Full text of DOI:10.1080/096708700415544

INTERNATIONAL JOURNAL OF PEST MANAGEMENT, 2000, 46(3) 205±209
Effects of phosphate fertilizer levels on cowpea pod-sucking bug populations and damage
(Keywords: control, phosphate, pod-sucking bugs, infestation, damage)
²³
³
OLUFEMI O. R. PITAN* , J. A. ODEBIYI and G. O. ADEOYE§
³Department of Crop Protection and Environmental Biology, University of Ibadan, Ibadan, Nigeria
§
Department of Agronomy, University of Ibadan, Ibadan, Nigeria
Abstract. The effect of different application levels of phosphate
fertilizer on cowpea pod-sucking bugs [Clavigralla sp., Riptortus sp.,
Anoplocnemis., Mirperus sp. and Negara sp. (Heteroptera)]was studied
using 0 (control), 20, 40 and 60kg/ha of single super-phosphate. The
results showed that the bug densities decreased significantly with
increase in rate of application. At 40 and 60 kg/ha, grain yields were
significantly higher than at the lower rates of application. Crude protein,
fibre, oxalate, phytin, potassium and magnesium contents as well as
seed hardness also increased with increase in the rate of fertilizer
application.
only increases seed yields but also nodulation (Luse et al.,
1975). Other workers have reported that phosphate application
influences the contents of other nutrients in leaves (Kang and
Nangju, 1983) and seed (Omueti and Oyenuga, 1970). Absorp-
tion of phosphorus occurs primarily at the end of the growing
period and is mainly transported to the seed (Singh and Rachie,
1985). At the present level of management in tropical Africa,
phosphate is the single most recommended fertilizer (P2O5; 20±
60kg/ha) (Rachie and Roberts, 1974).
The longevity, fecundity and damage by insects and mites
are greatly affected by the major plant nutrient components of
fertilizer (Anonymous, 1969).
1. Introduction
In most parts of Africa, the traditional method of maintaining
soil fertility and productivity has hitherto been the bush fallow
system, whereby available land is allowed to revert to fallow after
3±4 years of continuous cultivation. However, with agriculture
becoming more and more intensive, coupledwith the introduction
of high yielding and more nutrient-demanding crop varieties, it
became obvious that farmyard manure could not be obtained in
sufficient quantities to meet the farmers’ demand. Even where
available, transportation problems and labour costs unavoidably
limit its use on a routine basis. In this circumstance, attention was
givento mineralfertilizers as the only obvious alternative. The first
recorded indication of the potential value of inorganic fertilizers in
Nigeria was in 1937, when it was shown that the response of
cereal crops to small applications of farmyard manure was
matched by the use of single super phosphate-containing
quantities of phosphate equivalent to that in organic manure
(Federal Department of Agriculture, 1989). This, together with
similar studies in parts of southern Nigeria, marked the beginning
of fertilizer usage in the country. The maintenance of high yields
underintensive cultivation in present-day Nigeria is possible only
through the use of inorganic fertilizers. Next to soil acidity,
shortage of phosphorus is the major constraint upon food
production in the humid tropics (Federal Department of Agricul-
ture, 1989). In the semi-arid tropics, phosphorus deficiency is only
second to nitrogen as the most frequently encountered agro-
nomic problem.
Several conflicting observations have been reported on the
influence of fertilizers on the population of pests in fields. For
example, a high level of nitrogenous fertilizer makes corn plants
more succulent, rapid growing and attractive for oviposition by
the European corn borer moth (Ostrinia nubilasis Hubner)
(Lepidoptera: Nuctuiidea). Phosphorus applied alone or mixed
with nitrogen at planting time has been found to be closely
associated with the cutting activities of the wheat stem sawfly
both in winter and spring wheat (Anonymous, 1969). The same
author also reported that potassium, applied together with
nitrogen and phosphorus, decreased the amount of sawfly
cutting in wheat. Panchabhaviet al. (1974) noted that nitrogen in
combination with phosphorus increased the rate of infestation
damage caused by sorghum shootfly, Atherigona soccata.
