Apart from being consumed as a current vegetable, cabbage has been valued for medicinal purposes in treating headaches, gout, and diarrhea, inflammatory and gastrointestinal disorders [1-4]. Some researchers have focused their works on the capacity of cabbage to reduce the risks of some cancers [5,6], especially due to the content of glucosinolates and derived products, flavonoids and other phenolics in cabbage [7-9]. The antioxidant activity of these compounds has been shown to correlate with vitamin C and phenolic phytochemicals content [4,6,10,11].

The intensified growing of cabbage has led to a common problem of high pest infestation, which is caused mostly by the Diamondback Moth (DBM) Plutella xylostella [12-14]. DBM larvae are very difficult pests to control [15] and therefore, are the greatest threat to crucifer production in many parts of the world. The losses can reach the 90% [13,16]. This explains the large use of insecticides in crucifer production. Owing to its polyvoltine characteristics and serious overlap of generations, this pest can easily develop resistance to various kinds of insecticides [17-19], including biological one such as Bacillus thuringiensis [20-22], and particularly in sub-tropical and tropical countries [23]. To overcome resistance, farmers resort to increasing frequency and rates of pesticide applications and to mixed cocktails of pesticides [16]. In addition, the information about the chemical composition of these pesticides is not always publicly available [24]. In Togo, vegetable producers currently apply seven different chemical insecticides (fipronil, chlorpyrifos ethyl, cypermethrin, dimethoate, endosulfan, Bacillus thurgensis and acephate) on cabbage and in over 11 applications within 3 months of crop growth prior to harvest. Indiscriminate use of pesticides constitutes one of the main environmental and public health problems in developing countries leading to harmful effects on the ecosystems, the health of both farmers and consumers [25-29].

Biological options in an integrated pest management (IPM) approach offer a solution to sustainable control of DBM. In West Africa, farmers use botanical pesticides, such as plants extracts of Azadirachta indica A. Juss, Melia azedarach L. against DBM [30]. Cymbopogon schoenanthus is efficient in the biological control against pests [31,32]. Cymbopogon schoenanthus L. Spreng. (Poaceae) or lemon grass, originally from India, is a warm climate aromatic plant that grows in Togo [31]. It’s essential oils are very rich in piperitone [31,33,34] which is responsible for the insecticidal activity of this plant [31,35]. This present work aims at using aqueous leaf extracts and low concentrations of essential oil of C. schoenanthus as an integrated management approach to the control populations of P. xylostella and their larvae in vitro and the field.

Experimental site

Experiments were conducted from May 2009 to August 2010 at Agricultural Teaching Experimental Station (ATES) of University of Lomé (UL), Togo. Plant extractions, nutrition and contact tests on P. xylostella were carried out respectively in Chemistry and Plant Biology laboratories of the Ecole Supérieured’Agronomie (ESA) of UL.

Plant and materials

Cabbage cultivar “KK cross” purchased in a seeds shop in the outskirts of Lomé was used for the assay. Leaves of C. schoenanthus were collected at the Agricultural Teaching Experiment Station of UL where this plant was cultivated just for experimental purposes.

Source of insects

Fourth instars larvae of P. xylotella were collected in cabbage growing field and from a small rearing unit of P. xylostella of ATES.

Chemical insecticide dimethoate was purchased from a local supplier as ‘Calidim’ 400 EC (Caliope Chemical Industries Ltd, France).

P. xyllostella larvae control with emulsified oil of C. schoenanthus in vitro

Contact tests: Ten fourth instars larvae were introduced into a Petri dish, and were sprayed (using ULV apparatus) with the five formulations: DW (Distilled water); THE1 (essential oil 2 g, hand soap containing soda 2 g, distilled water 96 g); THE2 (essential oil 1 g, hand soap containing soda 2 g, distilled water 97 g); SW (hand soap containing soda 2 g, distilled water 98 g); and DT (40 μl aqueous solution containing 0.25 mg of active ingredient). The soap is an adjuvant used to support oils on the leaf surface and during larvae contact. The test was replicated four times with each formulation. Larval behavior was observed with a magnifying glass, and a chronometer was used to determine the duration of larval survival. The parameter measured was mortality, and larvae were considered dead if they did not move either their head or their thorax when touched.

