Bio-insecticidal and Grain Protectant Activities of Mixed Powder of Ocimum gratissimum and Vernonia amydalina against Callosobruchus maculatus (Coleoptera: Bruchidae)

The bio-insecticidal and grain protectant activities of mixed powder of Ocimum gratissimum and
Vernonia amygdalina upon Callosobruchus maculatus was carried out. The mortality rate of cowpea
weevil treated V. amygdalina: O. gratissimum at 100: 0 concentrations were 17.0%, 27.0%, and
47.0% for a period of 24 hrs, 48 hrs and 72 hrs, respectively. Concentrations 0:100 gave a percentage
mortality of 47.0%, 67.0% and 73.0% at 24 hrs, 48 hrs and 72hrs, respectively. 80: 20 and 20: 80
concentrations gave 33.3% and 47.0% at 24hrs and 48hrs, but 80:20 gave 47.0% while 20:80 gave
57.0% mortality rate at 72hrs. 50: 50 concentrations gave 73.0%, 77.0%, and 80.0% mortality rate at
24 hrs, 48 hrs and 72 hrs, respectively. The 0: 0 concentrations gave 0.0%, 3.3%, and 7.0% mortality
rate at 24 hrs, 48 hrs and 72 hrs, respectively. The effect on the oviposition of cowpea weevil revealed
that the mean value of eggs laid on seeds were 5.0, 2.3, 3.3, 2.8, 1.0 and 7.0 for 100:0; 0:100; 80:20;
20:80; 50:50 and 0:0 concentrations, respectively. The effects of mixture of V. amygdalina: O.
gratissimum powder on the emergence of cowpea weevil gave a mean value of adult weevil
emergence as 5.3, 3.0, 5.0, 3.3, 1.3 and 8.0 for 100:0, 0:100, 80:20, 20:80, 50:50 and 0:0
concentrations respectively. Mixed powder of V. amygdalina: O. grattisimun with 50:50%
concentrations conferred the highest bio-insecticidal potency against C. macullatus during the
experimental investigation. This was evident from the fact that treatment with V. amygdalina: O.
grattisimum (50:50%) concentration attained 80.0% mortality at 72hrs of exposure, which was
followed by V. amygdalina: O. grattisimum (0: 100%) concentration with 73.0% mortality. The results
showed that 50:50 and 0:100 ratio compounded V. amygalina and O. gratissimum had 85% and 76%
effect on the insects. The two treatments had similar effect with a significant difference at P<0.5
mortality. The highest percentage inhibition of reproduction/adult emergence was recorded at
Va50:Og50 treatment. When compared with the control, all plant material were effective in increasing
insect mortality and reducing oviposition, development and adult emergence thus reducing the overall
population of C. maculatus at 31 days after treatment. The potency of the combined concentrations of
V. amygdalina: O. gratissimum (50:50%) of these plant substances was due to the odour they
produce, which may have exerted a toxic effect by disrupting normal respiratory activities of the
cowpea weevil, thereby resulting in asphyxiation and subsequent dead; it may also have acted as
contact poison on the weevil. It equally may have ovicidal and larvacidal properties. It is
recommended that similar studies be conducted using separately other parts of the test plant like the
stem and bark of V. amygdalina and O. gratissimum to further evaluate their bio-insecticidal potency
against stored grain pests.