Milicia excelsa (Welw) C. C. Berg commonly known as Iroko is an important
timber trees species in Africa. It belongs to the family Moraceae and distributed
across the entire breadth of Africa (Keay, 1989). M.
excelsa wood is 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 high demand of M. excelsa timber and
other products from Milicia wood such as sliced veneer, rotary veneer
and profile boards for decorative and structural uses. Cultivation of Milicia
species is being constrained widely across West Africa by Iroko gall bug,
Phytolyma lata (Cobbinah and Wagner, 1992; Cobbinah
and Appiah-Kwarteng, 1996). The psyllid (iroko gall bug) attack the buds
and young leaves of Milicia excelsa plants especially the seedling which
later leads to formation of galls on the site of attack. The galls afterward
burst to release adults when they psyllid completes their life cycle inside
the gall. The bug lays eggs on young stems, leaves and shoots of the host tree
in high numbers (Nichols et al., 1998). When
the first instar nymphs hatches, it punctures plant surfaces and induces gall
formation which later develops to enclose the nymph inside the gall.
Generally, gall formation is followed by saprophytic fungi attack on the apical
region of the infested plant after the gall has ruptured to release adult leading
to foliage die-back down to the wood tissue (Cobbinah and
Wagner, 1995). However, the attacks decrease with age, as the old trees
tend to be more resistant. Phytolyma attack stunts its growth and affects
stem shape and size and also reduces the regeneration rate of the species.
Over the years, attention of researchers has been on natural regeneration (Nichols
et al., 1998) and evaluation of progeny for resistant lines of M.
excelsa (Cobbinah and Wagner, 1995) but a great
deal of success has not been recorded. Also several chemicals have been evaluated
in both Ghana and Nigeria for control of Phytolyma species but prove
ineffective due to the cryptic nature of the pest. The knowledge of the biology
of a pest gives an insight on the management approach that could be adopted
for its Integrated Pests Management (IPM). However, currently there is dearth
information on the biology of Phytolyma lata in Nigeria. Therefore this
work reports the reproductive and developmental biology of Phytolyma lata
on Milicia excelsa under laboratory condition.
MATERIALS AND METHODS
Experimental site: The experiment was conducted at the Entomology Laboratory
of Department of Crop Protection and Environmental Biology, University of Ibadan
under ambient temperature, 24-27°C, 88-92% relative humidity and 12:12 day
photoperiod. Treatments were arranged Completely Randomized Design (CRD) and
all measurement were made under microscope fitted with an ocular micrometer
Insect culture: Six month old Milicia seedlings were placed
inside wooden cage (170 cm heightx30 cm diameter) and a matured gall collected
from the wild were introduced inside the cage. The galls were allowed to release
adult P. lata inside the cage and the seedlings were infested. The plants
were watered daily and monitored for gall formation. The P. lata culture
was maintained in the cage until the end of the study (Fig. 1).
Life cycle study
Egg: Teneral adults were picked with the aid of a camel hair brush and paired
in oviposition cage (13 cm heightx6 cm diameter). The sexes of the insects were
determined by comparing their external genitalia with the aid of hand lens.
Fresh Milicia leaves and stem were introduced into separate cages daily
and sugar solution (25%) soaked in cotton wool was introduced to supplement
their food. The experiment was replicated four times. The paired adult P.
lata was monitored for oviposition, the female laid on the fresh leaves
and stem which was replaced daily until the female stopped ovipositing. Eggs
on the leaves and stem were removed, counted and placed in another cage for
incubation. The incubation cage was kept moist to prevent desiccation. The pre-oviposition,
oviposition period, fecundity, incubation period and egg viability were recorded.
The egg viability was determined by counting the number of first instar nymph
that hatched out from the incubated eggs after two days. A sample of 20 eggs
was collected with the aid of camel hair brush and measured for egg length and
Nymphal stages: Twenty (20) first instar nymph were transferred with
aid of camel hair brush from the incubation cage to the wooden cage (170 cm
heightx30 cm width) which enclosed a six-month Milicia seedling and were
replicated four times.
|| Culturing of Phytolyma lata under laboratory condition
The nymphs were allowed to feed on the young leaf tissues of the shoot apex
and thereafter monitored for gall formation. At the initiation of gall formation,
starting from the second day of gall formation, each gall was split open every
day to search for exuviae, which clearly indicated molting (Ewete
and Olagbaju, 1990). Twenty samples of each nyphal stage were collected
and preserved in 70% alcohol for measurement of the body length, body width,
head width, the developmental period of each nymphal stage and records of characteristics
of each nymphal stages. The observed head width was tested for conformity with
Dyar,s rule (Dyar, 1890) by comparing the observed and
calculated average head width. The calculated average head width of an instar
was obtained as a product of the mean head width of the succeeding instar and
its mean growth ratio. The growth ratio was calculated as the quotient of the
observed mean head width of the succeeding instar and the observed mean head
width of the previous instar.
