria are insect pest infestations. Many of the pests occurring on cotton are found to ravage okra: with up to 72 species of insects recorded on okra (Srinivas Rao and Rajendran, 2003). The most common insect pests are  Fasunwon and Banjo (2010.) To control the menace of insect pests of Okra, a number of chemical insecticides are liberally sprayed on the vegetable crop, which lead to several problems like, elimination of natural enemies, environmental hazards like air pollution, soil and water pollution and development of resistance by insect pests. Also, most of the synthetic chemicals are not available and when available are too costly for resource poor farmers to obtain due to the economic situation of the country.
To overcome the problem of synthetic insecticides, there is need to identify materials that are environmentally safe with insecticidal properties. Over the years botanicals have been discovered and used because they are biodegradable, low mammalian toxicity, safe to natural enemies, cheap and can be easily produced by skilled and unskilled farmers. Hence the aim of this study is to determine the efficacy of plant extracts and synthetic insecticide applied singly and synergist effect of the botanicals with the synthetic insecticide on some insect pests of okra.

1. Experimental site
Field trials were conducted during the two cropping seasons of 2011 at the Teaching and Research Farm, Ladoke Akintola University of Technology, Ogbomoso, (Latitude 07º 05'N, Longitude 04º, 50'E, 27.5 m) in the rain forest zone of Nigeria.

Experimental materials
Seed of an early maturing cultivar (NHAE 47-4) okra (Abelmoschus esculentus) was procured from the Oyo State Agricultural Development Project, Ogbomoso Nigeria, while Cypermethrin a synthethic pyrethrin, and plant extracts were obtained from Jubali Agro Allied Company, Lagos and LAUTECH Botanical Garden Ogbomoso respectively.

3. Agronomic practices
Plot size of 2.5 m x 1m was used for the establishment of the okra seeds. The gross experimental site was 27 m x 5 m replicated three times with a gap of 0.5 m in-between the rows, and with a planting distance of 0.5 m between plant to give a plant population of 18 plants per plot. The okra seeds were planted three (3) seeds per hole, weeding and thinning was carried out exactly two (2) week after germination. The experiment was laid out in a Randomized Complete Block Design (RCBD) with 8 treatments viz Z. officinales 25 %, L.camara 25 %, T. vogelii 25 %, Cypermethrin, Z. officinales + Cypermethrin, L. camara + Cypermethrin, T .vogelii + Cypermethrin and Control.

4. Preparation of plant extracts
Air dried leaves (500 g) each of Latana camara, Tephrosia vogelii and rhizomes of Zingiber officinales, were weighed on a scale and pounded separately using mortar and pestle. The paste of each of the plants were dissolves in 2 litres of distilled water and mixed thoroughly after which it was left for 24 hours. For each of the botanicals. The aqueous International Letters of Natural Sciences Vol. 19 solution was filtered, using muslin cloth and the extracts were kept in a refrigerator as stock solution.

5. Application of plant extracts
From the stock solution 250 ml of each of the solution was diluted with 500 ml of water to get 25 % concentration spraying volume. The control (unsprayed plots) and Cypermethrin were included for comparison. Foliar application was done using manually operated hand sprayers. The treatments were applied in the morning to prevent photodecomposition of the extracts and application of treatments commenced two weeks after planting before defoliation of okra leaves and was repeated weekly for over three weeks.

6. Data collection and Data Analysis
Data were collected on diversity and population of insect pest and yield of okra yield of okra. Data collected were subjected to analysis of variance (ANOVA) using Randomized Complete Block Design (R C B D) as explained by Gomez and Gomex (1987). Significant means were compared using Duncan Multiple Range test (DMRT) at 5 % probability level.