However, potassium in combination with phosphorus reduced
the infestation of potato aphids and leaf hoppers (El-Saadany et
al., 1977). Nitrogen, coupled with phosphorus, gave a higher
degree of incidence of gall midge and yellow stem borer. But
NPK lowered the incidence of rice bug, leaf folder, gall midge
and yellow stem borer (Pramanick et al.,1995).
Shri Ram et al. (1990) also showed that increase in
phosphorus concentration alone reduced leaf hopper population
consistently in wheat, they explained that the yield obtained in
the plot containing a high concentration of phosphorus was due
to some complex mechanisms, occurring in plant systems.
These mechanisms help in building up anatomical features of
the foliage so that plants develop resistance to the damaging
activity of the defoliators. The present study explored the effects
of use of single superphosphate fertilizer on the population of
Phosphorus, while not needed in large quantities, is critical
to cowpea yield (particularly for improved photoperiod insensi-
tive cultivars) because of its multiple effects on nutrition; it not
*To whom correspondence should be addressed at: PO Box 9643 (UI), Ibadan, Nigeria.
²
Present address: National Horticultural Research Institute (Nihort), Jericho Reservation Area, Idi Ishin, Ibadan, Nigeria. Fax: +234 2 2412230;
e-mail: nihort@infoweb.abs.net
International Journal of Pest Management
ISSN 0967-0874 print/ISSN 1366-5863 online Ó 2000 Taylor & Francis Ltd
http://www.tandf.co.uk/journals

206
O. O. R. Pitan et al.
cowpea pod-sucking bugs (PSBs) and the damage they cause.
The bugs studied were Clavigralla spp., Riptortus spp.,
Anoplocnemis spp., Mirperus spp. (Heteroptera: Coreidea) and
Nezara spp. (Heteroptera: Pentatomitidae).
Ten healthy seeds of about the same size were then selected
from each treatment and each was punched around the middle.
The pressure at which the seed snapped was read on the scale
attached to the tester. The mean seed hardness was measured
in kilogramms per square millimetre. The mean pressure (kg/
2
mm ) as determined by the hardness meter was calculated and
2. Materials and methods
subjected to analysis of variance and LSD.
The study was carried out at the Teaching and Research
Farm of the Faculty of Agriculture and Forestry, University of
Ibadan, Ibadan, Nigeria. `Ife Brown’, an improved local determi-
nate semi-erect cowpea variety of medium maturity, which is
highly susceptible to pod-sucking bug (PSB) damage, was used.
Each field experiment was carried out in the early and late
seasons of 1991 and 1992 using a completely randomized block
2.2. Chemical analysis of seeds
One hundred seeds fromeach of the treatments were ground
in a Christy-Norris hammermill using a 1/32inch mesh size sieve.
Thegroundmaterialwasdriedinthe ovenat105 C for6 handthen
8
stored in screw-capped bottles until needed for analysis. Two
grams of the dried samples were digested by the wet digestion
method. The digested materials were made up into four solutions
indifferent100 ml graduatedflasks. The nutrientcontents ofcrude
protein, crude fibre, oxalate, phytin, fat, moisture, ash, calcium,
phosphorus sodium, potassiumandmagnesiumwere determined
by methods described by IITA (1982, 1984).
´
design with four replicates. The plot size was 5 m 5 m and the
treatments were different application levels of single super
phosphate fertilizer of 0 (control), 20, 40 and 60 kg/ha (P2O5).
The fertilizer was applied by band application at planting. The
´
planting spacing was 30 cm 60 cm for all the treatments. The
cowpea seeds were planted at the rate of two seeds per hole. At
14 days after planting (d.a.p.) the plants were thinned to one per
stand. Deltametrin at 25g a.i./ha was applied at 15, 25 and
35d.a.p. against the leaf and flower pests of cowpea.
