The influence of certain factors on the reproduction of Sitophilus Oryzae (L.) (Coleoptera: Curculionidae)
Fourie, Gert Johannes Jacobus
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1. S. oryzac is a serious pest of cereals and cereal products throughout the world. 2. A study was undertaken in South Africa to determine the influence of various factors on the re-production of this weevil. 3. The weevils were collected at the Glen Agricultural College and reared on maize in a controlled temperature and humidity room (27°C and 70% relative humidity). The progeny of these weevils were used in the different experiments after random sampling. 4. Some of the experiments were conducted in the above mentioned constant temperature and humidity room, while others were conducted in desiccators stored in incubators set at different constant temperatures. Potassium hydroxido was used to control the relative humidity in the desiccators. 5. The parent weevils used in the different experiments were transferred fortnightly to fresh maize. The maize from which they had been removed was then kept in the controlled temperature and humidity room for another fourteen days and then sectioned for progeny counts. 6. (i) It was found that food consumption of the larvae increased as the temperature decreased. Larvae also required more food to complete their development at 12% than at 14% moisture content of maize. There was a highly significant positive correlation between the weight of food consumed by the larvae and the body weights of the corresponding adults. Larvae which consumed large amounts of food resulted in heavier adults. Thus it was concluded that the effect of temperature and moisture content of maize on the weight of food consumed by the larvae, was evident in the weights of the adults and it may be stated that larvae reared at lower temperatures and a lower moisture content, gave rise to heavier adults and vice versa. (ii) When the immature stages were reared at 12%, instead of 14% moisture content of maize, the resuIting adults produced significantly more progeny. In the case of both moisture contents, most progeny was produced when the immature stages were reared at 22 °C. 7. When the adults were exposed to the different treatments of temperature and moisture content, most progeny were produced at 26 °C. Adults which were kept at a moisture content of 14% produced significantly more progeny than adults which were kept at a 12% moisture content. 8. Females which copulated for long periods produced significantly more progeny than females which copulated for short periods. 9. Females that copulated once per month produced significantly more larval progeny than females which were allowed to copulate every fourteen days, every two months and once per lifetime. There was no significant difference in the number of larval progeny produced by females which copulated once per lifetime and females which copulated fortnightly. 10. More progeny was produced by heavier females. 11. The "best" male/female ratio for reproduction was 5 females and 2 males. The presence of too many males had a depressing effect on the oviposition rate of the females. It was found that 5 females, together with 10 males, produced significantly the least progeny of the different ratios. 12. The number of larval progeny in each experiment was counted at fortnightly intervals. The number of progeny reached a maximum when the adults were from 4 to 6 weeks old, irrespective of the different treatments, and then gradually fell away.