THE BIOLOGY OF THE RICE WEEVIL, CALANDRA ORYZAE (L.)

The length of the developmental instars of Calandra oryzae has been estimated at relative humidities of 50, 60, 70 and 80% at 21° C., at 70% at 18, 25 and 30° C., and at 80% at 18° C., by the dissection daily of samples of infested wheat-grains. The results are expressed as a median obtained by the method of probit analysis. Comparison of this method, with estimates of median and mean obtained by orthodox arithmetical methods from similar work on Rhizopertha , show that the probit method gives good estimates.
About 90% of the eggs laid are fertile. Normally only one adult will develop in a grain, all other individuals being destroyed by cannibalism. The sex ratio is unity. It was not possible to cross C. oryzae and C. granaria.
The daily oviposition rate of C. oryzae at 17, 21 and 25° C. increases with relative humidity. There is a critical point at about 60% r.h . below which egg laying declines rapidly, and mortality is high. At 100% r.h . the oviposition rates per female per day are approximately 1.3, 2.5 and 3.4 at 17, 21 and 25° C. respectively.
In experimental conditions most eggs per grain are obtained by giving isolated females one grain each, but more eggs are laid by females given more than one grain. Daily egg output is reduced by grouping females or including males. In culture, depths of grain up to 7 cm. do not discourage egg laying.