Adult: Thirty male and female newly emerged adults were collected and
preserved in 70% alcohol for the measurement of body length, body width and
head width. The sex ratio was determined by introducing a pair of teneral adult
to a six-months old M.
excelsa seedling inside a wooden cage (170 cm heightx30 cm diameter)
where they fed, mated and oviposited on the potted plant. Galls formed were
monitored until maturity and adult were released under natural condition. The
emerged adults were counted and the sex ratio was determined per each cycle.
The longevity of mated and unmated adults was also determined.
Data analysis: Data on the morphometric of the nymphal and adult stages
were subjected to Analyses of Variance and significant means were separated
using Duncan Multiple Range Test (DMRT). The relationship between the head width
and developmental period were subjected to Correlation analysis.
Phytolyma lata have seven life stages: Egg, five instars and the adult.
The female gall bug oviposited 24 h after emergence and oviposition lasted 3-6
days with an average of 4.00 days. The average number of eggs laid per female
was 41.2±5.35 with range from 32-45 eggs (Table 1).
The eggs were laid either in rows or sometimes scattered singly on the leaves
or stem of M. excelsa. About 35-66% of the eggs were laid on the first
day and within 2-3 days over 80% of the total number of eggs had been laid.
The pre-oviposition period of mated female P. lata under laboratory condition
on M. excelsa leaves was 1.4±0.54 and 2.00±0.50 on the
stem with a range of 1-2 days (Table 1). Eggs are oval in
shape, creamy white when just laid and anteriorly pointed (Fig.
2a). The egg was 0.4±0.50 mm in length and 0.2±0.50 mm in
width (Table 2). The average incubation period of the eggs
was 5.80 days. The newly emerged first instar nymph is cream colour, head quadrangle
with a pair of compound eyes, two antennae, the thorax and abdomen are not segmented
(Fig. 2b). The first instar nymph was 3.2±0.92 mm in
length and 1.90±0.74 mm in width (Table 2). Second
instar nymph is creamy white in colour, with two prominent antennae, the two
eye spot were dark brown in colour (Fig. 2c). The mean body
length was 9.50±0.50 mm, width 7.20±1.77 mm and the head width
4.45±0.60 (Table 2). Third instar is creamy in colour;
eye spot dark brown, the thorax and the abdomen were clearly defined, segmented
and with wing pad (Fig. 2d). The mean body length was 16.63±3.55
mm, body width 10.64±1.36 mm and head width was 6.59±0.91 mm (Table
2). Fourth instar nymph are greenish; the abdominal segment distinct and
clearly demarcated from the thorax, eye spot dark and reduced (Fig.
2e). The mean body length was 28.0±3.21 mm, mean body width 12.93±1.67
mm and head width 7.50±0.76 mm (Table 2). The fifth
instar is greenish, eye spot dark and reduced, wings are developed though very
tender and transparent, the abdominal segment and the genital organ appeared
distinct. The general body feature presented an appearance of miniature adult
(Fig. 2f). The mean body length was 30.00±0.67 mm,
body width 12.20±1.40 mm and head width 7.40±0.70 mm (Table
2). Adult is dark brown in colour with fully developed pair of wing adapted
for short flight.
|| Pre-oviposition, oviposition periods, fecundity and incubation
periods of P. lata on M. excelsa
|| Mean developmental periods and body morphometrics of developmental
stages of Phytolyma lata on Milicia excelsa
|Growth rate: The mean head width of a preceding instar divided
by the mean head width of a succeeding instar
||Life stages of Phytolyma lata, (a) Eggs x100, (b) 1st
instar x100, (c) 2nd instar x100, (d) 3rd instar x50, (e) 4th instar x50,
(f) 5th instar x25, (g) Adult male x25 and (h) Adult female x25
|| Head width for nymphal instars and t-test for conformity
to Dyars rule
|Growth ratio: Observed mean head width of nymphal instar divided
by observed mean head of succeeding nymphal instar, Calculated mean = observed
mean head capsule (mm) of a succeeding instar multiplied by the mean of
the growth ratio, Mean of the growth ratio = 0.75
There are distinct light yellow markings that separate each abdominal segment,
the colour of the markings of the female are more distinct and deeper in colour
than that of the male (Fig. 2g and h). The
female has ovipositor at the last segment which has dark spots at the tip while
the male has aedeagus which when pulled out during sex are bifurcated. The mean
body length of the male was 32.57±2.80 mm, female 35.43±2.82 mm;
body width of the male was 11.80±1.97, female 15. 40±1.43; the
head width of the male was 7.43±0.97 mm, female 9.47±0.73 mm (Table
2). The mean duration of developmental period of the nymphal instars were:
1st instar 3.50±0.25 days, 2nd instar 3.25±0.43 days, 3rd instar
2.00±0.00 days, 4th instar 2.00±0.00 days and 5th instar 1.00±0.00day.
