INTRODUCTION
The larvae of the Emperor moth Cirina forda is the most economically
important life stage in the insect’s life cycle; popularly consumed as
a rich source of animal protein supplement among native populations in Africa
in general and Nigeria in particular (Fasoranti and Ajiboye,
1993; Agbidye et al., 2009). According to
Ande (1991), the larvae of C. forda enjoys a high
level of acceptability and commercial value due to its appreciable size and
availability in large quantities thus, satisfying a major criterion for selecting
food insects (Ene, 1963). However, because the larvae
are pests of an equally economically important tree species, Vitellaria paradoxa,
the insect species development and production are often interrupted by the natives,
especially, due to the prolonged larval stage whose gregarious feeding normally
coincides with the reproductive season of the host plant. Therefore, there is
a need to strike a balance between sustainable production of both C. forda
larvae and the sole host plant, V. paradoxa. This may be achieved by
shortening the duration of the larval stage, such that the species is able to
complete its development within a short period, particularly outside the reproductive
season of its host plant. Though, the ecology and biology of C. forda
have been well elucidated (Ande, 1991); Odeyemi
and Fasoranti, 2000; Odebiyi et al., 2009),
little have been reported on sustainable strategies for improving the productivity
of the species. Over the years, however, different techniques have been employed
in inducing mutational changes that may result in improved performance of insect
species generally. One of such techniques is the use of gamma irradiation in
achieving desirable genetic changes including, shortening of life stage duration
(Elias, 1989; Gonzalez, 1994).
Therefore, in order to preserve natives’ culture of entomophagy in the
areas of distribution of C. forda, this study was carried out to elucidate
the potential of the use of gamma rays in enhancing growth rate and biomass
of C. forda larvae, as a prelude to eventual reduction and, perhaps seasonal
changes in the life cycle of the species.
MATERIALS AND METHODS
Source and collection of C. forda eggs: C. forda eggs
were collected from twigs of Vitellaria paradoxa in a rural community,
Enagi, about 100 km from Minna (longitude 6°33'E and latitude 9°37'N),
the capital of Niger state, Nigeria. The eggs were maintained in humid boxes
before transportation to Energy Laboratory for irradiation with Gamma rays.
Gamma irradiation of eggs and laboratory maintenance of larvae: Collected
eggs of C. forda were irradiated with gamma rays at the Centre for Energy
Research and Development, Obafemi Awolowo University, Ile Ife, Nigeria. Irradiation
doses used were 0, 10, 20, 50, 100, 150, 200 and 300 Gy. The irradiated eggs
were transported to the laboratory in humid boxes. The eggs were subsequently
incubated for hatching and the larvae were raised according to the methods of
Ande (1991).
Measurement of body weight and head capsule width: At the beginning
of each larval instar stage, 10 larvae were withdrawn randomly from each treatment
group and their weight and Head Capsule Width were measured, following standard
procedures (Ande, 1991).
Statistical analysis: Data obtained on growth performance indices were
statistically analysed following standard procedures. Differences in mean growth
performance parameters, among different groups of irradiated insects, were compared
using ANOVA.
RESULTS
The influence of increasing gamma irradiation dose on Body Weight (BW) of
C. forda larval stage is presented in Table 1. For all
irradiation doses tested, the eggs successfully hatched into larvae (i.e., 1st
instar larvae) and there was no significant difference (p>0.05) in 1st instar
larval BW between control group and irradiated treatments (range = 0.02±0.003
g/larva in the 200 Gy treatment to 0.028±0.003 g/larva in the control
group). However, increasing gamma irradiation inhibited subsequent larval instar
development to varying degrees; while, the 150 and 200 Gy treatments did not
permit larval development beyond the 1st instar stage, the 100 Gy treatment
allowed development to the 3rd instar. The remaining treatment groups (i.e.,
10, 20 and 50 Gy) supported larval development to the 5th instar and those of
the control group attained the final larval development stage (i.e., 6th instar).
Also, during the 2nd larval instar stage, mean Body Weight (BW) of the control
group of larvae were not significantly different from those of the irradiated
treatments (range = 0.105±0.003 g/larva in 20 Gy treatment to 0.172±0.009
g/larva in the control group). However, during the 3rd and 4th larval instar
stages, with the exception of the 50 Gy treatment, mean larval BW were lower
in the irradiated treatments than the control group.
