Acta agriculturae Slovenica, 116/2, 351–356, Ljubljana 2020
doi:10.14720/aas.2020.116.2.1689 Original research article / izvirni znanstveni članek
Effect of Tasmanian blue gum (Eucalyptus globulus Labill.) leaf extract on 
cowpea weevil (Callosobruchus maculatus [Fabricius, 1775], Coleoptera: 
Chrysomelidae)
Samuel Femi BABATUNDE
 1, 2
, Abdulrasak Kannike MUSA
 1
Received May 21, 2020; accepted November 27, 2020.
Delo je prispelo 21. maja 2020, sprejeto 27. novembra 2020.
1 University of Ilorin, Faculty of Agriculture, Department of Crop Protection, Ilorin, Nigeria
2 Corresponding author, e-mail: samfemmy2002@gmail.com
Effect of Tasmanian blue gum (Eucalyptus globulus Labill.) 
leaf extract on cowpea weevil (Callosobruchus maculatus [Fa-
bricius, 1775], Coleoptera: Chrysomelidae)
Abstract: A laboratory study was carried out to examine 
the efficacy of solvent extract of Eucalyptus globulus leaves in 
reducing post-harvest losses caused by cowpea weevil (Calloso-
bruchus maculatus) on cowpea seed variety RSH 256. 450 g of 
E. globulus leaf powder were put in 1000 ml flask and 450 ml of 
n-hexane was used as the solvent. The extract was tested at dos-
ages of 50, 100 and 150 µl 50 g
-1
 of cowpea grains on the weevil 
in stored cowpea. The different rates of treatment recorded sig-
nificant differences (p < 0.05) in causing adult mortality com-
pared to the untreated control. The different rates of treatment 
also recorded significant differences (p < 0.05) in emergence of 
F1 adults of each treatment compared to the control. The per-
centage masst loss and grain damage were also suppressed as a 
result of treatment with the plant material compared to the un-
treated control. However, among the treatments 150 µl 150 g
-1
 
cowpea recorded the highest adult mortality rate and lowest 
emergence while control had the lowest mortality rate and the 
highest emergence of the insect. The rates of application were 
indicative of bioactive characteristics of the plant extract.
Key words: cowpea; Callosobruchus maculatus; Eucalyp-
tus globulus; leaf exctracts; biopesticides
Vpliv listnega izvlečka modrega evkalipta (Eucalyptus globu-
lus Labill.) na skladiščnega hrošča Callosobruchus maculatus 
(Fabricius, 1775) (Coleoptera: Chrysomelidae)
Izvleček: Za preučitev učinkovitosti izvlečkov listov mo-
drega evkalipta (Eucalyptus globulus) na zmanšanje skladiščnih 
izgub, ki jih povzroča hrošč Callosobruchus maculatus na 
semenu kitajske vinje (‘RSH 256’) je bil izveden laboratori-
jski poskus. 450 g zmletih listov modrega evkalipta je bilo v 
1000 ml steklenicah prelitih s 450 ml topila n-heksana. Vpliv 
izvlečka je bil preiskušen v odmerkih 50, 100 in 150 µl na 
50 g semena kitajske vinje, naseljenega s hroščem. Različni 
odmerki so povzročili značilne razlike (p < 0,05) v smrtnosti 
hrošča v primerjavi s kontrolo. Različni odmerki so povzročili 
tudi značilne razlike v pojavljanju (p < 0,05) odraslih hroščev 
F1 rodu v primerjavi s kontrolo. Tudi odstotka izgube mase in 
poškodovanosti zrnja sta se zmanjšala kot posledica nanosa 
izvlečka listov modrega evkalipta v primerjavi s kontrolo. Med 
obravnavanji je imela uporaba 150 µl izvlečka listov modrega 
evkalipta na 150 g semena kitajske vinje največji vpliv na smrt-
nost škodljivca in zmanjšanje pojavljanja imagov, medtem ko 
je bila njihova smrtnost v kontroli najmanjša, pojavljanje pa 
največje. Uporabljeni odmerki izvlečkov dokazujejo bioaktivne 
učinke modrega evkalipta
Ključne besede: kitajska vinja; Callosobruchus maculatus; 
Eucalyptus globulus; listni izvlečki; bioinsekticidi

Acta agriculturae Slovenica, 116/2 – 2020 352
S. F. BABATUNDE and A. k. MUSA
1 INTRODUCTION
Cowpea (Vigna unguiculata [L.] Walp.) is an ed-
ible legume crop in many parts of the world especially 
in tropical and subtropical regions. Cowpea belongs to 
the family Fabaceae (Singh, 2002). Cowpea is the most 
economically important indigenous African legume and 
is of vital importance to the livelihood of several millions 
of people in West and Central Africa (Langyntuo et al., 
2003).
