




































In ternationa l
Scholars
Journa ls

 

African Journal of Environmental Economics and Management ISSN 2375-0707 Vol. 7 (2), pp. 001-010, 
February, 2019. Available online at www.internationalscholarsjournals.org © International Scholars Journals 

 

Author(s) retain the copyright of this article. 
 
 

 

Full Length Research Paper 

 

Toxic effects of five plant extracts against the larger 
grain borer, Prostephanus truncatus 

 
Mweshi Mukanga1*, Yakub Deedat2 and Felix S. Mwangala3

 
 

1
Mount Makulu Central Research Station, Private Bag 7 Chilanga, Zambia. 

2
Department of Crop Science, 

University of Zambia, P. O. Box 32379, Lusaka, Zambia. 
3
National Biotechnology Laboratories, National 

Institute for Scientific and Industrial Research, P. O. Box 310158, Lusaka, Zambia. 
 

Accepted 20 October, 2018 
 

Dried leaf powders of Eucalyptus, Guava, Neem, Tephrosia and Water hyacinth were evaluated for their 
insecticidal activity against Prostephanus truncatus Horn. The powders were tested at 0.1, 0.25, 0.5, 0.1, 
2.5 and 5 g/100 g of dried cassava chips or flour. Data was collected on larvae and adult mortality, 
larvae and adult emergence, antifeeding, and repellency. The effects varied with plant species and dose 
rate. Increased insect deformity and mortality and reduced fecundity were observed among the leaf 
powder treatments compared to untreated control. The percentage reduction in the adult insect 
numbers ranged from 37.2 to 99.2% and was highest in the Neem, Tephrosia and Water hyacinth 
treatments. Neem had the highest larvicidal effects though not significant different (P > 0.5) from the 
other four plant species. On the other hand, Tephrosia and Guava leaf powders exhibited both strong 
repellent and anti-feeding effects. The petroleum ether extracts of the leaf powders of Neem and Guava 
were highly toxic on P. truncatus. The results suggest that these materials tested have the potential in 
development of post-harvest protection technology against, P. truncatus, the major pest of stored 
grains and dried cassava. 

 
Key words: Prostephanus truncates, antifeeding, mortality, repellency, toxicity, leaf powders, petroleum ether 

extracts. 
 
 
INTRODUCTION 

 
The larger grain borer, Prostephanus truncatus Horn 
(Coleoptera: Bostrichidae), is a serious pest of farm-
stored maize (Golob, 2002; Vowotor et al., 2005) and 
dried cassava (Hodges et al., 1985). Losses to maize and 
dried cassava caused by this pest could be as high as 36  
– 40% and 70 - 80%, respectively over a period of 6 
months of storage (Wright, 1984; Golob, 1988). Damage 
of this magnitude is extraordinarily high and demon-
strates the destructive nature of this pest, which can 
threaten food security at both household and national 
levels (Mallya, 1992). The recent introduction and spread 
of the larger grain borer into Africa has increased dried  
 
 
 
*Corresponding author. E-mail: mmweshi@gmail.com. 

 
 
 
 
 
cassava storage problems (Schulten, 1996) . Hence, 
effective storage protection strategies are urgently 
required. Current control measures for the P. truncatus 
include chemical insecticides, fumigation and biological 
control using of Teretrisoma nigrescens Lewis 
(Coleoptera: Histeridae), a predator of P. truncatus 
(Richter et al., 1997; Hell et al., 2006). These methods 
are expensive and cannot be afforded by the small-scale 
farmers in developing countries.  

