Abstract: Cabbage, Brassica oleraceae var. capitata is an important vegetable grown and consumed in Ghana. Apart from infestations of the lepidopterous Plutella xylostella and Hellula undalis resulting from continuous cultivation, a new pest Zonocerus variegatus has been reported to causing damage to the crop. The efficacy of a novel pyrrolizidine alkaloid (PA) based novel PA-attracticide developed from treating the roots of the neophyte, Chromolaena odorata which contains PAs with Carbofuran 3G for the management of Z. variegatus was tested using cabbage as a test crop in field caged plots. Field caged plots of cabbage artificially infested with Z. variegatus were treated with and without PA-attracticide in addition to a control treatment of caged cabbage plot with no insect and PA-attracticide. The experimental design was a RCB replicated three times in two ecological zones. The results showed that the establishment, leaf and head damage of cabbage was statistically the same in the PA-treated plots and the control treatment. These treatments, however, performed significantly better than the treatment without PA-attracticide. The results obtained holds promise for mitigating the menace of the grasshopper on cabbage using PA based attracticides.
INTRODUCTION
Cabbage, Brassica oleraceae var. capitata (L.) Alef. (Brassicaceae) is a very important vegetable for both the rural and urban inhabitants of Ghana. The crop has replaced many indigenous green vegetables and is now cultivated all year round (Timbilla, 1997). The continuous cultivation of the crop has, however, led to the proliferation, particularly of Hellula undalis (Fabricius) (Lepidoptera: Pyralidae), Plutella xylostella (L.) (Lepidoptera: Plutellidae) and aphids, Brevicoryne brassica (L.) (Homoptera: Aphididae) (Timbilla, 1997).
Surveys conducted in 2003 revealed that late instar nymphs and adults of Z. variegatus cause economic damage to cabbage within a fortnight of attack (Timbilla, 2006). The damage was observed to primarily occur in cabbage fields close to hatching sites of both dry and wet season populations of Z. variegatu and up to 26 insects could be counted per plant. Damage attributed to vegetables including cabbage by Z. variegatus is documented in Ghana (Timbilla, 1997; Afreh-Nuamah, 2004).
A number of sex pheromone attracticides are available for the management of lepidopteran pests of vegetables including cabbage (Mitchel and Mayers, 2001; Mayers and Mitchell, 2002; Reddy et al., 2002; Cork et al., 2003; Badenez-Perez et al., 2004; Trimble et al., 2004). However, no known attracticides are available for the management of orthopteran pests, particularly, a peculiar insect such as Z. variegatus.
Recent chemoecological studies by Boppré (1991) have revealed that Z. variegatus is strongly attracted to volatile derivatives (PAs) from the dry roots and fresh flowers of C. odorata which contain pyrrolizidine alkaloids. It is thought that a pest management strategy could be developed based on the ‘hunger’ of Z. variegatus for PAs (Boppré and Fischer, 1994; Fischer and Boppré, 1997).
This study was conducted to assess the effectiveness of the pyrrolizidine alkaloids in the root of C. odorata used in combination with Carbofuran® 3G as attracticide bait for the management of Z. variegatus on cabbage field.
MATERIALS AND METHODS
Cultivation of cabbage seedlings: The experiment was conducted on field plots located in Cape Coast and Kumasi. Cape Coast has coastal savannah vegetation and Kumasi occurs in the moist Semi-deciduous forest zone. The trial in Cape Coast was conducted from August-November, 2003 and the one in Kumasi from February-May, 2004.
Cabbage seeds of the variety oxylus were nursed on two field beds, measuring 2x1 m close to each experimental site. The nursery beds were sterilized by burning wooden pieces gathered on each bed for about 30 min and allowed to cool overnight. The seeds were then drilled with a wooden stick along lines, spaced 15 cm apart on the beds, covered with a thin layer of sand and adequately watered. The plots were then covered with palm fronds to preserve moisture for germination.
The palm fronds were removed after 10 days and the seedlings were thinned out to a population of about 500 healthy seedlings per bed. Subsequently, the seedlings were covered with a nylon mosquito mesh (1 mm2 size) built to the specifications of the plots to protect them from insect damage and watered as and when necessary until they were ready for transplanting onto the experimental plots.