3. Results
Before planting in both seasons, the soil was sampled from
the planting site and chemical analysis was carried out for pH,
organic carbon, total nitrogen, available phosphate, exchange-
able calcium, magnesium and sodium as described by IITA
(1982). The assessment of the number and species of the PSBs
started at the budding stage (50 d.a.p.). This was done by visual
counting on ten randomly selected plants in the middle row of
each plot. Sampling was done at weekly intervals till pod
maturity (72 d.a.p.) between 7.30 and 9.30a.m. when the insects
were relatively less active. The data collected were subjected to
ANOVA. Least Significant Difference (LSD) was used to assess
significance of difference of the means of treatments. Data from
The fertility status of the soil at the experimental site is
shown in table 1. There was a general decrease in the PSB
population with increase in phosphate level irrespective of the
season (table 2). There were no significant differences (P > 0.05)
in the population of Clavigralla sp., Riptortus sp. and Anoploc-
nemis sp. between 0 and 20kg/ha levels of phosphate fertiliza-
tion. Species populations at these levels were significantly
higher (P < 0.05) than populations on plots treated at 40 and
60kg/ha which were not significantly different from each other.
Compared with other bugs, the populations of Mirperus and
Nezara spp. were generally low in both seasons and did not
respond to fertilization (table 2). Again, irrespective of seasons,
there was no significant difference in the damage to pods and
seeds between the control and plots treated at 20 kg/ha. These
were however, different from those of 40 and 60kg/ha, which
were not significantly different from each other (table 3). There
were no significant differences between the number of pods/
plant and number of seeds/pod between the control and the
other treatments, irrespective of seasons. The phosphate
fertilizer only affected seed weight; thus yield/plant and yield/
ha increased significantly (P < 0.05) with increase in level of
phosphate fertilizer (table 4). The yield/plant in the 40 and 60 kg/
ha plots were about twice as high as those of the control and 20
insect counts were transformed ( + 0.5) before analysis.
X
At maturity, all the pods were harvested and dried to a
moisture content of 12%. Damaged and undamaged seeds and
pods were sorted out for each of the treatments. A pod was
considered damaged if it had one or more constrictions along its
length, showing seed failure due to the bug attack, or if it was
shrivelled. Seeds damaged consisted of aborted seeds, wrinkled
or shrivelled seeds or seeds showing necrosis and/or feeding
lesions. Pods damaged by PSBs, when opened, contained
damaged seeds with darkly stained `feeding lesions’ compared
with physiologically aborted, wrinkled or shrivelled seeds.
Evaluation of damage by the PSBs was based a 100 pod-
samples per replicate per treatment. For the seed damage, 1000
seeds were selected randomly and the damaged and unda-
maged seeds were sorted out for each of the treatments. The
data on yield collected were subjected to ANOVA and LSD was
then used to assess the significance of differences between
means of treatments.
Table 1. Some chemical characteristics of the experimental site before
planting in the early and late 1991 cropping seasons
Early season,
1991
Late season,
1991
PH
6.2
1.8
1.1
7.7
2.8
0.4
0.2
5.9
2.1
1.6
8.9
2.2
0.5
0.1
Organic carbon (g/kg)
Total nitrogen (g/kg)
Available phosphate (mg/kg)
Exchangeable calcium (cmol/kg)
Exchangeable magnesium (cmol/kg)
Exchangeable sodium (cmol/kg)
2.1. Determination of seed hardness
Hardness of seeds was determined by using a hardness
tester to which a punching pin 5 mm in diameter was attached.
One hundred seeds from each of the 0, 20, 40 and 60 kg/ha
plots were dried in the oven for 48 h at 105 C in the laboratory.
8

207
Effects of phosphate on cowpea pod-sucking bug
kg plots. The results of the proximate analysis of the seeds
indicated that crude fibre, oxalate, phytin and moisture contents
did not increase with increase in phosphate levels. However,
crude protein, fat, ash, calcium, phosphorous, potassium and
magnesium contents increased at higher phosphate levels
especially between 40 and 60kg/ha treatments (table 5).
There was no significant difference in terms of hardness
betweenthe seeds inthe controlandthose of20kg/ha plots (table
6). But the seeds from 40 and 60 kg/ha plots were significantly
harder (P < 0.05) than those of the control and 20kg/ha.
and Pillai (1979) and Singh et al. (1981) on cowpea. These
workers all concluded that most tropical soils were deficient in
phosphorus in relation to other nutrients. This was also observed
in this study. It was reported that the yields, growth rate, leaf
area index, nodules per plant and dry matter production were
improved by phosphate fertilization. This improvement was
considered responsible for the significant increase in grain and
stalk yield as well as the increase in harvest index at high
phosphate levels (Singh et al., 1981). In addition to this, the
populations of PSBs and seed weight were found to be affected
by the high phosphate level in this study. Seeds from 40 and
60kg/ha phosphate-treated plots were significantly heavier and
suffered less damage than those of the control and 20kg/ha.