Use of naturally occurring plant materials to protect 
agricultural products against insect pests is an old-age 
practice in many parts of the world (Dales, 1996; 
Belmain, 1999). Obeng-ofori et al. (1996) have indicated 
that the use of locally available plant materials is a 
common practice in traditional African communities, 
mainly for medicinal purposes (herba l use) and in 



 
 
 

 

agriculture. 
Extracts from different plants have been known to 

possess insecticidal properties against a wide range of 
insect pests (Abdullahi and Muhammad, 2004). Plants 
with insecticidal properties offer a cheaper sus-tainable 
alternative to synthetic insecticides, store design, 
fumigation and thermal distribution methods. They could 
be an abundant source of locally available pest control 
agents that can be grown at the village level. The 
insecticidal specificity of some of the plant extracts and 
their lack of negative impacts on the food and the envi-
ronment make them ideal candidates for incorporation 
into an integrated pest management strategy. 

Exact quantities of botanicals from these plants that 
give optimum insecticidal effects are unknown. It is thus 
desirable to quantify the amount of the plant derived 
materials that provide adequate protection against insect 
pests and to determine how these affect insect behaviour, 
growth and reproduction (Jilani, 1992). Equally, plant 
species that are found to be effective and popular locally 
with the farmers need to be subjected to safety testing, at 
least involving basic toxicological studies (Jilani, 1992). 
The principal advantage of botanicals is that farmers are 
able to provide their own protectants (Isman, 2008). The 
objectives of this study were: 
 
 
1. To determine the effect of leaf extracts from Tephrosia, 
Tephrosia vogelii; Neem Azadirachta indica; Guava, 
Psidium guajava; Eucalyptus Eucalyptus globulus and 
Water hyacinth, Erchhornia crassipes  
2. To test for insecticidal properties of the extracts such 
as toxicity and morphological abnormalities and 
3. To determine whether these extracts could be used as 

protectants against P. truncatus 
 
 
MATERIALS AND METHODS 
 
Study location 
 
The experiments were conducted at Entomology laboratory, Mount 

Makulu Central Research Station Chilanga, Zambia (S 15°28’, 

E28°14’). 

 

Test plant materials 
 
The materials used were dry leaves of Tephrosia, T. vogelii Hook 
F.; Neem A. indica A.Juss; Guava, P. guajava L.; Eucalyptus E. 
globules Labill. were collected from the field s surrounding Mount 
Makulu and Water hyacinth, Erchhornia crassipes (Mart.) Solms. 
The dried leaves were ground and sieved repeatedly to obtain the 
finest particles using a 300 m sieve. The leaf powders were stored 
in 10 g glass bottles for later use. Ground cassava chips (2 – 5 cm) 
were used as a food substrate for P. truncatus in all these studies. 
The cassava chips were sterilized by freezing in sealed plastics for 
four weeks to kill any residual insect.  

Petroleum ether leaf extract was prepared using Soxhlet 

extraction method. Fifteen grams of leaf powder of each plant 

 
 
 
 

 
species were extracted separately 40 - 60°C for 8 h in 300 ml of the 
above solvent. The extract thus obtained was filtetred through a 
sterilized Whatman No. 1 filter paper. After which, the solvent was 
evaporated in a rotary evaporator at 30°C to dryness and the weight 
of the extract determined. Guava leaf powder yielded 251 mg, 
Tephrosia 477.3 mg, Eucalyptus 719.7 mg, Water hyacinth 196.8 
mg and Neem 260.4 mg of extract.  

The extract obtained was then made to different doses that is  
0.1, 0.25, 1 and 1% concentration) respectively by acetone. A 1% 
concentration stock solution was obtained by dissolving 100 mg of 
petroleum ether leaf extract from each plant in 10 ml of acetone. 
Lower concentrations of 0.1, 0.25 and 0.5% were prepared by 
further dilution of the stock solution with acetone. 
 

 
Test insects 
 
Adult P. truncatus used in the experiments were reared in 1L glass 
jars in the Protection Insectary at Mount Makulu Central Research 
Station, Chilanga on previously sterilized maize grain at 28 + 2°C 
temperature and 70 + 5% relative humidity and with alternating light 
and dark periods of 12 h. The adults and larvae were separated 
after sieving out the maize. They were placed in different vials for 
later use. The second-generation adults that emerged were used in 
all these trials. 
 

