MATERIALS AND METHODS

Experimental site: Field trials were carried out at Teaching and Research Farm of Federal College of Forestry Ibadan, for two years. The Federal College of Forestry Site is located on the latitude 7.50°N and longitude 3.90°E. The climate condition of the area is tropical with an annual rainfall range of 180-700 mm annum^-1 while the annual temperature is 34.40°C and the daily humidity is about 60% (FRIN, 2011).

Construction of experimental materials: Wooden plank was used to construct a cage-like structure measuring 2x1 m (height and breadth) and was covered with mosquito net (16 mm) mesh leaving the bottom side open. Two colours of net were used (blue and green). A total of 12 cages were constructed with each colour of the net.

Field establishment: The experimental plot measuring 15x15 m² was manually cleared with hoe and cutlass. Six months-old uniform sizes of M. excelsa seedlings were collected from the screen house at Forestry Research Institute of Nigeria and transplanted on un-tilled land at the spacing of 2x2 m at the rate of 1 seedling per stand.

Fig. 1:	Effect of different treatments on the infestation of Milicia excelsa by Phytolyma lata

The experiment was laid out in a Randomized Complete Block Design (RCBD) and replicated four times. The net barrier cages, colours blue and green were used to cover the plants immediately after transplanting except the control plot. The cages were randomly placed in four replicates per block (Fig. 1). N.P.K fertilizer was administered once to the plants four weeks after transplanting. The plots were weeded manually at four weeks intervals.

Treatment procedure: The procedure of integrating carbofuran (3 G), dimethoate and barrier nets was adopted from Onekutu (2011).

The treatments were as follows:

The integration of the treatments commenced 12 months after transplanting. The dimethoate was applied at the rate of 2.0 mL L^-1 of water as a foliar spray while carbofuran (3 G) was applied to the soil at the rate of 10 g plants^-1 using ring methods both at 4 weeks interval. The application of the two insecticides was done separately at two weeks interval. The control plot were sprayed with only dimethoate at two weeks interval.

Data collection: Data were collected on plant height, stem diameter, numbers of galls, Pytolyma lata density and plant mortality at monthly interval for two years.

Data analysis: Data collected were subjected to Analysis of Variance (ANOVA) and significant means were separated using Duncan Multiple Range Test (DMRT).

RESULTS

Effect of different treatments on the infestation of Milicia excelsa by Phytolyma lata: Monthly application of dimethoate and carbofuran on the M. excelsa seedling after 12 months of covering with blue and green nylon nets reduced the infestation of P. lata significantly (p<0.05) by 25.74 and 19.31%, respectively compared to dimethoate only (control). The plants covered with blue net in addition with application of dimethoate and carbofuran recorded least number of galls (7.89%), followed by green net+carbofuran (14.32%). Plots treated with green net+dimethoate+carbofuran recorded least P. lata density (13.59%) followed by blue net+carbofuran+dimethoate (15.13%). The plots treated with only dimethoate (control) had highest number of galls and P. lata density 33.63 and 28.73%, respectively (Fig. 1).

Effect of different treatments on the growth of M. excelsa: The treatments showed a significant differences (p<0.01) on the growth of M. excelsa in plants height and stem diameter but there was no significant difference (p>0.05) on the number of branches. The integrated treatments of monthly application of dimethoate and carbofuran separately at two weeks interval on plants covered with blue net had the highest plant height (122.50 cm), followed by plants covered with green net (105.13 cn), however, they did not significant differ from each other. Similarly, integration of dimethoate and carbofuran on the plants covered blue net had the highest stem diameter (1.03 mm) followed by the green net (0.98 mm). The treatments did not show any significant difference on the number of branches. However, the plant covered with blue net in addition with the application of dimethoate and carbofuran had the highest number of branches (8.75) (Table 1).

