MATERIALS AND METHODS

Experimental site: Field trials were conducted at the Teaching and Research Farm of Federal College of Forestry Ibadan, for two years (2010-2011). 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 per annum while the annual temperature is 34.40°C the daily humility is about 60%¹⁷.

Evaluation of mixed planting with other tree species for the control of P. lata on M. excelsa:The method for evaluating the effect of mixed planting of tree species with M. excelsa for control of P. lata infestation was adopted from Forrester et al.¹⁸. An experimental plot 30×30 m² was cleared manually with cutlass and divided into four sub plots. Six-months old M. excelsa seedlings of uniform heights (20 cm) collected from screen house and six months old seedlings of Azadirachta indica, Cedrela odorata and Pierreodendron africanum collected from Forestry Research Institute of Nigeria (FRIN) nursery were planted in binary mixture.

A row consisted of five seedlings and was replicated five times giving a total 25 Milicia plants and 25 plants of other tree species in each sub plot in a Randomized Block Design (RBD) The control plot was mono plantation of Milicia excelsa at the same spacing of 3×3 m². The plots were maintained by weeding manually at 6 weeks intervals. Data were collected on the plant height (cm), stem girth (mm), number of branches at monthly intervals and on number of galls and P. lata population at 2 weeks interval until two years.

Plant height (cm) was measured from the soil level to the terminal bud using meter rule. Stem girth (mm) was measured with the aid of veneers caliper. A mark was made on the plant at 5 cm from the ground level and all the measurements were taken at that marked portion until the end of the experiment. The number of branches was assessed by direct counting of the number of branches observed on each M. excelsa plant. Number of galls on the test plants was assessed by direct counting of the galls at 2 weeks intervals while size of gall was measured with the aid of thread which was later stretched on meter rule to determine the actual length. The population of P. lata was assessed by close observation on the leaves, stem and branches of the Milicia plant and direct counting of P. lata adult.

Evaluation of weeding regimes for the control of Phytolyma lata on Milicia excelsa:Six-months old healthy seedling of M. excelsa raised in the screen house were transplanted in a 10×30 m² experimental plot at the spacing of 2×2 m at the rate of one seedling per stand and five seedlings per row in three replicates. The experiment was laid out in Randomized Block Design (RBD) in four replicates. The treatments were four weeding regimes: Two weeks weeding regime, 4 weeks weeding regime, 6 weeks weeding regime and control (3 months weeding interval). Each block was allotted to one weeding regime and labeled accordingly. The plots were weeded manually with hoe and cutlass according to the weeding regime stipulated for it. Horse dung manure collected from polo club Ibadan when horses were fed fresh grasses only were applied to the M. excelsa seedlings after 2 weeks of transplanting at the rate of 5.0 t ha^–1. Data were collected on Milicia height, stem girth and number of branches at monthly intervals and on number of galls and P. lata population density at 2 weeks interval until the end of the experiment following the same procedure applied on the previous experiment.

Statistical analysis: Data collected on growth parameter were analyzed by one way analysis of variance (ANOVA) followed by Turkey’s test while data on number of galls and population density of P. lata were transformed using square root transformation. All the analysis were performed using ASSISTAT version 7.6 beta statistical assistance¹⁹.

RESULTS

Effect of mixed planting with companion plants on P. lata infestation on M. excelsa:Field infestation by P. lata was significantly (p<0.01) lower on M. excelsa seedlings mixed with Cedrela odorata and neem compared to control. The percentage infestation of M. excelsa mixed with C. odorata. and neem were 12.94 and 22.01%, respectively (Fig. 1).

Mixed planting of M. excelsa with different tree crops also showed a significant difference (p<0.05) on the growth of M. excelsa. The highest plant height, stem girth and number of branches of Milicia was obtained on plot mixed with Cedrela odorata with mean value of 72.80 cm, 0.94 mm and 1.79, respectively (Table 1).

Table 1:	Mean growth of M. excelsa inter-planted with companion plants 2 years after transplanting
a,bMean values followed by the same letter in a column are not significantly different at 5% level of probability. *Significant at 5% level of probability, CV (%): Percentage coefficient of variation, NS: Non-Significant

Fig. 1:	Effect of mixed planting on the Phytolyma lata infestation on Milicia excelsa

Fig. 2:	Effect of weeding regimes on the Phytolyma lata infestation on Milicia excelsa

Table 2:	Percentage survival and mortality of M. excelsa 2 years after transplanting with three companion plant species
a,bMean values followed by the same letter in a column are not significantly different at 5% level of probability

Table 3:	Mean growth of M. excelsa on different weeding regimes 16 weeks after transplanting
a,dMean values followed by the same letter in a column are not significantly different at 5% level of probability. **Significant at 1% level of probability. CV (%): Percentage coefficient of variation

Similarly, the plots mixed with C. odorata and neem had the highest percentage survival 24 months after transplanting. Approximately 52.23% of M. excelsa survival was recorded while the least M. excelsa survival recorded in sole plantation (control) was 24% (Table 2). There was no significant difference (p>0.05) between the effect of mixed planting with C. odorata and neem on the percentage mortality and survival of M. excelsa. There was a significant (p<0.05) increase in plant height of transplanted seedlings of M. excelsa weeded at 2 weeks interval (48.42 cm) at 16 weeks after transplanting, followed by 4 weeks weeding interval (41.62 cm). These translated to an increase of 14.83 and 10.18% in seedlings weeded at 2 weeks and 4 weeks weeding regime (interval) respectively above the control by 16 weeks after transplanting.