 
Reproduction inhibition test 
 
Dry leaf powders were applied as admixtures in six serial doses of 
0.1, 0.25, 0.5, 1, 2.5 and 5 g of ground leaves per 100 g of sterilised 
dried cassava chips. One hundred grams of treated cassava chips 
were placed in 1L glass jars. The synthetic insecticide, Actellic-
super dust (1.6% Pirimphos-methyl and 0.3% permethrin) treated 
dried cassava chips as standard check while untreated cassava 
chips served as control. Actellic super was applied at recommended 
dosage at 50 g per 90 kg shelled maize grain (Mwaya, 1997).  

The treatments including the control were replicated thrice. 
Twenty (20), 1 - 3 days old, P. truncatus adults (10 male, 10 
female) were introduced in each glass jar. After 10 days, the adults 
were removed. On the 14th, 28th, 42nd, 56th and 70th day after 
infesting the dried cassava chips. The number of live, dead and 
malformed P. truncatus larvae and adults in the ensuing progeny 
were counted after sieving. The response of the test insects in the 
treated cassava were corrected using the modified Abbott’s 
formula: 
 
100 (Number. of insects in control – Number of insects in treatment) 
/ (Number of insects in control) 
 
Henceforth, percentage reduction in the progeny was calculated. A 
treatment was judged to give complete protection against P. 
truncatus if no progeny was produced. Only the live and normal 
larvae were returned to the jars. Dead and malformed larvae and 
adults were removed.  

Square root (x + 0.5) transformations were made of the data to 
compensate for skewness and to stabilise variance before the 
analysis by ANOVA was done. The total insect counts at each 
dosage were regressed against the time interval (days) at which 
insect counts were made, to determine the residual persistence of 
the leaf powders. The treatments and control were replicated three 
times for all the experiments. The data from the experiments were 
subjected to Analysis of Variance (ANOVA). The separation of 
means was done by New Duncan’s Multiple Range Test (DMRT). 
Significance was taken at 5% level (Steel and Torries, 1980). 



 
 
 

 
Repellency effects 
 
Two and a half grams of treated cassava flour with 1, 0.25, 0.5, 1, 
2.5 and 5 g of ground leaves per 100 g dried cassava chips and 
untreated cassava were separately placed on opposite sides on 
filter paper, with 10 cm space in between on a plastic Petri dish (14 
cm diameter, 2 cm height), which served as an arena. Ten, 1 - 2 
weeks old P. truncatus adults starved for 48 h were introduced in 
the centre of the arena using an aspirator. The number of insects 
found on the treated cassava was recorded twice daily, at 09:00 
and 16:00 h, for five (5) consecutive days (Dales, 1996). Average 
insect counts for each day were converted to percent repellency 
(Jilani et al., 1992; Saxena et al., 1989), which was calculated after 
Gillenwater and McDonald (1975): 
 
% Repellency = No. of insects on control half - No. on treated half / 

No. of insects on control half + No. on treated half x 100 

 

Anti-feeding study 
 
The potency of antifeedant effect of the dry leaf powders was 
determined using filter paper bioassay. The leaf powder was dusted 

evenly on both sides of the filter paper (113.1 cm
2
), in which 1.00 g 

cassava flour was placed. The leaf powder was applied on the filter 
paper as proportion of the dose per 100 g to that per 1.00 g dried 
cassava, that is, 0.001, 0.0025, 0.005, 0.01, 0.025 and 0.05 g.  

The wrapped cassava was placed in 35 ml vial. Cassava flour 
wrapped in the untreated filter paper served as control. Five, 1-
week-old adults of P. truncatus , starved previously for 24 h were 
introduced in each vial and left for 7 days. The wrapped cassava 
served as sole food source. The numbers of holes in filter paper 
produced by boring insects were recorded at end of the 7 – day 
exposure period. 