Table 2 highlights larval Head Capsule Width (HCW) responses
of C. forda to gamma irradiation. The pattern of distribution of HCW
in relation to increasing irradiation doses was different from that of Body
Weight, though irradiation treatment had no significant effect on HCW during
the 1st instar larval stage (range = 0.926±0.014 mm in control, to 0.955±0.012
mm in the 100 Gy treatment). Unlike BW, Head Capsule Width increased with irradiation
dose from 10-50 Gy during the 2nd and 3rd larval instar stages but not significantly
different in the 4 and 5th instars. Also, HCWs during the 5th larval instar
of the irradiated treatments (range = 5.256±0.072 to 5.662±0.026
mm), were not higher than that of the 6th instar in the control group (6.065±0.10
mm).
| Table 1: |
Mean body weight (g) of variously irradiated C. forda
for all Instars |
 |
| Table 2: |
Mean head capsule width (mm) of variously irradiated C.
forda for all Instars |
 |
DISCUSSION
Gamma irradiation did not affect egg-hatching and Body Weight (BW) of hatched
larvae (1st instar) were not significantly different from those of the control
group. Since it was the egg stage of the insect that was exposed to gamma rays,
then this result indicates that irradiation had no lethal effect on embryogeny
in the species. On the other hand, however, increasing gamma irradiation dose
had significant inhibitory effects on subsequent larval instar development.
From the economic standpoint, irradiation doses above 50 Gy proved lethal to
improved production of C. forda for entomophagy, as such treatments could
not support larval development to the 5th instar; a stage considered threshold-acceptability
for human consumption (Agbidye et al., 2009).
Though, irradiation treatments (i.e., 10-50 Gy), that produced entomophagy-threshold
larval stage (i.e., 5th instar), could not support total larval development,
they nevertheless produced significantly heavier larvae at the 5th instar (as
much as 16.390±0.961 in the 50 Gy treatment), than those recorded for
the 6th instar larvae in the control group (i.e., 13.231±0.169 g/larva).
The significant effects of gamma irradiation on the development of C. forda
was extended to the 2nd instar stage; but beyond this stage, the effects became
more apparent. However, while the effects on Body Weight were mostly subtractive
during the 3rd and 4th larval instars when compared with the control, the reverse
was the case during the 5th instar stage. These observations, in addition to
confirming the non-lethal effects of gamma rays on embryony and egg-hatching,
may mean that gamma irradiation disrupted larval growth metabolic activities
during the early larval stage (i.e. 3rd and 4th instars), especially feeding
and food processing and/or utilization activities. This finding has great ecologic
and economic implications. The 3rd and 4th larval instars are the most voracious
and disruptive phase of C. forda against Vitellaria paradoxa.
Therefore if gamma irradiation checks feeding activities of the insect pest
during its most destructive phase but still produce much heavier larvae during
the 5th instar, as indicated by the results of this study, may mean effective
pest control with an added advantage of enhanced yield for entomophagy. Again,
Head Capsule Width during the 1st instar larval stage were not significantly
different between the control and irradiated groups of larvae. This result,
perhaps, further confirms that gamma irradiation has no lethal effect on embryonic
development of C. forda.
The results of this study showed that larval Body Weight and Head Capsule Width
responded differently to increasing gamma irradiation dose. This disparity may
be explained by the differential physiological bases of the two variables. While,
biomass in wet organic matter is highly influenced by its moisture retentive
capacity, Head Capsule in insects is more of a function of the degree of sclerotization
during ecdysis. Therefore, it may mean that these two physiological processes
respond differently to gamma irradiation. Also, unlike its effect on body weight,
gamma irradiation treatment did not produce wider Head Capsule in its terminal
larval stage (i.e., 5th instar), than that of the control group (i.e., 6th instar).
Fortunately, such an effect may not diminish from the potentials of gamma irradiation
in producing increased C. forda biomass for entomophagy. According to
Agbidye et al. (2009), the nutritional constituents
of C. forda larvae contained mainly in the thorax and abdomen and not
the head capsule.
CONCLUSION
The findings of this study showed that gamma irradiation has little or no effects
on embryony and, perhaps, early larval development in C. forda. Its effects
on older larval instars may, however, be subtractive or otherwise, depending
on irradiation dose. On the whole, Gamma irradiation shows promising potentials
for improving the biomass of C. forda larvae. However, the dose of irradiation
must be carefully selected to balance the subtractive effects observed in the
early larval stages against the anti-ecdysis effects of high gamma irradiation
doses. The optimum irradiation dose (i.e., that will promote increased biomass
accumulation and complete larval development), may lie between 20 and 50 Gy.
The findings of this study should provide baseline information for sustainable
production of C. forda larvae for entomophagy, though gamma irradiation
genetic improvement.
ACKNOWLEDGMENT
We are grateful to the Management of Energy Research and Development Center,
Obafemi Awolowo University, Ife, Nigeria, for assistance rendered regarding
gamma irradiation of C. forda eggs.
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