Experimental plot: The seedlings were transplanted at 4 week onto field plots measuring 2.1x0.75 m at 0.45x0.45 m within and between rows with a plant population of 10 per plot. This was done in the evening to avoid heat shock to the seedlings. The treatments were as follows:
| • | Cabbage seedlings with only Z. variegatus |
| • | Cabbage seedlings with Z. variegatus exposed to PA-attracticide |
| • | Sole cabbage seedlings with no Z. variegatus and no attracticide (control) |
Each treatment plot was covered with a nylon mesh and the insects were introduced 2 week after transplanting. The experiment was conducted in a randomised complete block design with 3 replications. Data was taken from all the 10 plants in each treatment plot. The attracticide treatment was replaced 3 times at 3 weekly intervals. Insects used for the experiment were collected as early hatchlings from the field and reared on cassava foliage in the laboratory following the method of Cobbinah and Tuani (1992) till they reached the 3rd instar stage.
Fifty 3rd instar nymphs of Z. variegatus were counted into Kilner jars and introduced into each plot except for the control. The attracticide consisted of 300 g weight of the dry chopped roots of C. odorata soaked in 1.0% (w/v) Carbofuran® 3G solution and dried in the sun for 3 h (Timbilla, 2006). The bait consisted of a hexagonal wire mesh (about a cm wide) containing the attracticide and placed 10 cm above the soil, at two positions (the 1st and 2nd third) in each treatment plot. Weeds were controlled by hand picking by opening the sides of the net at 3 and 6 weeks after transplanting. Data on surviving cabbage plants was taken at the end of each experiment. Leaf damage of each individual plant per plot was estimated using a 1-5 point scale (Timbilla, 1997) presented below, at weekly intervals after the introduction of the insects. The yield of the plants as indicated by weight of head was also measured at harvest.
Damage score scale:
| • | 0-5% leaf defoliation |
| • | 6-15%,, |
| • | 16-25%,, |
| • | 26-50%,, |
| • | >50%,, |
Additionally, damage of cabbage head (using the same scale above) and data on surviving nymphs were determined when the experiment was terminated. The trapped insects in the nets were removed and counted before harvesting.
Data analysis: All count data was transformed by the formula y = √(x + 0.5) and subjected to One-Way Analysis of Variance (ANOVA) using GraphPad Prism® statistical package version 4.00 (GraphPad Prism® 4.00, 2003). Mortalities encountered in the treatment with cabbage and only Z. variegatus were corrected with Abbott’s correction formula (Abbott, 1925).
RESULTS
Cabbage leaf damage: The estimated damage done to the crop by Z. variegatus is presented in Fig. 1. The results clearly indicated that significant damage was done by Z. variegatus in the treatment without attracticide.
The damage was more pronounced from the 6th and 3rd week for the trials conducted at Cape Coast and Kumasi, respectively. The mean leaf damage score recorded at the end of the trials at Cape Coast were 4.3, 2.8 as against 1 and 5.0, 2.7 and 1 at Kumasi for treatments without attracticide, treatment with attracticide and the control, respectively (p<0.05).
Surviving plant population: The surviving cabbage plants at the end of trial 1 conducted in Cape Coast was 36.7, 73.3 and 100% in respect of the treatments without attracticide, with attracticide and control (Fig. 2). The percentage surviving plants on the control treatment was significantly higher than the treatment with attracticide which was also significantly higher than the treatment without attracticide (p<0.05).
| Fig. 1: | The damage done to cabbage by Z. variegatus caged with
and without C. odorata attracticide and sole cabbage Control, Cape
Coast and Kumasi. Bars indicate standard error of means |
In trial 2, planted in Kumasi, the surviving plants at the end of the experiment in the control and treatment with attracticide were statistically the same i.e., 100.0 and 96.7%, respectively (p<0.05) (Fig. 2). These two treatments, however, recorded significantly higher plant population than the treatment without attracticide which recorded a plant population of 49.0% (p<0.05).
Yield of cabbage head: The mean yields recorded for trial 1 (Cape Coast) were 0.9, 1.0 and 1.2 kg per cabbage head for treatments without and with attracticide and control, respectively (Table 1) with yield on the control treatment being significantly heavier than on the treatment without attracticide (p<0.05).
Treatment without attracticide was statistically the same as the one with attracticide which was also statistically the same as the control (Table 1).
In trial 2 (Kumasi), the yields were very low but followed a similar pattern in the order 0.3, 0.5 and 0.5 kg plant-1 in respect of treatments without and with attracticide and control (Table 1). Treatment with attracticide and control were statistically the same and both were significantly higher than the plots without attracticide (p<0.05) (Table 1).
| Fig. 2: | Surviving cabbage plants caged with Z. variegatus and
with or without C. odorata attracticide and a sole cabbage control
8 weeks after attracticide application at Cape Coast and Kumasi. Bars indicate
standard error of means |
| Table 1: | Yield of cabbage under artificial infestation of Z. variegatus caged with and without C. odorata attracticide and sole cabbage control |
| n = 10 | |
| Table 2: | Head damage score of cabbage caged with Z. variegatus and with and without C. odorata root attracticide and sole cabbage control |
Head damage of cabbage: At Cape Coast, cabbage head damage scores were 4.3 and 1.9 for treatments without and with attracticide, respectively, with the former being significantly higher than the latter treatment (p<0.05). However, the treatment with attracticide recorded a statistically higher head damage than the control treatment which did not suffer any damage (p<0.05) (Table 2).