 

Toxicity effects of petroleum ether leaf extract vapours 
 
Ten P. truncatus adults (24 – 72 h old) , were placed inside a vial 
(35 ml) which acted as a gas chamber with a plastic lid in which 

minute slits were made. A filter paper disc (15.9 cm
2
) impregnated 

with the test solution, was suspended from the plastic lid where it 
was in position by one of the slits in the plastic lid. Insects exposed 
to the acetone impregnated filter paper disc served as control. 
Mortality was recorded after 24 h exposure period. Petroleum ether 
leaf extracts were applied at 0.1, 0.25, 0.5 and 1% concentrations.  

Mortality was confirmed by exposing the insects to a light source 
and those that did not move were considered dead. Insect mortality 
data were analysed using the POLO procedure (Russell et al., 
1977) using the probit analysis (Finney, 1971) to obtain the median 
lethal concentration of the petroleum ether leaf extracts at 50% kill 
(LC50). 

 

Contact toxicity by topical application of petroleum ether leaf 

extracts 
 
The experiment was conducted using the method described by 
McDonald et al. (1970) with slight modifications. Using a micro-
syringe applicator, 1 µl of the petroleum ether leaf extracts of 
selected plant species were applied at 0.1, 0.25, 0.5 and 1% 
concentrations were applied to the dorsum of thorax of each insect. 
Ten pairs of newly emerged P. truncatus adults were treated with 
each dose.  

After treatment, insects were transferred to glass Petri-dishes (9 

cm diameter, 1 cm height) containing a food substrate – untreated 

 
 
 
 

 
cassava flour. The insects were examined daily for five days; those 

that did not move or respond to the gentle touch were considered 

dead. Percentage kill of the insects was recorded every 24 h. 
 

 

RESULTS 

 

Effect on larvae and adult emergence 

 

The effect of leaf powders on the emergence of larvae 
and adult P. truncatus are presented in Figure 1. Results 
showed that there was between 37.2 to 99.2% reduction 
in the number of larvae recovered from the leaf powder 
treatments compared to untreated cassava. The larvicidal 
effect was of the order: Neem leaf powder > Tephrosia> 
Eucalyptus> Water hyacinth> Guava. The percentage 
reduction in adult progeny in the leaf powder treatments 
followed similar pattern with the least recoveries 
occurring in Neem leaf powder treatment, followed by 
Water hyacinth. No insect recoveries were made in the 
standard check, Actellic super treatment. 

There was a preponderance of lower dosages (0.1, 
0.25 and 0.5 g /100 g) compared to higher doses (0.1,  
0.25 and 0.50 g /100 g) being less effective in reducing 
both larvae and adult population (Figure 2). The 
regression analysis indicated the existence of a positive 
linear relationship between insect count and the time 
interval (days) at which the counts were made (Table 2). 
The strongest relationship occurred in Neem applied at 5 
g/100 g. The slopes for regression lines at each dosage 
were observed to be close suggesting similarity in the 
rate of decay of the leaf powders. 

 

Effect on larvae and adult deformity and mortality 

 

The materials tested significantly affected the longevity or 
survival of the P. truncatus larvae and adults compared to 
the untreated control (Table 1). The mean number of 
deformed larvae ranged 0.21 to 0.34 larvae per 100 g 
dried cassava chips in the leaf powder treatments, while 
the number of dead larvae were between 0.21 – 0.34. 
Though the analysis of variance revealed no significant 
difference (P > 0.05) among the leaf powder treatments 
for larvae mortality and deformity, numerically the highest 
larvae mortalities were recorded in the Neem treated 
cassava chips. Water hyacinth produced the lowest 
larvae mortality.  