Effect of different treatments on mortality and survival of Milicia excels seedlings: The barrier nets protected the Milicia seedlings from Phytolyma attack as to ensuring 100%. Milicia excelsa seedling survival at the end of the experiment resulting to zero mortality for the treatments integrated with net of both blue and green colours. The control experiment dimethoate only recorded 54% Milicia survival and 46% mortality (Fig. 2).

Table 1:	Effect of different treatments on the growth of Milicia excelsa
Mean values followed by of the same letter in a column are not significantly different at 5% level of probability, **Significant at 1% level of probability, NS: Not significant, GA: General average, CV%: Percentage coefficient of variation

Fig. 2:	Effect of different treatments on the mortality and survival of Milicia excelsa seedlings

DISCUSSION

Integration of barrier nets of blue and green colours with synthetic inscticides (dimethoate and carbofuran) against P. lata infestation on Milicia excelsa showed a very highly promising control measure of P. lata on M. excelsa at the early stage in the field. The integrated approach of blue and green net barrier recorded a significant success over the use of synthetic insecticides only (control) on the rate of infestation by P. lata. The M. excelsa plants covered with the blue and green nets did not have any infestation for a period of one year. This results corroborate the earlier report by Rahaman and Prodhan (2007) that zero pests infestation was observed in cucumber plants under the net barrier. The M. excelsa attained a height of 1.12 and 1.10 m on the blue and green nets, respectively at the end of one year. This result validates the earlier report by Ofori and Cobbinah (2007) that the less susceptible ones may grow up to 1.8 m high in a year. It also implies that the shading by the net did not affect the plant growth through reduction in light intensity that may be required by the plant. This study also revealed that nylon net (16 mesh) used is adequate in preventing the adult P. lata from gaining access to the plant to lay eggs. Moreover, it implies that the first instar nymphs that usually initiate gall formation is not active crawler which can migrate from one plant to another. Furthermore, the colours of the nets (blue and green) could serve as deterrence to P. lata in identifying M. excelsa plant. Moreover, the covering of Milicia seedling with the nets in the field reduces the intensity of sunlight on the plant thereby enhancing the increase in plant height. This view is in line with the report by Wagner et al. (1996) that at the lowest levels of sunlight (18%) seedlings were 50% taller than in 57% sunlight and 100% taller than in full sunlight. This implies that M. excelsa seedlings do not require full sunlight for growth. There was 100% survival of Milicia seedlings covered with nets after two years. This implies that net barriers (16 mesh) can adequately protect Milicia plants from P. lata attack for a very long period if proper maintenance and other control measures are incorporated.

Therefore, use of barrier nets in the field is a promising approach in the management of P. lata infestation for successful plantation establishment. The barrier will protect the plant from early attack that subsequently leads to death of the plant in severe situation. At a year old, the root system has developed and it will facilitate regeneration after the attack and subsequently prevent die back of the plant stem shoot. Ofori and Berg (2007) reported that the regeneration rate of Milicia in Ghana during the 1980s was anticipated to be about 29,000 m³ year^-1. This implies that M. excelsa has ability to regenerate.

Use of barriers are environmentally friendly way of keeping pests large or small from attacking valuable crops. Use of barrier nets has been successfully used by several scientists to control insect pests of vegetable and fruits in the garden. Apart from the environmental and health safety, net barrier are cost efficient compare to use of chemicals.

CONCLUSION

The use of net barrier was found to provide adequate protection from P. lata attack. A physical barrier preventing the adults from reaching the stem to lay their eggs is a potential control method and since mosquito netting is widely available in Nigeria it could be easily procured by farmers for such purpose. Subsequent incorporation of the systemic insecticides such as carbofuran after some times when the net is getting worn out is ideal and is environmentally safe and should be adopted for raising Milicia excelsa seedling in the field.

ACKNOWLEDGEMENT

This study was part of the first author’s postgraduate work in the Department of Crop Protection and Environmental Biology, University of Ibadan. We are grateful to ETF for sponsoring part of this study and to the Forestry Research Institute of Nigeria for granting the enabling environment towards the success this study.