Cowpea plays a critical role in the lives of millions of 
people in Africa and other parts of the developing world, 
where it is a major source of dietary protein that nutri-
tionally complements staple low-protein cereal and tuber 
crops, and is a valuable and dependable commodity that 
produces income for farmers and traders (Singh, 2002; 
Langyintuo et al., 2003).
Cowpea is a drought tolerant and short warm-
weather crop well adapted to the drier regions where 
other food legumes do not perform well. It requires an-
nual rainfall of about 750-1100 mm (Hall et al., 2002; 
Hall 2004). As much as 1000 kg ha
-1
 of dry grain has been 
produced in a Sahelian environment with only 181 mm 
of rainfall and high evaporative demand (Hall and Petal, 
1985).
Several diseases, insect pests, nematodes, and para-
sitic weeds cause significant losses in cowpea yield on 
field and in storage (Iheanacho et al., 2000). Cowpea 
is susceptible to a number of fungal, bacterial and vi-
ral diseases such as leaf spot, ashy stem blight, bacterial 
blight, black-eyed cowpea mosaic potyvirus (BICMV), 
cowpea aphid-borne mosaic potyvirus (CABVC), and 
cowpea mosaic comovirus (CPMV) (Shaw, 1988). All of 
these factors, singly or combined are responsible for the 
low grain yield obtained from the cowpea fields (Singh, 
2002). Control by one or two applications of insecticide is 
invariably necessary. Infestation of insect pest on cowpea 
for commercial production will lead to downgrading of 
grain (Monfankye, 2014).
There has however been serious concerns about the 
long-term negative effect of continued or excessive use 
of synthetic formulations in insect pest control (Praveen 
& Dhandapani, 2001). The work therefore aims at evalu-
ating the effect of Eucalyptus globulus Labill. leaf extract 
on cowpea weevil (Callosobruchus maculatus [Fab-
ricius,1775 ]).
2 MATERIALS AND METHODS
2.1 EXPERIMENTAL SITE
This study was conducted in the Department of 
Crop Protection laboratory, and Ir. Leo Vande Mierop 
Biotechnology laboratory of the University of Ilorin. The 
extraction process was carried out in the Central Re-
search and Diagnostic Laboratory, Tanke, Ilorin, Nigeria.
2.2 SOURCE AND PREPARATION OF SEEDS
The cowpea seed variety RSH 256 used was obtained 
from the International Institute of Tropical Agriculture 
(IITA), Ibadan, Oyo State, Nigeria. The seeds are white in 
colour with a maturity period of 60 days. The seeds were 
wrapped in a polyethylene bag and kept in the freezer 
compartment of a refrigerator in the Ir. Leo Vande Mi-
erop Biotechnology laboratory, University of Ilorin, to 
kill immature stages of insects. The seeds were removed 
10 days after freezing and then spread on a laboratory 
desk to thaw.
2.3 INSECT CULTURE
A culture of cowpea weevil, C. maculatus, was pre-
pared at laboratory ambient temperature (30 ± 3°C) and 
relative humidity (68 ± 3%0), respectively. The weevils 
were collected from already existing stock in the Crop 
Protection laboratory, University of Ilorin, Nigeria. Fifty 
(50) unsexed adults of C. maculatus were picked with the 
aid of hair brush to infest cowpea seeds in a transparent 
plastic container which was covered with muslin cloth 
held tightly by perforated lid to ensure aeration and pre-
vent escape of the insects. Freshly emerged adults were 
used for the study.
2.4 APPARATUS AND EQUIPMENT
The apparatus, equipment and materials used in-
clude: transparent plastic containers, cowpea seeds, wa-
ter, n-hexane, measuring cylinder, conical flasks, Euca-
lyptus globulus leaves, muslin cloth, a pair of scissors, a 
razor blade, paper tape, hair brush, digital weighing bal-
ance, spatula, micro-pipette (0-200 µl), Soxhlet appara-
tus, a plastic sieve, electric blender, rubber band, labora-
tory desk, refrigerator, foil paper and beakers.
2.5 COLLECTION AND PLANT IDENTIFICATION
Mature leaves of Eucalyptus globulus were collected 
from the parent plant at the University of located in the 
southern Guinea Savannah agro-ecological zone of Ni-
geria between latitude 8°25’N and longitude 4°67’E in 

Acta agriculturae Slovenica, 116/2 – 2020 353
Effect of Tasmanian blue gum (Eucalyptus globulus Labill.) leaf ... (Callosobruchus maculatus [Fabricius, 1775], Coleoptera: Chrysomelidae)
the morning at sunrise. The leaves were taken to Plant 
Biology Department of the University for for proper 
identification.
2.5.1 Processing of the plant leaves
Fresh mature leaves of E. globulus were air-dried, 
ground with the aid of electric blender, sieved by using 
a 2 mm sieve to obtain uniform particles. The pow-
der was stored in a vial under room condition until 
required for use.