With respect to the number of deformed and dead 
adults recovered, no significant differences (P < 0.05) 
were found among powders of Eucalyptus, Guava, 
Neem, Tephrosia and Water hyacinth -treated cassava. 
However, on average 0.7 to 1.5% of adults in leaf powder 
treated cassava were deformed, and 1.7 to 3.5% died. 
This means that leaf powder has exhibited some amount 
of insecticidal effects and lowered insect reproductivity 
especially at higher concentrations, 2.5 and 5 g (Figure 3) 



  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 1. Mean percent reduction in the number of larvae of the larger grain borer, P. truncatus in dried cassava 

treated with different doses of five leaf powders after 70 days of storage.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

Figure 2. Percent reduction in the number of adults of the larger grain borer, P. truncatus in dried cassava 

treated with different doses of leaf powder during 70 days of storage. 



 
 
 

 
Table 1. Relationship between total insect count and the sampling interval for different leaf powder 

treatments.  
 

    Mean number/100 g dried cassava 

 Treatment   Deformed    Dead 

  Larvae Adult Larvae Adult 

 Eucalyptus 0.22 A 0.23 AB 0.36 A 0.66 C 

 Guava 0.31 A 0.29 A 0.37 A 0.74 C 

 Neem 0.34 A 0.01 C 0.39 A 0.34 C 

 Tephrosia 0.23 A 0.03 C 0.27 A 0.66 C 

 Water hyacinth 0.26 A 0.12 BC 0.37 A 0.69 C 

 Actellic super 0.00 B 0.00 C 0.00 B 20 A 

 Untreated 0.00 B 0.00 C 0.00 B 0 D 

 Mean 0.19  0.09 0.249 3.29 
 C.V% 29.6  21.7 33.7  32.1 

 
Figures followed by the same letter in the same column are not significant different at P<0.05; Duncan 

multiple range test on square root (x+0.5). 

 
Table 2. Mean number of dead and deformed larvae and adult of the larger grain borer, P. runcates in dried cassava treated 

with leaf powders.  
 

    Mean number/100g dried cassava   

 Treatment   Deformed    Dead  

  Larvae Adult Larvae Adult 

 Eucalyptus 0.22 A 0.23 AB 0.36 A 0.66 B 

 Guava 0.31 A 0.29 A 0.37 A 0.74 B 

 Neem 0.34 A 0.01 C 0.39 A 0.34 B 

 Tephrosia 0.23 A 0.03 C 0.27 A 0.66 B 

 Water hyacinth 0.26 A 0.12 B 0.37 A 0.69 B 

 Actellic super 0.00 B 0.00 C 0.00 B 20.0 A 

 Untreated control 0.00 B 0.00 C 0.00 B 0.00 C 

 Mean 0.19 0.09 0.25  3.29 

 C.V% 29.6 21.7 33.7  32.1 
 

 

(Figure 3). The highest insect mortality occurred at 5 
g/100 g dried in the Guava treated cassava, followed by 
eucalyptus at the same rate, though not statistically 
different. No adult mortalities were recorded in the Neem 
treatment at 0.1 and 0.25 g/100 g dried cassava. All the 
insects in Actellic super, died within 7 days of its appli-
cation while no deformity and mortality were observed in 
the untreated control. 
 

 

Repellency bioassay 

 

P. truncatus demonstrated negative orientation response 

to the cassava powder treated with leaf powder. 
Repellency increased significantly (P < 0.05) with 
increase in dosage (Figure 4). Higher dosages (2.5 and 5 
g/100 g) were superior to lower dosages (0.1, 0.25, 0.5 
and 1 g/100 g). Neem leaf powder exerted strong 

 

 

repellency (59.1%). The leaf powder of Guava was next 
best (50.5%). The decreasing order of efficacy of the five  
leaf powders was as follows: 

Neem>Guava Tephrosia>Water hyacinth>Eucalyptus. 
 

