LIFE CYCLE , MORPHOMETRICS AND DAMAGE ASSESSMENT OF THE KHAPRA BEETLE , Trogoderma granarium Everts ( COLEOPTERA : DERMESTIDAE ) ON STORED GROUNDNUT

The life cycle of the Khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae) on stored groundnut was studied under fluctuating laboratory conditions of 25±5°C and 70±5% relative humidity. There were five larval instars and the total development time from egg to adult ranged from 37 to 40 days with an average of 37.95 days. The duration of each developmental stage was egg: 6.05 days; first instar: 3.8 days; second instar: 4.7 days; third instar: 5.6 days; fourth instar: 6.2 days; fifth instar: 6.8 days and pupa: 4.8 days. Females had an average fecundity of 80.2 eggs. Mean adult longevity was 12.4 days.

a wide range of cargo including non-agricultural goods, which appear to become infested through contact with infested goods (A n o n y m o u s , 2001).The beetle breeds most rapidly under hot, dry conditions and has a potential for spread.These spreads around the world in international trade are shown by its regular interception on oil cake and other products imported into Great Britain from Burma, Sudan, Senegal, India and Nigeria (F r e e m a n , 1974).
Khapra beetle is a cosmopolitan, multivoltine and polyphagus pest throughout the tropics (H i l l , 1983; H i l l and W a l l e r , 1988; O d e y e m i , 1989).The bionomics of T. granarium developing on groundnuts has not been vigorously examined in the laboratory, and optimal conditions for development are still a matter of debate (W i g h t m a n et al ., 1990).
The life cycle of dermestid beetles shows a typical holometabolous development; the larvae being the destructive stage of the insect pest.The adults possess wings but are not capable fliers (O f u y a and Lale, 2001) and do not feed.The methods commonly used to control T. granarium in the regions of its natural occurrence and distribution include chemical (mainly fumigants), non-chemical (legislative policy) and integrated pest management approach.However, it has been observed that to use these control methods effectively, the life cycle, morphometrics and extent of damage caused by T. granarium on stored groundnut kernels were studied as shown in this investigation.

Insect rearing:
The experiment was carried out in the laboratory under fluctuating ambient conditions of 25±5° C and 70± 5% relative humidity.A culture of T. granarium was established on a groundnut (RRB) variety kept in a 250 ml Kilner jar to ensure availability of the insect pest.
Experimental procedure: Plastic containers, 12 cm in diameter, contained 100 g groundnut kernels into which four adults T. granarium (sex ratio 1:1) were introduced.A muslin cloth was used to cover the containers to allow aeration and prevent escape of the insects.The experimental set-up was replicated four times.
Life history: The adult beetles were allowed five days to lay eggs before they were removed and discarded.The number of eggs laid on the surface of twenty groundnut kernels randomly selected were counted and then recorded.There was keen observation of the experiment to obtain the first day of larval emergence and development was followed to the adult stage by daily observation.The number of larval instars was examined and studied by direct counting of larval moults under the high-power microscope.Ten newly emerged adults (irrespective of sex) were kept separately in a rearing unit and observed daily to obtain the longevity of unmated adults.
Morphometric studies: Morphometric data (length of eggs, body length and head capsule width of every instar and the body lengths of pupa and adult) were measured under the occular micrometer.Estimates of body length and head capsule width are means and standard errors of the number of specimens used (n=10).
Damage assessment: Five freshly hatched larvae of each of the five larval instars were introduced into a container of 100 g groundnut kernels and then left undisturbed for three months.The experimental set-up was replicated four times.
The groundnut weight damage was determined by the standard weight method.The initial weight of the groundnut (100 g) was compared with the final weight of the groundnut after three months storage period.The percentage damage done was evaluated by the difference in the initial weight and the final weight of the groundnut divided by 100.Thus, Data analysis: The data were subjected to one-way Analysis of Variance (ANOVA) and significantly different means were separated using Student -Newman -Keuls (SNK) test.

Results and Discussion
Table 1 shows developmental duration and morphometrics of T. granarium raised on stored groundnut.
Life history and morphometrics: The female T. granarium laid their eggs on the surface of the groundnut kernels.There were considerable variations in the egg-laying capacity of the females.This could be attributed to the food provided ad libitum and pairing of the females with the males.Total number of eggs per female ranged from 40 to 85.When laid, the eggs were 1.5±0.01mm long (n=10), creamy white in colour.The number of observed moults (four) indicated that T. granarium passed through five larval instars (Table 1).From measurement of head capsule width a mean growth ratio of appropriately 1.40 was calculated.Developmental duration of the first larval instars was 3-7 days, the duration increased with increase in the number of instars.Newly emerged 1 st instar larvae were tiny and white.They could move briefly but the body was sensitive to disturbance.Though the antennae were not very visible but the hairs were conspicuously visible.The first instar larva measured 1.59±0.14mm in length and had 0.18±0.01mm head capsule width.The first larva emerged on the seventh day of infestation.
The newly hatched 2 nd instar larva was yellow in colour with visible setae.The antennae became conspicuous.In the 3 rd , 4 th and 5 th instars, the antennal segments were more conspicuously visible and the barbed hairs were well developed.The 4 th and 5 th larval instars showed brownish appearance and erect setae dorso-laterally and urogomphi.Table 1 shows the mean body lengths and mean head capsule widths of the larvae.The mean body lengths of the 2 nd , 3 rd , 4 th and 5 th instars were 1.73±0.13mm, 2.92±0.40mm, 4.62±0.33mm and 5.34±0.50mm respectively while the head capsule widths were 0.25±0.03mm, 0.45±0.02mm, 0.53±0.05mm, and 0.64±0.04mm respectively.The latter instars were more active, had greater head capsules and longer body lengths with profuse hairs on the body.They were easily resilient when disturbed compared to the more sensitive first instar larvae.
The last larval instar later developed a protective case for transformation into the pupa.The pupa appeared shorter than the larval forms, quiescent and did not feed.The pupa was of dark brown colour.The head and the thoracic segments were visible, the remaining being covered up by the exuviate.The mean pupa period was 4.80+0.79days (Table 1).

Damage assessment:
Table 2 shows mean weight damage of groundnut kernels caused by T. granarium larval instars after three months storage period.The fifth larval instars caused significantly highest mean weight damage of 33.9% compared to the lowest mean weight damage of 22.7% caused by the first larval instars.Among the five larval instars, the first larval instar was significantly different (PO.05) from the fifth larval instar and caused least groundnut kernel damage.However, there was no significant difference (P>0.05)among the damage caused by the second, third, fourth and fifth larval instars.

C o n c l u s i o n
T. granarium completed its development from egg to adult on stored groundnut kernels in the laboratory in 37.95 days.The egg-laying pattern varied between 45 and 80 eggs per female.The varying developmental period, the high fecundity and high groundnut kernel consumption of the larval instars make the insect species a major primary pest.
The head capsule measurements increased by an average factor of approximately 1.40 which agreed with Dyar's law (Ewete, 1990; Emehute and Odiette, 1991; Imms, 1970).Dyar's law can be used to determine the number of actual instars in a species, particularly in a species where the individual moults are not readily observed (Emosairue, 2002).Measurement of head capsule width would make separation of larvae into instars more accurate than other morphometric parameters (Anikwe and Okelana, 2006.)The 5 th larval instar was the most destructive with 33.9% of groundnut kernels consumed after three months storage period.This corroborates the observation made by Magar (1973) of the destructive capacity of T. granarium on groundnut in shell.Since larvae were frequently intercepted in export crops, control action should be instituted immediately to prevent development and further spread under bulk and favourable food supply.