Field evaluation of plant extracts for the control of diamond moth ( Plutella xylostella Linnaeus) infesting Cabbage ( Brassica oleracea Linn)

Cabbage ( Brassica oleracea Linn.) is one of the important vegetables grown worldwide. Insect pests have been a major constraint to its production with some reported to have developed resistance to commonly available synthetic insecticides. In order to find alternative control for these pests, field experiments were conducted in the early seasons of 2011 and 2012, to evaluate the efficacy of three plant extracts, Tephrosia vogelli Hooks, Zingiber officinales Rosh and Lantana camara Spanish flag, applied at 5, 10 and 25 % w/v concentrations Delthamethrin (a synthethic insecticide) and an unsprayed plot were included as positive and negative controls respectively. Plots were arranged in a Randomized Complete Block Design (RCBD), with three replications. Mean Data over the two planting seasons were subjected to the analysis of variance and significant means were compared using the least significant difference (L.S.D) at P < 0.05. Z. officinales extract effectively control P . xylostella larvae at all concentrations with 25 % w/v gave the best yield of 0.50 kg/ha while T . vogelii and L. camara extracts were as effective as Deltamethrin at different concentrations and hours on diamondback-moth ( P. xylostella ) adult The present findings indicated that effective botanicals against Diamondback moth adult and larvae of cabbage can be formulated from extract of T. vogelii , L. camara and Z. officinales at 25 % concentration respectively.


INTRODUCTION
Cabbage is a popular cultivar of the species Brassica oleracea Linn. of the family Brassicaceae. It is one of the most important leafy vegetables worldwide and is a crop grown by smallholder farmers in Africa (Talaker, 2000). It has medicinal properties and it is an excellent source of vitamin C, minerals, dietary fibers, folates and especially phytochemicals (Jagerstad et al., 2004: Kyung Young Yoon et al., 2006: Rodriguez et al. 2006). Cabbage also reduces risk of heart disease and stroke, alleviate rheumatisms and skin problems. Fresh cabbage juice has been shown to promote rapid healing of peptic ulcer (USDA 2009).
Management of the pests is heavily reliant on the use of chemical insecticides especially in the developing countries of the tropics (Tabashmik et al.,1987;Dandang et al, 2003a). Synthetic insecticides have been reported to have some side effects such as mammalian toxicity, environmental pollution, insect resistance and resurgence high cost and unavailability at critical periods (Duke, 1990). Arising from these highlighted problems, there is the need to search for alternatives for synthetic insecticides. Such agents should be pestspecific, non-toxic to humans and other biota, biodegradable, less prone to pest resistance and resurgence, relatively less expensive and with relative ease of application (Devlin and Zettel, 1999;Adebayo, 2003).
The basis for this research is an attempt to solve some of the problems facing farmers, like problem of low productivity due to the insect pests attack. Protecting consumers from eating pesticide-contaminated vegetables and exposure of farmers to the toxic synthetic chemicals. Therefore, the objective of this research is to determine the effect of Tephrosia vogelli Hooks, Zingiber officinales Rosh and Lantana camara L. extracts on two developmental stages of P. xylostella infesting cabbage insect pest diversity and population, head damage and yield of cabbage.

1. Experimental site
Field study was conducted at Ladoke Akintola University of Technology (LAUTECH) Teaching and Research Farm, Ogbomoso, Nigeria. Ogbomoso is located on longitude 4°30' and latitude 10°5' N. The climate of the region could be described as hot humid tropical falls in Southern Guinea Savanna of Nigeria with a mean temperature of 27 °C and annual rainfall of 1400 mm, marked with dry and wet seasons and characterized by a bi-modal rainfall pattern with peaks in July and September. each of botanical separately and crushed with a mortar and pestle. The crushed plant parts were put in separate plastic buckets containing two liters of water. These were allowed to settle for 24 hours and the aqueous suspension was filtered through muslin cloth. The filtrate was stored in plastic containers as stock solution for bioassay and kept in the refrigerator until use.