The significant reduction in the level of infestation agrees
with the reports of Shri Ram et al. (1990). It has been reported
that phosphorus, in combination with nitrogen and potassium,
4. Discussion
The significant differences in the yields of cowpea at higher
fertilizer rates are in agreement with the reports of Luse et al.
(1975), Singh and Lamba (1971), Swami and Pal (1974), Kumar
Table 2. The mean number of different species of PSB at different levels of phosphate fertilizer application on cowpea in the late 1991 and early 1992
cropping seasons
Phosphate level in kg/ha
0
20
40
60
LSD(0.05)
0
20
40
60
LSD(0.05)
PBS
Late season, 1991
Early season, 1992
Clavigralla sp.
Riptortus sp.
Anoplocnemis sp.
Mirperus sp.
Nezara sp.
41.4
7.1
7.2
1.1
1.5
37.4
7.1
7.3
1.1
1.8
29.6
5.6
3.9
0.7
1.1
27.3
4.1
4.3
1.0
0.9
4.2
1.1
2.6
0.4
0.9
29.4
4.0
4.0
0.7
1.1
28.6
4.6
5.3
0.4
1.3
20.3
2.3
1.8
0.4
1.3
19.5
1.8
1.4
0.1
0.7
2.7
1.4
2.1
0.4
0.6
LSD = Least significant difference.
Values were transformed ( x+ 0.5) before analysis.
X
Table 3. The percentage pod and seed damage by the PSD at different application levels of phosphate fertilizer
Late season, 1991 Early season, 1992
Pod damage Seed damage
Application levels
(P2O5 Kg/ha)
Pod damagea
Seed damageb
0
20
40
60
92.0
90.0
73.0
75.0
2.4
68.3
66.9
41.3
40.1
5.6
62.0
63.0
50.0
51.0
4.8
57.0
50.0
37.0
35.0
6.0
LSD(0.05)
aBased on 200 pods per treatment.
bBased on 1000 seeds per treatment.
LSD = Least significant difference.
Values were transformed arcsin (x) before analysis.
Table 4. The effect of different application levels of phosphate fertilizer on cowpea yield in the late 1991 and early 1992 cropping seasons
Phosphate level in kg/ha
0
20
40
60
LSD(0.05)
0
20
40
60
LSD(0.05)
Yield
Late season, 1991
Early season, 1992
Mean no. of pods/plant
Mean no. of seeds/pod
Mean 100 seed weight (g)
Mean yield/plant (g)
10.6
10.7
17.7
5.2
10.7
10.7
17.8
6.2
10.6
10.5
19.9
10.7
10.6
19.9
0.3
0.3
1.8
10.0
10.0
16.5
5.5
10.0
9.9
16.6
5.9
9.8
10.1
18.1
10.3
10.0
18.0
0.4
0.2
1.1
10.9
10.7
2.2
10.5
10.8
2.8
Mean yield/ha (kg)
287.9
345.3
604.9
593.4
128.3
302.3
323.3
583.0
596.0
63.4
LSD = Least significant difference.

