Effects of Net Barrier and Synthetic Insecticides on Phytolyma lata
(Homoptera: Psyllidae) Infestation, Growth and Survival of Milicia excelsa
in the Field
The effect of net barrier and synthetic insecticides on Phytolyma lata
infestation, growth and survival of Milicia excelsa seedlings was investigated
during the year 2010-2011 in Ibadan Southwest Nigeria. In a complete randomized
design with three replications, a 2 m high netted barrier were laid out in unit
plots of 15x15 m2 using five treatments: Blue net+dimethoate+carbofuran,
Green net+dimethoate+cabofuran, Blue net+dimethoate, Green net+carbofuran and
dimethoate only (control). Plant height, stem diameter, number of branches,
numbers of galls, Pytolyma lata density, plant mortality and survival
of Milicia seedling were recorded at monthly interval for two years.
The treatments showed a significant difference (p<0.05) over the control.
The net barrier plants were free from P. lata infestation 12 months after
planting. The combined treatments of blue net barrier, dimethoate and carbofuran
recorded least number of galls (7.89%) on Milicia seedlings while combined
treatments of green net, dimethoate and carbofuran recorded least population
density of P. lata (13.59%). There was no significant difference (p<0.05)
between the combined treatments of blue and green barrier nets on the plant
growth, although blue net+dimethoate+carbofuran recorded highest plant height
and stem diameter with mean of 122.50 cm and 1.03 mm, respectively. Integrated
treated plots recorded 100% seedling survival while control plot recorded 54%
seedling survival after two years. Integration of net barrier with carbofuran
and dimethoate is economical and should be adopted for successful establishment
of Milicia plantation.
Received: August 29, 2013;
Accepted: March 22, 2014;
Published: April 18, 2014
Milicia excelsa (Welw.) C.C. Berg. Iroko Moraceae is an important timber
tree species in Africa. It is distributed across the entire Africa from Sierra
Leone, through Central and East Africa to Mozambique (Taylor,
1960; Irvine, 1961; White, 1966;
Keay, 1989). M. excelsa requires substantial moisture to grow well
but not necessarily a high rainfall. Areas with 760-1520 mm mean annual rainfall
are suitable for the species (Pukkala, 2000). However,
the species has also been reported to grow well under mean annual rainfall as
low as 700 mm.
M. excelsa is considered as a light-demanding pioneer tree species which
regenerates in disturbed open areas and in logged forest (Hawthorne,
1995). M. excelsa wood is one of the most valuable timber species
from all of West, Central and East Africa. The wood is extensively used due
to its high durability and good working properties. There is considerable demand
of M. excelsa timber products for decorative and structural uses. The
wood is also used for making fences, equipment and other construction purposes,
they represents a significant part of the timber and furniture export income
to Ghana and Côte dIvoire (Nichols et al.,
1998). The species also plays an important role in erosion control and soil
fertility improvement. M. excelsa leaves are used as mulch and the species
is a tremendous source of shade or shelter and is sometimes used as an avenue
tree (ICRAF, 1992).
Several efforts to establish plantations of Milicia species by many
farmers as well as researchers have been constrained severely by the gall forming
psyllid, Phytolyma lata which attacks young growing tissues. P. lata
attack is followed by foliage die-back down to the wood tissue which subsequently
leads to total death of the plant in severe situation (Cobbinah
and Wagner, 1995). However, the attacks decrease with age, as the old trees
tend to be more resistant. According to Cobbinah (1993),
a number of chemical have been evaluated for the management of Phytolyma
attack on Millicia species in Ghana but little or no success was
recorded. Moreover, other pest management strategies like cultural and biological
methods have been implemented to lessen the damage caused by P. lata
in many West African countries but none of the strategies has had satisfactory
impact. In Nigeria, management of P. lata to achieve successful establishment
of M. excelsa plantation has also proven difficult due to the hidden
nature of the pest. According to Knipling (1972), integrated
approach in Pest Management (IPM) has been encouraged by entomologists and ecologists
for adoption in pest control over the years as it allow for a safer means of
controlling pests. Therefore, this study assessed the effects of net barriers
and synthetic insecticides on Phytolyma lata infestation, growth and
survival of Milicia excelsa in early plantation.
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.
||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
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.
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.
|| Effect of different treatments on the growth of Milicia
|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
|| Effect of different treatments on the mortality
and survival of Milicia excelsa seedlings
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.
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
This study was part of the first authors 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.
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