2.5.2 Extraction process
Extraction from the E. globulus leaf powder was 
done using the following methods:
Cold extraction, and Soxhlet extraction
Cold extraction:
450 g of E. globulus leaf powder were put in 
1000 ml flask and 450 ml of n-hexane was used as the 
solvent. Mixture of the powder and the solvent was 
shaken thoroughly and allowed to settle for 4 days. The 
flask was covered with paper sellotape. The extract was 
separated from the sediment into a 1000 ml beaker us-
ing a sieve and then placed on laboratory worktable for 
excess solvent to evaporate. The concentrated extract 
was kept in a vial until required for use in evaluating 
the insecticidal potential of the plant material.
Soxhlet extraction:
150 g of E. globulus leaf powder was wrapped in 
a muslin cloth and tied with rubber band before in-
troducing it into the thimble of a Soxhlet extractor. 
300 ml of n-hexane was introduced into a round bot-
tom flask. The electric cooker served as source of heat 
which lasted for 6-7 h. The excess solvent was allowed 
to evaporate in order to concentrate the extract. The 
extract remaining after evaporation was kept in a vial 
until required for use in the determination of bioactive 
components of the plant material.
2.5.3 Determination of bioactive components of E. 
globulus leaves using n-hexane extract
Plant material consisted of leaves was put into 
distillation apparatus using Soxhlet apparatus. Mass 
of plant material was taken before loading in the Sox-
hlet apparatus and water is heated so that the steam 
passes through the plant material vaporizing the vola-
tile compounds. The vapour flows through a coil where 
they condense back to liquid which is then collected in 
the receiving vessel.
2.5.4 Experimental procedure
13 g of the sticky extract of E. globulus leaves 
was dissolved in 4 ml of n-hexane. The mixture was 
stirred with the aid of spatula. Different volumes of the 
concentration i.e., 50, 100 and 150 µl, were measured 
using a micro pipette. Aqueous extract of E. globulus 
leaves was added to 50 g cowpea seeds in transparent 
plastic containers at three levels, 50, 100 and 150 µl 
using a micro-pipette. Each treatment was replicated 
three times including the untreated control. Ten fresh-
ly emerged adults of C. maculatus from the stock cul-
ture were added to the treated cowpea seeds in each 
plastic container and covered with muslin cloth held 
tightly with rubber band. The effect of the plant extract 
on the survival of adult C. maculatus was assessed by 
recording adult mortality on daily basis.
2.6 DATA COLLECTION
Data were collected on various parameters includ-
ing adult mortality, total number of emerged progeny 
(adults), number and weight of damaged seeds (seeds 
with holes) and undamaged seeds (seeds without 
holes) and percentage seed weight loss, and average 
number of seeds per 50 g in a container. Counts of the 
emerged weevils commenced 23 days after infestation 
(DAI) and continued at intervals of 48 h until 100 % 
adult emergence was recorded.
The percentage seed mass loss was computed fol-
lowing the method of Haines (1991) as follows:
2.6.1 Data analysis 
Data were subjected to analysis of variance using 
Genstat 12th edition and means were separated using 
Duncan`s Multiple Range test (DMRT) at 5% level of 
significance
3 RESULT AND DISCUSSION
The results of the study revealed that the vari-
ous treatments used in the experiment had effects on 
mortality increased with increase of dosages of extract. 

Acta agriculturae Slovenica, 116/2 – 2020 354
S. F. BABATUNDE and A. k. MUSA
The E. globulus leaf extract caused adult mortality of C. 
maculatus at the high and low rates when compared 
to the control, which was indicative of bioactive char-
acteristics of the plant part. This is in agreement with 
the report of Oluma and Garba (2002) that E. globu-
lus has potential as source of insecticide for protection 
of stored grains against attack by C. maculatus. They 
concluded that E. globulus extract had toxicity against 
adults of C. maculatus. The extract used in this study 
could have caused insect mortality due to their physi-
cal action on respiration through blockage of the spira-
cles of the C. maculatus. The differences in the adult 
mortality could be attributed to the active ingredients 
of E. globulus extract. The shortcomings of the use of 
synthetic chemicals which include development of re-
sistance by insects, adverse effect on non-target spe-
cies, pollution of the environment including soil, water 
and air and hazard of residues necessitated the evo-
lution of natural insecticides of plant origin (Deedat, 
1994).
The E. globulus leaf extracts reduced progeny 
emergence of C. maculatus in treated cowpea seeds 
(Table 2). This could be attributed to the adult mor -
tality already observed (Table 1) and the inhibition of 
oviposition as well as the remarkably high reduction in 
survival to adulthood of mature stages of C. maculatus 
compared to the control. This result corroborates that 
of Okonkwo and Ewete (1999) in pepper fruit, Ogun-
wolu and Idowu (1994) in root powders of Zanthoxy-
lum spp. and neem seed.