Nutrition test: A randomized complete block design five treatments each replicated 4 times was used. Fresh cabbage leaves discs (diameter 5 cm) were sprayed with the formulations DW, THE1, THE2, SW and DT and then placed in Petri dishes. Ten fourth instar larvae were introduced into each Petri dish. The development of the larvae feeding on sprayed leaves discs was observed at 24, 48, 72 and 96 hours. Three parameters were recorded: mortality, larval stage and adult emergence.

Aqueous extracts preparation: Aqueous extracts of Cymbopogon schoenanthus leaves were obtained according to the following methodology: 1) three years old leaves were harvested, chopped to fine particles size and shade dried at room temperature for four days; 2) three formulations, TS50, TS100, TS150, were obtained with 50 g, 100 g and 150 g respectively of chopped dried leaves infused in one liter of water for 24 hours.

Effects of C. schoenanthus leaves aqueous extracts on the field: In our investigation, different concentrations of aqueous extracts of C. schoenanthus leaves were used as an integrated management approach to the control of P. xylostella under field conditions. An untreated small plot of cabbage was placed about 15 m from the test plots to serve as a control. After one month in seedbed, the seedlings were transplanted to the plots. Plot size was 3.50 m × 1.20 m each, with a spacing of 75 cm between plants and 60 cm between rows, to create two rows of 5 plants each. Spacing between plots and replications were 1.5 m and 1.5 m, respectively. Before the planting, 30 kg of organic manure collected from extensive poultry farming was applied to each plot. The seedlings were sprinkled with water two times per day. No insecticide was used in the seedbed, but after the planting, the organophosphorus insecticide dimethoate was used weekly at 400 g of active ingredient per hectare (the dose indicated by manufacturer) to control DBM on every plot until the pre-heading stage. After this phase, three doses of C.schoenanthus aqueous extracts (TS50: 50 g/l, TS100: 100 g/L and TS150: 150 g/L) were applied weekly at a rate of 6 liters for the 4 plots during 4 weeks. One liter of dimethoate prepared solution was used for 4 plots, and the treatment was stopped 15 days before harvest to respect persistence time. Sprays were applied with a manually operated knapsack sprayer at 1.5 liter per treatment. A randomized complete block design (RCBD) with four treatments replicated four times was used. At harvest, variables recorded for analysis were the yield, the circumference, the weight of the heads. For damage analysis, the cabbage leaves were classified according to the level of the perforations caused by the larvae (Table 1).

Table 1: Classification of cabbage leaves damaged.

Statistical analysis

Significant differences among data concerning cabbage head circumference, weight, yield, level of damage, and P. xylostella larval mortality and adult eclosion were determined with analysis of variance using Systat 5.0 software. Pairwise comparisons were done using the Fisher LSD at p < 0.05. All data are presented as means ± standard deviation.

Toxicity of C. schoenanthus oil to P. xylostella larvae (contact test)

The formulations THE1 and THE2 of C. schoenanthus emulsified oil caused a faster mortality of the larvae than dimethoate suspension. The mean survival times in a group of 10 larvae were 22, 33 and 44100 seconds respectively for THE1, THE2 and dimethoate (Table 2). Larvae mortality time was significantly lower in the C. schoenanthus emulsified oil treatment compared to the control dimethoate (ANOVA: F_0.05(4)=333.73; P<0.001). Thus C. schoenanthus emulsified oil acts more quickly than synthetic insecticide used in the present study. Larvae treated with the soap preparation (SW) survived for 330 seconds.

Table 2: Mean survival time of the larvae of P. xylostella after treatment with emulsified oil of C. schoenanthus (100% mortality).

The results in Table 3 showed that different concentration of pesticide tested affected differently the instars of P. xylostella larvae. Feeding damage was observed on the leaf discs after 24 hours indicating that the larvae fed on the leaves. Generally, THE2, SW and DT treatments caused similar death rates, which ranged from 5-12.5%. This value turned around 10%. These formulations did not differ significantly from each other. However, larval mortalities were 25% and 60%, respectively, at 24 h and over 48 h for THE1. A significant effectiveness against larvae of P. xylostella was observed when cabbage leaves were treated with THE1 dose (ANOVA: F_0.05(4)=26.25; P<0.001). As in the contact or feeding tests, the formulation THE1 appeared more effective. Figure 1 reports percentage of P. xylostella adult eclosion at 96 hours. Concerning the absolute control by the DW and THE2 treatments, 95% and 65% adults, respectively, have evolved. THE1 (7.5%) significantly (ANOVA: F_0.05(4)=9.5; P<0.005) prevented the adult evolution than DW, SW and THE2. The adult eclosion rate recorded with dimethoate was 37.5%.

