THE RELATION BETWEEN THE HEADING DATE OF THE HOST PLANT AND THE PUPATING LEAF SHEATH OF THE RICE STEM MAGGOTS WHICH INJURE THE DEVELOPING EARS OF THE RICE PLANT

Abstract

The rice stem maggot, Chrolops orizae MATSUMURA, in summer blood bores near the growing point of the rice plant and injures the developing leaves and ear. After the heading of the host plant the maggot which has eaten the developing ear pupates inside the leaf sheath. The leaf sheath in which the maggot pupates has been observed by some investigators but the results of their observations did not always conform with each other. The author conducted a detailed investigation on this problem. The results are summarized as : (1)In the variety which headed on about the 20th of August, 80 per cent of the pupae of the rice stem maggots were found inside of the top leaf sheath, but in the varieties which head later, the individuals which pupated in the lower leaf sheath gradually increased as the heading date became later. In the varieties which headed earlier than the 20th of August, the individuals pupating in the top leaf sheath decreased as the heading date became earlier, but this relationship was not so close as in the later varieties. (2) When the heading date of the same variety was shifted by means of sowing in different days, the relation of heading date and pupating leaf sheath was nearly the same in many different varieties. (3)In a single variety, the higher the per cent of pupating individuals becomes, the higher the per cent of individuals pupating inside of the top leaf sheath becomes. From this, it is reasonable to conclude that the percentage of the individuals which pupate soon after heading are found at the second or the lower leaf sheath more commonly than in those which pupate later. The pupating date of the maggots differed with the heading date of the host variety. But the duration from heading to pupation gradually becomes shorter as the heading date of the host variety becomes later, that is, in later heading varieties the duration from heading to pupation is shorter that in earlier heading varieties. From the above two facts, it is clear that, in varieties which head later than the 20th of August, the later becomes the heading date, the more the number of individuals pupate at the lower leaf sheath. (4) In early heading varieties, pupation was likely inhibited after a few maggots had pupated. It is reasonably considered that, because in early varieties the number of individuals pupating before the inhibition and mortalty of maggots waiting for pupation were different among varieties, the relation of pupating leaf and heading date was not so close as the late varieties. (5) It was discussed, based upon an example, that if the pupating date fluctuated among years, the relation written in (1) would also differ among years.