Basic HTML Version
Relative Susceptibility and Proximate Composition of Rice Varieties by
Sitophilus oryzae
L.
20
The rice varieties were sterilized in a deep freezer for
21 days. This was done to destroy any mites, larva,
pupa or eggs of insect and adult insects that may be
contained in the rice samples (Wee and Nick, 1998).
The rice varieties were placed in jars and then left in
the laboratory for seven (2) days so that the grains will
acclimatize to ambient conditions.
1.3 Insect culture and maintenance
Adult rice weevils,
S. oryzae
were obtained from
naturally infested rice grain from a cereal store in Ado
Ekiti, Nigeria. Cultures for the study were established
on a clean highly susceptible local rice variety. The
weevils were introduced into the jar containing this
rice variety and they were allowed to oviposit for a
period of 18 days after which they were removed from
the medium and discarded. The grains were returned
to the culture jar and left undisturbed until the weevil
completely emerged after 40 days.
1.4 Determination of grain hardness
Twenty grams (20 g) of each of the 5 local varieties
and 5 imported varieties (Table 1) were weighed into
Petri dishes, in 4 replications. The seed hardness was
estimated using a California Bearing Ratio (CBR)
Machine, which measured the force needed to crush
the seeds in Newton. Five randomly selected grains of
each of the rice varieties were individually crushed
using the CBR machine and the readings recorded.
The mean value of the force required to crush each of
the varieties (in Newton) was then computed.
1.5 Mortality, F1 Progeny emergence and damage
to rice varieties by
S. oryzae
Four replicates of 20g each rice variety were placed in
50ml transparent containers. The containers were
covered with lids with mesh perforation to allow for
aeration. Five freshly emerged male and five female adults
S. oryzae
were introduced into each 20g lot of each variety.
The weevils were allowed to feed and oviposit for a period
of 28 days. During this period, mortality of
S. oryzae
was
determined from daily counts of dead adults after which
all surviving adults were removed, according to
Onolemhemhen and Oigiangbe (1991).
Adult
S. oryzae
was presumed dead, if it failed to
move when touched with a spatula. Thereafter, all
weevils were sieved out. The experimental containers
were then kept in the laboratory undisturbed until
emergence of F
1
generation. Thereafter the samples
were inspected every other day for the emergence of
F
1
adult
S. oryzae.
Newly emerged F
1
adults of
S.
Oryzae
were counted and removed until no emergence
was recorded for five (5) consecutive days to prevent
interference with F
2
generation. Eight (8) WPI, the F
1
generation had completely emerged. The adult
emergence for each of the varieties was recorded as F
1
adult emergence and the weight obtained for each of
the samples.
The experimental containers were then left undisturbed
for the F
2
generation to emerge. The emergence was
monitored until no emergence was noticed for about
five days. The adult emergence for each of the
varieties was recorded as F
2
adult emergence and the
weight loss of each of the ten varieties obtained.
Percentage mortality of introduced weevils was
calculated at 4 WPI. The total number of adult
emergence at 8 weeks and 12 WPI of parent weevils
and grain loss at week 8 and week 12 were calculated.
1.6 Calculation of index of susceptibility
The index of susceptibility was determined as follows:
Index of susceptibility (SI) = (Natural log F x 100)/D,
where F is total number of F
1
adult insects counted, D
is median developmental period in days. Median
developmental period is defined as the time from the
middle of oviposition period until the emergence of
50% of the F1 generation. (Dobie, 1974)
1.7 Laboratory analysis of rice samples
Proximate Analysis was carried out to determine the
ash, fat, mineral, crude fibre, moisture content and
crude protein contents of each of the varieties.
1.8 Determination of moisture content
The moisture content was determined by using
oven-drying method. Clean and dry petri-dishes were
weighed by using Mettler balance and their respective
weight were recorded (W
1
). 5g of each the sample was
weighed into the dishes (W
2
) spreading as much as
possible. The petri-dishes containing the sample were
transferred into the oven maintained at 105°C and
dried for about three hours. After three hours, they