208
O. O. R. Pitan et al.
Table 5. The proximate analysis of cowpea seed samples at different application levels of phosphate fertilizer in the late 1991 and early 1992 cropping
seasons
Phosphate application rates (kg/ha)
0
20
40
60
0
20
40
60
Nutrient
Late season, 1991
Early season, 1992
Crude protien
Crude fibre (%)
Oxalate (%)
21.0
10.5
0.2
22.0
11.0
0.2
24.5
11.0
0.3
26.5
11.8
0.3
24.1
12.5
0.1
24.3
12.5
0.1
25.8
12.8
0.1
27.1
12.9
0.1
Phytin
0.5
0.5
0.6
0.6
0.6
0.7
0.7
0.7
Fat
3.5
5.0
5.2
5.8
4.5
5.4
5.8
7.0
Moisture content (%)
Ash content (%)
Calcium (mg/kg)
Phosphorus (mg/kg)
Sodium (mg/kg)
Potassium (mg/kg)
Magnesium (mg/kg)
8.1
2.6
8.0
2.8
7.8
3.6
8.0
3.6
7.8
2.0
7.8
2.5
7.6
3.4
7.8
3.6
82.0
490.0
9.0
1010.0
280.0
84.0
520.0
9.0
1080.0
290.0
85.0
640.0
9.6
1101.0
290.0
86.2
690.0
10.0
1090.0
300.0
76.6
320.0
8.1
860.0
210.8
79.1
410.0
8.0
890.0
230.0
83.9
580.0
8.7
950.0
230.6
85.5
650.0
7.4
980.0
250.0
level probably indicated a difference in the iron compound
formed and stored in the seeds. Other compounds could also be
formed with Zn, Cu, Co and Mb whose concentration were also
raised. The rise in protein levels with increasing soil phosphate
level may be due to the role of phosphorus in energy
metabolism providing more energy for protein synthesis and it
is possible that more phospho-protein is also formed.
Table 6. The hardness of cowpea seeds at different application levels of
phosphate fertilizer as determined by the use of the hardness tester in
the late 1991 and early 1992 cropping seasons
Fertilizer rate
(P2O5 kg/ha)
Late season,
1991 (kg/mm2)
Early season
1992 (kg/mm2)
0
20
40
60
0.652
0.659
0.689
0.690
0.01
0.654
0.060
0.694
0.697
0.009
Tests for the hardness of seeds revealed that the seeds from
40 and 60 kg/ha were significantly harder than those of 0 and 20
kg/ha. Thiery (1984) reported different chemical properties in
some fabaceous seeds (Phaseolus vulgaris L., P. cocineus L.
and Lablab purpureus L. in relation to the strength of their seed
coats. He found out that the seed coat of P. lanatus contained
some complex substances and was harder than other species
investigated and consequently, Acanthoscelides obstectus
(Say) nymphs were unable to penetrate through it. In the
present study it is also possible that hardening of the seeds may
be due to changes in the internal chemical composition. PSBs
do not attack dry seeds; they initiate attack when the pods and
seeds are still maturing and succulent, therefore, toughness of
the seed coat at the early part of seed development may be
responsible for reducing the PSB population at higher phos-
phate levels.
LSD(0.05)
LSD = Least significant difference.
decreased the amount of sawfly cutting in wheat (Anonymous,
1969). Shri Ram et al. (1990) also reported that the increasing
concentration of phosphorus alone consistently reduced leaf-
hopper populations. They postulated that the high yield obtained
was due to mechanisms that help build up anatomical features
of the foliage which confer resistance to the damaging activity of
the defoliators.
Investigation of these mechanisms was further attempted
through the proximate analysis of the seeds from various
phosphate levels in the experiment. The results from this study
showed that increase in phosphate level increased the crude
protein, fat and calcium contents of the seeds. In addition,
reports have shown that there is an increase in the concentra-
tion of Co, Fe, Zn, Cu and Mb as phosphate concentration
increased while that of sodium was not influenced significantly
(Omueti and Oyenuga, 1970). These results suggest that since
phytin content was not significantly raised with increase in
phosphate level, more of the calcium, phosphorus and
magnesium would now be available at higher levels of
phosphate application. Calcium and magnesium are known to
form salts of oxalate and phytates (FAO, 1981; Oke, 1965). We
suggest that the increase in phosphate rate raised the
concentration of these elements without simultaneously raising
the oxalate and phytin contents. This means that high levels of
phosphorus in the soil would tend to increase the proportion of
available calcium and magnesium in seeds. In addition to this,
Omueti and Oyenuga (1970) pointed out that the increase they
observed in iron as a result of the increase in the phosphate
In conclusion, the higher yields observed at 40 and 60 kg/ha
could have been due either to reduction in damage by PSBs or
to improved growth and seed set as a result of the direct effects
of phosphorus fertilizer or, indeed, to a combination of both
factors.
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