RESULTS
Combination of plant extracts with Cypermethrin effectively controlled Podagrica. sjostedti when compared with control and plant extracts at 1 st and 2 nd season. Also Z. officinales 25 % was more effective than L. camara 25 % at 1 st and 2 nd season. It was observed that T. vogelii 25 % at 4 th week and 5 th week after planting were more effective than control in both season. As number of weeks of spraying increases, no of insect pest reduces for the sprayed plots. (Table 1). Unsprayed plot recorded the highest number of P. uniforms at 3 rd and 4 th WAP and the same trend was observed in 1 st season and 2 nd season planting. (Table 2) Z. officinales + Cypermethrin and Cypermethrin gave the highest control of P. uniformis population followed by T. vogelii + Cypermethrin and L. camara + Cypermethrin at second spraying. Z. officinales 25 % effectively controlled the insect when compared with control. There were no significant difference at 5 WAP among the treatments. There were no significant difference between 4WAP and 5WAP on Cypermethrin and mixture of Cypermethrin with plant extract, and these treatments they effectively controlled Zenocerous variegatus than plant extract only 1 st and 2 nd season planting. Z. officinales, L. camara and T. vogelii at 25 % to control Z. variegatus concentration were not statistically different at 4WAP and 5WAP while T. vogelii 25 % and the ability of Z. officinales 25 % were significantly higher than L. camara 25 % which shows that they were more effective than L. camara 25 %. (Table 3). The same trend was observed in the second season. Control plot had the highest population of insect pests in both seasons when compared with other treatments.
Data on okra yield revealed that at first season there were no significant difference among the treated plots while at second season Z. officinales 25 % and combination of different plant extracts with Cypermethrin had significantly higher yield than Cypermethrin alone. T. vogelii and control were not statistically different from each other.    Mean followed by the same alphabet in a column are not significantly different (p < 0.05).

DISCUSSION AND CONCLUSIONS
Generally, it was observed that the unsprayed plot (control) had the highest number of insect pests and Cypermethrin with botanicals effectively controlled the observed insect pests Similarly, T. vogelii and Z. officinales controlled the insect pest more than L. camara at different spraying week. The mixture of plant materials with Cypermethrin effectively controlled insect pest of okra. The high effectiveness of Cypermethrin could be attributed to its knock down effect on target organism (Hill, 1988). Also the synergistic effect of Cypermethrin and plant materials enhanced its performance thereby reducing okra insect pest population. This corroborates the work of Adebayo (2003) who reported that T. vogelii + Lamdacylothrin was more effective than using Lambdacylothrin at recommended rate. This finding indicates the possibility of reducing the amount of synthetic insecticides that will go into the ecosystem when botanicals are incorporated as synergist with Cypermethrin.
Z. officinales 25 %, T. vogelii + Cypermethrin, Z. officinales + Cypermethrin L. camara + Cypermethrin produced the highest yield of okra followed by Cypermethrin. This is in agreement with the work of Adebayo (2003) who reported that combination of locust lotion + T. vogelii gave the higher okra yield than Labdacylothrin. The high population of Podagrica species and Z. variegatus recorded on control plot (unsprayed plot) as compared with those treated with Cypermethrin, T. vogelii, Z. officinales and Cypermethrin + T. vogelii and Z. officinales + Cypermethrin showed that application of the treatments on okra seedlings constituted a toxic material or an antifeedant to the insect. This study corroborates the work of Miller and strickler (1984) who reported that suitability of any refuge for habitation or feeding by insect is determined by nutritional quality, presence or absence of toxins and exposure to natural enemies.
The extracts and insecticides exhibited different levels of toxicity. Z. officinales and T. vogelii effectively controlled insects especially at 4 th and 5 th week after planting this agrees with work of Adebayo 2003 that T. vogelii effectively controlled Podagrica species on okra as Lamdacylothrin.
Use of botanicals in pest management of okra can help the farmers to reduce the sole reliance on synthetic insecticides. Moreover, this eco-friendly approaches of pest control with minimum disturbance to natural beneficial insects and non target organisms should be highly encouraged.