4. Agronomic practices
The Experimental site was manually cleared and divided into -plots. Each plot was divided into beds having 2 m x 3 m dimension with 1 m gap between adjacent plots. A gap of 2 m was used to separate different blocks. A total of thirty three (33) plots were made to accommodate the treatments. Each plot had 5 rows with 6 plants per row spaced out at 0.6 m x 0.5 m. which amounted to 30 plants per plot and 50,000 plants per hectare. The plants were Four plant stands from the middle rows were tagged for data collection.
Cabbage seeds were planted in early season of 2011 and 2012 on 12 May 2011 and 23 May 2012 respectively at the rate of three to four seeds per stand. Thinning was done two weeks after planting (WAP) to achieve one plant per stand. Weeding was done as at when due.
From the stock solutions, 50, 100 and 250 ml each of the solution was diluted with 1000 ml of water to have 5, 10 and 25 % concentration. The unsprayed plots and synthetic insecticides (Deltamethrin used at the rate of 0.8 litres /hectare) were included for comparison. The treatments were applied by spraying the plants very early in the morning on weekly basis. This was done according to the method of Owolade et al. (2004) and Oparake (2005) in order to reduce or eliminate pesticide drift. Foliar application was done using hand sprayer which commenced 4 WAP and was repeated at weekly intervals which were made weekly over a 4 -week period given a total of four applications.

5. Data collection and Data Analysis
Data were collected on insect pest population, level of damaged caused by the insect pest, vegetative, head damage and yield parameters. Data collected over the two years were pooled and subjected to analysis of variance (ANOVA) using Randomized Complete Block Design (RCBD) according to by Gomez and Gomez (1987). Significant means were compared using least significant difference (LSD) at 5 % probability level.

RESULTS
Z. officinales extract was significantly better at (p = 0.05) in the control of P. xylostella larvae with mean number of 0.71 ±0.00 compared with Deltamethrin (1.71 ±0.34 ). The best control against P. xylostella larvae was observed at 25 % concentrations w/v. Application of T. vogelii and L. camara extracts were as effective as Deltamethrin at different concentrations on diamondback-moth (P. xylostella) adult. The mean number of P. xylostella adult on T. vogelli and L. camara were (0.91 ±0. 20

DISCUSSION AND CONCLUSIONS
Among the plant extracts, Z. officinales extract proved effective in controlling P. xylostella larva at different concentration but gave the best at 25 % concentrations. Insect population decreased as exposure hours increased and when compared with synthetic insecticides, it performed better. This agrees with the work of Dadang et al. (2009) who reported that Piper retrofractum and Annona squamosa extracts were more effective than synthetic insecticides (Deltamethrine) in controlling Crocidolomia pavonana (F.) and P. xylostella (L.) which are insect pests of cabbage. Prakash Rao (1997) and Sridhar et al (2002) reported that the active ingredients in Z. officinale which are pungent makes it a very good insecticides with repellent mode of action against insect pests such as aphids, leaf miner, bollworm and thrips. Gingerols and shogaols are the active insecticidal materials in Z. officinale rhizome (Goto et al., 1990).
Conversely, it was observed from this study that Z. officnales was ineffective against P. xylostella adult and as its concentrations increased the population of P. xylostella adult increased. This shows that it does not have effect in controlling adult insect population of cabbage. This corroborates with the work of Hala et al. (2010) who reported that plant extracts of Z. officinale showed good level of efficiency against nymphs of Bemisia tabaci and Aphis craccivora. Dadang et al. (2007) reported that extract of Aglaia odorata was effective in controlling several agricultural insect pests including P. xylostella and Crocidolomia pavonana larvae.
Deltamethrin and L. camara -treated plots had higher population of P. xylostella larva compared with the other insect at different concentrations and HAS. This showed that L. camara was ineffective as Deltamethrin in controlling the P. xylostella larva. Report shows that synthetic insecticides do not have much effect in reducing insect population but rather increased. Ninsin (1997) observed that P. xylostella was resistant to many conventional insecticides and so spraying DBM infested cabbage plot often has little effect on the pest. Youdeowei (1988) and  reported that DBM can cause serious damage even with application of several different insecticides because of its ability to develop resistance to almost all insecticides.
Prevalence of leave damage pointed out that as the concentrations increases in T. vogelii, L. camara leaves damage reduced. This corroborates the work of Isirima (2010 ) that leaf damage decreased with increasing application of the plant extracts (garlic and ginger). Throughout the period of this study the effectiveness of the extracts of Z. officinales kept on decreasing with increase in the time after application. This may be due to photo -degradation of its bioactive composition. It is recommended that extract of Z. officinales at 25 % concentration should be evaluated for its bioactive against other field insect pests of cabbage in order to establish its spectrum of activity.