Similarly, there were significant differences (p<0.01) among the treatments on the stem girth . Two weeks weeding regime had the highest stem girth (0.58 mm), followed by 6 weeks weeding regime (0.45 mm) and the least was the control (0.35 mm) (Table 3). Field infestation by P. lata in M. excelsa seedling was lower in 12 and 6 weeks weeding interval by 15.6 and 3.34%, respectively compared to 2 weeks weeding interval (Fig. 2). The control experiment had the least number of galls, sizes of galls and population density of P. lata. However, there were not significant differences (p<0.05) among the different weeding intervals and the control.

DISCUSSION

In this study, mixed planting of Milicia with Cedrela odorata, A. indica (neem) and Pierreodendron africanum did not have significant effect in reducing P. lata infestation. There was no significant difference (p<0.05) on P. lata infestation between mono plantation and mixed plantation. This corroborate report by Ratnadass et al.²⁰ that vegetation diversification does not necessarily reduce the incidence of pests and diseases. Similarly, Plath et al.²¹ reported that higher herbivore damage to Tabebui rosea was found in mixed tree diversity than in mono plantation/stands. In contrast, Wagner et al.¹⁴ reported that Milicia planted in mixture with Gliricidium sepium reduced gall formation in the mixed plots compared to the pure Milicia stands (Mono plantation). Also, Bosu et al.¹³ found that planting M. excelsa or M. regia in a mixture with Terminalia superba was effective in reducing damage from P. lata attack. On arable crops, Mutisya et al.²² reported that agronet covers and companion cropping with a row of basil planted between adjacent tomato rows significantly (p<0.05) lowered B. tabaci infestation in tomatoes by 68.7%. Legaspi et al.²³ also reported that intercropping mustard (Brassica juncea) as a companion crop for collards (Brassica oleraceae var. acephala) successfully reduced whitefly infestation. Milicia excelsa planted in mixture with C. odorata significantly (p<0.05) showed higher plant height and stem diameter than those in pure stands. This is in support with report by Paul and Weber²⁴ that Astronium graveolens, Cedrela odorata and Terminalia amazonia planted in mixture with Zea mays and Cajanus cajan showed significantly superior growth performance over those in pure plantation.

Mixed planting of Milicia could probably reduce the P. lata infestation as reported by earlier researchers if transplanted in existing/old mono plantation of other tree crops not in a new plantation where the mixed trees are of the same age with the Milicia plant. The existing tree crop which provides shade environment will serve as deterrence to P. lata from locating its host plant Milicia and consequently reduce infestation. This assumption corroborate the report by Jonsson et al.²⁵ that of coffee berry borer infestation on cocoa were reduced in shade plantation than on sun-exposed plantations. Similarly, Wagner et al.¹⁴ also reported that mixed planting with other trees species and the use of different shade environments reduces the pest population, activity, abundance of galls and enhance growth of Milicia spp. Wagner et al.²⁶ reported that agro forestry and mixed species plantation approach could be used to successfully manage Phytolyma lata, implying that the over story shade is beneficial to M. excelsa at some stages. Wagner et al.¹⁴ also reported that deep over story shade during the early stages of growth can reduce Phytolyma gall formation, prevent dieback and associated loss of seedlings. In contrast, Bosu et al.¹³ reported that deep shade can minimize the quantum of photosynthetic radiation needed for growth, which can result in seedlings becoming etiolated and eventually dying.

Moreover, this study showed that 12 weeks weeding regime reduced the incidence of P. lata infestation. This indicates that shading effects of different vegetation affect the population of P. lata, their activity, abundance of galls and growth of Milicia excelsa. Guyot et al.¹² reported that infestation by Dryocosmus kuriphilus on chestnut trees Castanea sativa was lower on stands with higher tree species richness or diversity .

On the contrary, Nowak et al.²⁷ reported that competing vegetation was an important factor in population stability of some insect species such as the pine tip moths. Moreover, Sun et al.^28,29 reported that low levels of competing vegetation are often associated with higher tip moth infestation rates. Similarly, Pinus silvestris and Quercus humilis were found favored by the presence of a dense under storey, particularly when shrubs were higher than seedlings^30,31. This implies that companion plants encourages development of some insect pests infestation rather than reduction.

CONCLUSION

This present study reveals that planting of M. excelsa with companion plants of the same age was not highly promising in reducing Phytolyma lata infestation on Milicia seedlings. Twelve (2 week) weeding intervals minimized P. lata attack on Milicia excelsa during early growth. Therefore, 12 weeks weeding interval can be recommended to reduce P. lata infestation on Milicia plantation at the early stage and further studies on planting of Milicia in mixture with old companion plants in plantation is required to confirm their potential in reducing P. lata infestation.

SIGNIFICANCE STATEMENTS

This study revealed the potential of using companion plants as a control option for Phytolyma lata infestation on Milicia excelsa at the early stage of plantation establishment. The study will help the researchers to expose the ecological relationship between P. lata and Milicia and its cryptic nature that contributes to their complexity in management. Thus a new assumption for their control may arrive.

ACKNOWLEDGMENTS

The authors are grateful to Education Trust Fund 2008/2009 ETF- AST and D for sponsoring this study and to the Forestry Research Institute of Nigeria for granting the enabling environment towards this study.