Cabbage head damage score of 4.8 was recorded for the treatment without attracticide in the trial conducted in Kumasi which was significantly higher than the score of 1.3 for the treatment with attracticide (p<0.05) (Table 2).
| Fig. 3: | Survival of Z. variegatus caged with and without C. odorata root attracticide, 8 week after attracticide application. Bars indicate standard error of mean |
Cabbage head damage score for treatment with attracticide was significantly higher than the control treatment (p<0.05).
Survival of Z. variegatus: Results from trial 2, conducted in Kumasi, indicated a percentage survival of 14.0% of Z. variegatus in the treatment with attracticide. This was significantly lower, compared with the corrected survival in the treatment without attracticide which was originally 8.3% (p<0.05) (Fig. 3).
DISCUSSION
The results of the experiments have demonstrated clearly that the PA-attracticide developed trap could be used to reduce damage caused by Z. variegatus to the crop plants. The bait lured the insects from the plants on which they were caged, thus reducing the damage done. The construction of the bait is simple, non-hazardous, of low-cost and could easily be produced at the cottage level. The bait could, therefore, be used to help control the damage of the insect to crops such as cabbage which is often cultivated on small scale.
The head damage score above 4.0 observed in the treatment without attracticides, equivalent to damage between 25 and 50% is ranked below marketable standard (Timbilla, 1997) and confirms the assertion that Z. variegatus can cause serious crop loss if they are not controlled. Cabbage is known to suffer serious attack by H. undalis and P. xylostella which sometimes cause total yield loss (Anonymous, 1992). Thus any feeding damage by Z. variegatus will increase its potential threat to the cabbage industry.
On the other hand, the score of less than 2.0 for the treatment with attracticide indicates a damage below 15% which is acceptable by elite consumers who are conscious of pesticide contamination of vegetables (Timbilla, 1997).
The literature reports on work done on the management of lepidopteran pests of cabbage with attracticides (Mitchel, 2002; Khrimian et al., 2002; Millar et al., 2002) with promising results for their potential use (Jacobson et al., 1976; Baur et al., 1998; Reddy and Guerrero, 2000; Badenes-Perez et al., 2004). Unfortunately, however, there is no basis for comparison of the available literature with this novel PA-attracticide developed since the behaviour of Z. variegatus is completely different and secondly the mode of action of pheromone attracticides is different from the PA-attracticide which depend on gustatory response and not contact as in the case of pheromone traps.
The suggested use of attracticides in the management of Z. variegatus in cabbage fields is compatible with the current management of the crop with biological and chemical insecticides and this will be more compliant in the foreseeable future when pheromone attracticides catch up with farmers in the sub region. Perhaps a few poisoned roots scattered in and around infested fields could substantially reduce the pest population below damaging levels during their entry and exit of the cultivated land.
In a study on cabbage, the cost of applying Karate® 2.5 EC to control lepidopteran pests sprayed at weekly intervals according to farmer practice on a similar plot size as used in the present study was $8.9 (Timbilla, 2006). In the present study, the cost of preparing a 300 g weight of PA-attracticide with Carbofuran® 3G is estimated at $0.02. With the same plot size, it will require (2x$0.02.00x3 plots) yielding $0.12 in addition to the cost of the metal mesh, estimated at $0.56 each for each plot. Thus, the overall cost for usage of the PA-attracticide is estimated at $1.8.
Notwithstanding the fact that it would require a higher concentration of chemical insecticide to control Z. variegatus on a similar plot, the comparison with lepidopteran pests still makes the usage of the PA-attracticides cheaper. Also, considering the environmental hazards posed to operators, consumers and beneficial arthropods attributed to the use of chemical insecticides, PA-attracticides are specific and thus environmentally friendly, adding to its advantage as a cheaper, environmentally friendly and novel way of managing the grasshopper.
It is concluded from the results of this study that PA-attracticides hold promise in mitigating the menace of Z. variegatus in infested cabbage and other vegetable fields.
ACKNOWLEDGMENTS
The authors are very grateful to Prof. Dr. Michael Boppré and O.W. Fischer, both of the Forstzoologisches Institut, Freiburg, Germany for their useful comments during preparation of this manuscript.