The susceptibility of sorghum to attack by the weevil Sitophilus oryzae (L.)

Extract

Factors affecting the susceptibility of seeds of sorghum (Sorghum vulgare) to attack by Sitophilus oryzae (L.) were investigated in the laboratory. It was known that varieties in which a majority of the seeds have a predominantly vitreous endosperm (vitreous varieties) are less attacked than those in which the majority have a predominantly mealy endosperm (mealy varieties). A method of measuring the relative amounts of vitreous and mealy endosperm in a seed was devised, and was used to classify samples from ten varieties of sorghum that had been grown in East Africa. The varieties ranged from vitreous to mealy, and the seeds from them formed the test material.

Adult weevils confined singly in tubes with seeds of either a vitreous or a mealy variety consumed more per day of the latter than of the former (about 0·2 and 0·15 mg., respectively, per mg. body-weight of weevil). When females were allowed to oviposit in seeds and the next-generation adults were counted, greater numbers were obtained from mealy than from vitreous varieties, the extreme results from a range of five being in the ratio of about 20:1. Seeds of vitreous varieties have a lower moisture-content than those of mealy ones at the same humidity, but this was not the cause of the differing numbers of adults, for the results were similar when the experiment was repeated with seeds of a vitreous and a mealy variety treated so that the moisture contents were identical. Attempts to compare the numbers of eggs laid in seeds of these two varieties gave inconclusive results; it was shown that none of the methods used was entirely reliable; in two experiments, no difference could be detected, but in a third, in which the seeds were examined by means of X-rays as well as by other means, significantly more eggs were laid in seeds of the mealy than of the vitreous variety. There was no evidence of a preference for any particular size of seed of either.

The hardness of the seeds of all ten varieties was estimated by grinding equal quantities in a mill and weighing the amounts that then passed through a 60-mesh sieve. This was done twice, with seeds at 10 and 14 per cent, moisture content. The order of hardness was not appreciably affected by moisture content, and was clearly correlated with the proportion of seeds with predominantly vitreous endosperm; the more vitreous varieties had harder seeds than the more mealy ones. It is inferred that hardness of the seed is the principal factor responsible for resistance to weevil attack.