 

Anti-feeding test 
 

The leaf powder treatments produced significantly (P < 
0.05) higher anti-feeding effect on the adult P. truncatus 
than the control (Figure 5). Among leaf powder 
treatments, the mean differences were statistically 
insignificant (P > 0.05) however, the highest feeding 
deterrence was in the Neem followed by Tephrosia where 
the test insects made fewer holes in the filter paper at all 
the doses. The least effect was in Guava. An increase in 
anti-feeding activity was observed with increasing dose 
though, not significant different (P > 0.05). 



  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 3. Mean number of dead larger grain borer, P. truncatus adults in dried cassava treated with different doses 

of leaf powder during 70 days of storage.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 4. Mean percent repellency of leaf powders to the larger grain borer, P. truncatus 

 
 

 

Contact toxicity of petroleum ether leaf extracts 

vapours 
 
The petroleum ether extracts were moderate toxic. The 

median lethal concentration (LC50 ) of the petroleum ether 

leaf extract vapours are shown in Table 3. Neem vapours 

 
 
 
 
were the most toxic against P. truncatus adults followed 

by the Tephrosia. The LC50 ranged from 0.384% for 

Neem to 4.027% for Guava vapours. Vapours from Neem 
were 8 times more toxic to the test insects than those of 
Guava. The confidence limits and slopes of dosage-
mortality curves obtained indicated no difference in the 



  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 5. Antifeeding effect of different doses of leaf powders on the feeding behaviour of the adults of the larger grain 

borer, P. truncatus 
 

 

toxicity of Neem, Tephrosia and Water hyacinth. Toxicity 
of the vapours that emitted from the impregnated filter 
papers on the test insects were of the following order: 
Neem > Tephrosia > Water hyacinth > Eucalyptus > 
Guava. 
 

 

Contact toxicity through topical application 
 

The probit statistics, estimate of the LC50 values of insect 
mortality data are presented in Table 4. Comparison of 

the LC50 values showed that Neem was most toxic (LC50 
0.079%) within 48 h after treatment, while Guava was the 

least toxic (LC 50 0.260%). After 96 h, Tephrosia was 
observed to be more toxic extract (LC50 value: 0.033%) 
followed by Water hyacinth (0.036%). At 120 h after 
application, Tephrosia and Water hyacinth extracts were 

most toxic (LC50 0.027%) and least was Guava, LC50 
value of 0.092%. Increased P. truncatus mortality was 
observed in all five extracts, during the 120 h observa-
tional period. The closeness of the dose-mortality curves 
as indicated by the slopes indicates there were no 

 
 

 

marked differences in contact toxicity amongst the 

extracts, except with Guava. 
 
 

DISCUSSION 

 

Leaf powders clearly suppressed the emergence of P. 
truncatus populations in dried cassava chips. The 
reproduction potential was reduced but not completely 
inhibited. The effect was observed to be plant-specific 
and dose-related. Several workers (Pierce and Schmidt, 
1992; Niber et al., 1994; Osiptian et al., 2010; Mulungu et 
al., 2010) have also reported the ability of some leaf 
extracts to inhibit the reproductive capacity of P. 
truncatus. The reduction in the numbers of P. truncatus 
progenies in the leaf powder treated cassava may be 
attributed to anti-oviposition, delayed egg-hatching and 
insect growth disrupting effects of the leaf powders. The 

absence of F1 insects at 14 DAT in the leaf powder 

treatments could have been as a result of these effects.  
The insect species sensitivity for a same plant extract 

might be different for different dosages (Illoba and 



 
 
 

 
Table 3. Toxicity effect of vapours of petroleum ether leaf extracts on the adults of the larger grain borer, P. truncates.  