Table 3: Mean percentage of larvae mortalities induced by the insecticides and larvae developing to pupal stage.

Larvae were nourished with fresh cabbage leaves treated with formulations DT, SW, DW and the essential oil of C. schoenanthus at 2 doses (THE1 and THE2). Data in percent are Means ± SD. *P < 0.05 (SW vs DW). **P < 0.05 (THE1 and DT vs DW).
DW: distilled water; THE1: essential oil 2g + hand soap containing soda 2 g + distilled water 96 g; THE2: essential oil 1 g + hand soap containing soda 2 g + distilled water 97 g;
SW: hand soap containing soda 2 g + distilled water 98 g; DT: 40 μl aqueous solution containing 0.25 mg of active ingredient.
Figure1: Evolution of the larvae of P. xylostella towards the adult forms at 96 hours.

Overall, insect populations were low. Generally, it was observed that the cabbage plants thrived well, and all the harvested heads of cabbage from each treatment plot were marketable. First attacks began at pre-heading stage, and an ave rage of 3-5 larvae were recorded per 10 plants. By comparing the various treatments at the harvest period, a strong presence of larvae (more than 4 larvae per 10 plants) was observed on certain heads harvested from the dimethoate-treated plot. However, the heads treated with dimethoate were slightly larger than those treated with the botanical aqueous extracts: 61.70 ± 3.71 cm versus 59.85 ± 5.86 cm; 59.53 ± 1.96 cm and 60.32 ± 2.86 cm for TS50, TS100 and TS150 respectively. The head weight also showed a slight difference between the dimethoate treatment and the botanical extracts: 1.69 ± 0.32 kg versus 1.41 ± 0.36; 1.57 ± 0.31 kg and 1.56 ± 0.19 kg for TS50, TS100 and TS150, respectively (Table 4). There were no statistically significant differences between the dimethoate and C. schoenanthus aqueous extract treatments (ANOVA: F_0.05(3)=0.38; P>0.7). The yield was slightly higher and the level of damage concerning the heads harvested from control plot (dimethoate treatment) was relatively of low quality compared to the treatments the aqueous extracts the botanical aqueous extracts (4.22 ± 0.34 kg/m² and 2.30 ± 0.14 versus 3.45 ± 0.40 to 3.93 ± 0.28 kg/m² and 1.80 ± 0.72 to 2.22 ± 0.73 respectively). The ANOVA test for yield and damage parameters showed also no significant difference between the insecticide and the botanical treatment (ANOVA: F_0.05(3=0.84; P>0.4).

Table 4: Effects of aqueous extracts of C. schoenanthus and dimethoate on cabbage Yield and quality.

The objective of this study was to investigate the activities of aqueous leaves extracts and low concentrations of essential oil of C. schoenanthus to control populations of P. xylostella and their larvae. The aqueous leaf extracts of this plant were quite effective against P. xylostella, and achieved the similar protection of cabbage to dimethoate. The results of our investigation confirm the reports of Idrissou [36] which revealed that both Azadirachta indica and C. schoenanthus extracts had equivalent efficiency against DBM of cruciferous plants. The observations showed that the essential oils of C. schoenanthus can cause effective mortality of P. xylostella larvae. The essential oil extracted from C. schoenanthus contained a high percentage of monoterpenes. Studies carried out in Togo showed that the major component of C. schoenanthus extract was piperitone with a value bordering 70% [37,33], and was responsible for the insecticidal activity [35]. This component, isolated and purified by Author’s name [38], had strong insecticidal activity against eggs, neonate larvae and adults of Callosobruchus maculatus at very low concentrations [31]. These observations corroborates the low concentrations of the formulation THE1 (2 g/l) effective effect on larvae of P. xylostella in our study. This formulation would ensure a good protection of cabbage against DBM. It can validly used as alternative in P. xylostella management.