Observations on Grain-Weevils, Calandra (Col., Curculionidae), General Biology and Oviposition.


  • O. W. Richards.


1. Methods for handling grain and weevils for experiments with Calandra are described. Nearly all the experiments were conducted at 25°C. 70% R.H.

2. The dimensions of the eggs and the head-widths of the four successive larval instars of C granaria are recorded.

3. The rate of sexual maturation and the effects of this process on the readiness of weevils to pair is described in both strains of C. oryzae and in C granaria.

4. Records are given of frequency of pairing, choice of mate in relation to weight; effect of ovarial development on readiness to pair, length of the oviposition-period and number of eggs laid, length of life, effect on ovipositionrate and on fertility of one or more pairings.

5. The rate of development of weevils at 21°C. and 50, 60, 70 and 80% R.H. is recorded. Normally, only one larva can develop in each grain and the elimination of supernumerary ones is described. Each larva in its development at 25° C., turns about 14 mg. of grain into CO2 and water, and produces about 14 mg. of frass to produce a weevil weighing 2˙4 mg.

6. The emergence of adult weevils from the grain in which they have developed is stimulated by shaking the grain.

7. In both strains of C. oryzae and in C. granaria the sex-ratio is not far from unity. There is some evidence that cultures derived from a single female parent produce rather fewer males than mass-cultures.

8. It is shown that the oviposition-rate, though variable, is reasonably constant over ten-day periods. There is little evidence for any periodic variation.

9. The weight of the female, her ovarial score (number of eggs in ovary) and oviposition-rate are all more or less correlated. There is some evidence that the relation is due to the environment which has acted on the female rather than to her genetic constitution.

10. If weevils are matured in crowded conditions it is apt to reduce their oviposition-rate afterwards.

11. Single weevils lay better in large quantities of grain than in one or two grains. With as many as 600 grains it makes no difference whether it is deep or shallow. But groups of 20♀ lay fewer eggs on 600 grains than 40 ♀on 1,200. This effect could not be demonstrated in single weevils.

12. With groups of 20♀weevils, oviposition falls significantly if there are fewer than 10 grains per female and there is some evidence that about 50 grains per female are really required for maximum egg-laying. The depressing effect of population-density is partly due to the saturation of grain with eggs, partly to mutual interference, in which virgin females are as effective as fertilized ones. Males have a still more depressing effect.

13. In a normal grain-sample (grain-size normally distributed), oviposition is effectively random though grain-size determines which grains shall receive the higher numbers of eggs. Grains containing fourth stage larvae are avoided.

14. At 21° C., oviposition increases slowly as the relative humidity rises from 70 to 100% and falls rapidly if it is reduced below 70%. The same is true at 17° C. and 25° C. though the data at these temperatures is less reliable. The amount of oviposition at 17°, 21° and 25° C: is probably in the proportions of 43: 100: 268. Oviposition ceases at about 9˙5°C.