 
Extracts LC50 95% C.L Slope + SE Chi-square Df Heterogeneity 

Eucalyptus 1.3256 0.678 – 2.72 0.931 + 0.346 1.651 10 .17 
         

Guava 4.0279 1.228 – 6.821 0.783 + 0.371 1.940 10 .19 
         

Neem 0.3841 0.227 – 0.692 1.138 + 0.327 3.324 10 .33 
         

Tephrosia 0.5120 0.331 – 1.058 1.288 + 0.339 6.116 10 .61 
         

Water Hyacinth 0.5149 0.281 – 0.870 0.824 + 0.321 2.791 10 .28 
 

LC50 = concentration (%) calculated to give 50% mortality, S.E = Standard Error, C.L=Confidence limit, Df = Degree. of freedom 
 

 
Table 4. Response of the adults of larger grain borer, P. truncatus to the petroleum ether leaf extracts after topical application.  

 
    LC 50   

Extract   Observational period (h)  

  24 48 72 96 120 

Eucalyptus 0.379 0.158 0.083 0.048 0.034 

(0.000 ± 3987126) (2.221 ± 0.911) (0.719 ± 0.348) (0.323 ± 0.320) (0.300 ± 0.319) 

Guava 0.573 0.260 0.171 0.108 0.092 

(0.875 ± 0.228) (0.775 ± 0.225) (0.806 ± 0.226) (0.788 ± 0.230) (0.739 ± 0.231) 

Neem 0.135 0.079 0.057 0.042 0.040 

(0.666 + 0.225) ().767 ± 0.231) ().585 ± 0.232) (0.575 ± 0.235 ) (0.594 ± 0.236) 

Tephrosia 0.189 0.110 0.042 0.033 0.027 

(0.737 + 0.224) (0.020 ± 0.226) (0.597 ± 0.234) (0.506 ± 0.237) (0.492 ± 0.239) 

Water hyacinth 0.197 0.105 0.048 0.036 0.027 

(0.567 ± 0.222) (0.664 ± 0.226) (0.629 ± 0.233) (0.623 ± 0.236) (0.560 ± 0.240) 
 

LC 50 = concentration (%) calculated to give 50% mortality, Slope + Standard Error are in the parenthesis. 
 

 

Erakene, 2006) however, physiological responses 
ought to be noted for different plant extracts. The 
use of plant extracts powders were observed to be 
slightly detrimental to the development of later 
stages of the P. truncates as seen by the low 

 
 

 

numbers of deformed and dead adults. 

Increased insect F1 population in the leaf 
powder treatments with time suggests possible re-
adaptation of the adult progeny to presence of the 
plant extracts and the decay of the leaf powder, 

 
 

 

which resulted in reduced potency. It was 
observed that the higher the dose of leaf powders, 
the lower the numbers of offspring in the 
subsequent generations. These findings are in 
agreement with those of Akob and Ekwete (2007) 



 
 
 

 

and Nukenine et al. (2010) on Sitophilus species.  
Neem leaf powders showed excellent repro-duction and 

growth inhibitory effect, followed by Tephrosia and Water 
hyacinth. The closeness of regression coefficients (Steel 
and Torries, 1980) calculated for different leaf powder 
treatments when regressed against the time intervals at 
which the P. truncatus counts were made, indicates that 
the rate of breakdown of these leaf powders was similar. 
Though the relationship was strongest in Neem when 
applied at 5.0 g/100 g dried cassava, the significantly low 
numbers of P. truncatus recorded in Neem treated 
cassava at each sampling time indicates that Neem was 
more persistent in its effect than other leaf powders. The 
high P. truncatus populations in the Guava and 
Eucalyptus treatments could be a result of the rapid loss 
in toxicity most probably caused by the strong breakdown 
of the leaf powders of these two plants that belong to the 
same genera (Sharaby, 1988).  