The E. globulus leaf extract was also observed to 
have effects in reducing the damage on cowpea seeds 
by C. maculatus (Table 3). Damage on cowpea seeds 
may have been reduced as a result of the extracts act-
ing as a deterrent to C. maculatus, keeping them from 
infesting and damaging the seeds. A similar trend of 
eucalyptus oil activity in preventing grain damaged to 
cowpea by C. maculatus was observed by Longe (2011). 
The study reveals that E. globulus leaf extract 
could be very effective for use as biopesticides for pro-
tecting cowpea seeds from C. maculatus infestation 
and damage. It has been reported by the pest manage-
ment specialists that botanicals are not known to leave 
any residue in any crop they are used to protect and the 
protective ability of essential oils could be attributed to 
interspecific insect responses to oil constituents (Enan, 
2001).
The use of natural toxicants from plants as insecti-
cides had been existent since the ancient times (Bohinc 
et al., 2013) and is investigated in recent times (Bohinc 
et al., 2020) The natural insecticides which require low 
cost to prepare, are readily available, environmentally 
and ecologically friendly are best suited for use in the 
storage of produce.
Adult mortality of C. maculatus (DAT)
E. globulus leaf extract (µl) 1 2 3 4 5 6 7 8
50 1.33
b
3.33
b
4.33
b
6.33
b
7.67
b
  b 8.67
b
9.33
b
10.00
100 3.00
a
5.67
a
7.67
a
9.33
a
9.67
a
  a 10.00
a
10.00
a
10.00
150 3.33
a
6.00
a
8.00
a
9.67
a
10.00
a
10.00
a
10.00
a
10.00
Control 0.00
b
0.00
b
0.00
c
0.00
c
0.00
c
0.00
c
0.00
c
00.00
S.E.M 0.55 0.80 0.94 0.41 0.24 0.33 0.17 Ns
Table 1: Mean adult mortality of C. maculatus exposed to E. globulus leaf extract (x n out of 10)
Values with the same superscript in the same column are not significantly different at 5 % level of significance using Duncan`s multiple range test.
DAT: Days after treatment
Progeny emergence of C. maculatus (DAT)
E. globulus leaf extract (µl) 23 25 27 29 31 33 35
50 2.00
a
3.00
ab
2.33
a
3.67
ab
4.67
a
3.33
a
1.00
a
100 0.33
b
3.00
ab
3.33
a
2.67
b
3.33
a
1.33a 0.00
b
150 0.33
b
1.33
b
2.00
a
2.00
b
2.67
a
0.33a 0.00
b
Control 2.67
a
4.33a 4.33
a
6.00
a
4.00
a
2.00
a
0.00
ab
S.E.M 0.41 0.62 0.82 0.80 0.76 0.87 0.289
Table 2: Effect of cowpea seeds treated with E. globulus leaf extract on progeny emergence of Callosobruchus maculatus
Values with the same superscript in the same column are not significantly different at 5 % level of significance using Duncan`s multiple range test
DAT: Days after treatment

Acta agriculturae Slovenica, 116/2 – 2020 355
Effect of Tasmanian blue gum (Eucalyptus globulus Labill.) leaf ... (Callosobruchus maculatus [Fabricius, 1775], Coleoptera: Chrysomelidae)
4 CONCLUSION 
Botanical control is said to be the best pest control 
of stored grain pests because it is biodegradable, envi-
ronmentally friendly and does not leave toxic residues. 
Therefore, plant extracts can be another source of in-
secticides/pesticides against stored grain pests. The re-
sult from this research shows that Eucalyptus globulus 
have insecticidal potential for protecting stored cowpea 
against C. maculatus. The E. globulus extract prove to be 
effective on adult mortality. Furthermore, 150 µl was the 
most effective and this was shown in the suppression of 
progeny emergence. Thus farmers can use E. globulus ex-
tract in place of expensive synthetic pesticides used against 
the cowpea weevil.
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E. globulus leaf extract 
(µl)
% Seed mass 
loss
Mass of 
damaged seeds
Mass of 
undamaged seeds
Number of 
damaged seeds
Number of 
undamaged seeds
50 18.3
b
17.8
a
23.1
ab
100.3
ab
108.3
a
100 15.9
b
15.8
a
26.2
a
86.3
b
130.0
a
150 12.3
b
15.8
a
22.7
ab
77.3
b
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a
0 31.4
a
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a
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b
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a
74.7
b
S.E.M 2.52 3.01 2.54 10.01 10.28
Table 3: Effect of cowpea seeds treated with Eucalyptus globulus leaf extract on Callosobruchus maculatus activity
Values with the same superscript in the same column are not significantly different at 5 % level of significance using Duncan`s multiple range test.

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