The total developmental period of the nymphal instars was 11.75 days (Table
|| Relationship between the nymphal stages of P. lata and
their head width
|| Mean longevity (days) of adult P. lata fed with sugar
solution under laboratory condition
|Means followed by same letters in a column are not significantly
different at 5% level using duncan multiple range test *: 5% level of significance;
**: 1% level of significance
There was significant (p<0.05) correlation (r = 0.973) between the stages
of nymphal development and the head width (Fig. 3). Ninety-five
percent of eggs laid hatched under ambient temperature 24-27°C, 88-92% relative
humidity and 12:12 h day photoperiod.
The observed values of head width of various larval instars when compared with
calculated values showed conformity to Dyars
rule (Table 3). The mean longevity of mated adult males was
4.00 days, unmated 7.00 days while for mated female it was 4-75 days, unmated
8.75. Mated male lived significantly (p<0.05) shorter than the mated female
under laboratory condition. Moreover, unmated male lived significantly (p<0.01)
shorter than unmated female (Table 4).
The female Phytolyma lata were oviparous under laboratory condition.
Eggs were laid in batches of 3-15 on the leaves and stem of young Milicia
seedlings, a higher batches of eggs were reported by Wagner
et al. (1991).
The mean incubation period of P. lata eggs in this study under laboratory
condition was 5.8 days ranged (4-8 days). This was in line with the earlier
report of Wagner et al. (1991) that the incubation
period of P. lata is about 8 days. White (1966),
Browne (1968) and Cobbinah (1986)
also reported approximately 8 days of incubation. However, when eggs were kept
in the cages and covered with leaves, the incubation period was shorter. This
indicates that temperature is a major factor in the incubation of the egg. It
also suggests that the incubation period of P. lata may vary from one
location to another depending on the prevailing environmental conditions. The
little variation in the incubation period of P. lata in this study and
that reported by Wagner et al. (1991) could be
attributed to the variation in the environmental conditions of the different
study areas. Ninety five percent of egg viability observed in this study indicates
the successful reproductive ability of the insects. This also attributes to
the high population of the insects that account for the constant infestation
on M. excelsa throughout the year. The viability of the eggs enhances
their chances of multiple generations that were earlier reported. Cobbinah
(1986) reported that Phytolyma is multivoltine and ten or more generations
can be recorded in a year. It was observed that P. lata completes it
life cycle on M. excelsa between 24-26 days (3-4) weeks. This was in
the same range with the earlier report by Cobbinah (1986),
that it takes 2-3 weeks for P. lata to completes it life cycle. Female
P. lata are moderately fecund, with an average of 41.2 eggs being laid
by a female under laboratory condition. In contrast, Wagner
et al. (1991) reported that approximately 9-546 eggs are laid by
female. The environmental factors could be responsible for the variation in
the fecundity of the female P. lata. Higher number of eggs were laid
by the female at the beginning of the rainy season (March-June) and the sizes
of adult P. lata were also bigger. The variations are probably due to
variation in the environmental conditions. This suggested that environment was
more favourable for the insect during the rainy season. There were five nymphal
instars as was previous reported by Cobbinah (1986).
The ratio of the previous instars nymphal head width to the succeeding
one obeyed Dyars rule (Dyar, 1890). The head size
increased at each molt by an average of 1.48 approximating the expected constant
ratio of 1.4 for lepidopterous (Wigglesworth, 1974).
A linear relationship was obtained between the head width and with a high correlation
coefficient of 0.91. This indicates that variation in the developmental period
for each nymphal instar was not strong enough to vacate Dyars rule.
The problem of P. lata on Milicia excelsa can be curtailed with
identification of appropiate Integrated Pest Management (IPM). The proper understanding
of the biology of a pests provides the pedestal for a more sustainable control
method. Therefore, this work has provided a base for the researcher to design
a sustainable Integrated Pest Management(IPM) for Phytolyma lata.
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 of this study.