The repellency effect of the powders was sustained 
throughout the study period. The strong repellent effect of 
this leaf powders admixed cassava may have been 
largely dependent on olfactory and gustatory sensation 
(Schmutterer, 1990) of the test insects. Properties like the 
repellent odour may have caused the insects to be 
restless (Ogendo et al., 2003). The insects were 
observed to be turning away and settling on the untreated 
cassava. The choice for the untreated cassava was 
because of the repellent chemicals inherent in the leaf 
powders (Jilani et al., 1988). Insect repellents are 
secondary metabolites which have been identified to be 
alcohols, alkaloids, phenolics, flavonoids and terpenes 
(Dales, 1996). Tephrosia leaf powder showed excellent 
repellency activity followed by Neem. In the preference 
test, the insect were seen to be crawling more towards 
untreated cassava than leaf powder treated cassava. 
Neem and Tephrosia treated cassava were the least 
preferred food media.  

The antifeedant effects of the leaf powders might be the 
result of different kinds of insect anti-feeding 
allelochemicals (Saxena et al., 1989) in the extracts that 
may have been working together, they deterred the 
insects from penetrating the filter papers. The anti-
feeding effect of the leaf powders may have also resulted 
from the bitter taste that rendered the wrapped cassava 
unpalatable to the test insects. The Neem followed by 
Water hyacinth leaf powders were more effective feeding 
deterrents. The outstanding effectiveness of Neem leaf 
powders have also been reported by other scientists 
(Saxena, 1993; Niber, 1994; 1995; Schmutterer, 1990; 
Facknath, 2006; Illoba and Erakene, 2006).  

Petroleum ether leaf extracts produced considerable 
high mortality and growth regulatory effects on almost all 
the treated insects. Significantly higher effects were pro-
duced by Tephrosia, Neem and Water hyacinth extracts. 
P. truncatus mortality was dose-dependent, increasing 

with an increase in dose. The levels of mortality caused 

 
 
 
 

 

by vapour from the leaf extracts were comparably low to 
those reported on pulse beetle, Callosobruchus chinensis 
by Pajni and Gill (1991). However, Neem vapours were 
most toxic to P. truncatus adults, followed by Tephrosia. 
In topical treatment, the active fractions in the petroleum 
ether leaf extracts may have caused mortality by direct 
interference with insect physiological balance (Pierce and 
Schmidt, 1993). The restlessness in the treated insects 
before death suggests hormonal involvement in the 
action of these compounds (Rani and Jamil, 1989). 
Similar observations have been reported for other plant 
extracts (Ogendo et al., 2004). These morphological 
features may be due to the failure of the wings to expand 
and flatten after adult insect emergence.  

The LC50 values of the petroleum ether extracts 

indicates that Tephrosia, Neem and Water hyacinth 
displayed the highest potency against test insects while 
Guava was the least toxic. The toxicity effect may be 
attributed to the secondary metabolites (Dales, 1996), 
which have been isolated from various plant parts. These 
tend to affect insects in several ways such as disrupting 
major metabolic pathways and causing rapid death, 
acting as deterrents, phagostimulants or anti-feedants, or 
modifying oviposition (Jilani, 1992). These secondary 
compounds may also retard or accelerate development, 
or interfere with the life cycle of the insects. The 
differences in the toxicity of Eucalyptus and Guava to P. 
truncatus, although, belonging to the same family, 
Myrtaecae, may be attributed to a species-specific factor 
(Kamal et al., 1988). In general, the amounts and toxicity 
of these compounds in the leaf powders will depends on 
the maturity of the plants, the season (temperature, 
photoperiod, and hygrometry) and the geographical and 
pedological conditions. 
 

 

Conclusion 

 

Besides exhibiting the repellency, antifeeding and 
reproduction inhibition effects, the leaf extracts exhibited 
vapour and contact toxicity on P. truncatus. These plant-
derived materials appear to be effective in reducing P. 
truncatus infestation. The presence of toxicants and 

growth inhibitors in these candidate plants suggest good 
potential for their use in storage pest management 
especially farm stored grain and pulses. The present 
findings indicate a piquant toxic nature of Neem, 
Tephrosia, Water hyacinth and Guava extracts on P. 
truncatus that can be utilized in farm stores against this 
pest. However, further work is required to investigate the 
isolation of their insecticidal bio-molecules. 

 
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