It has been said that infestation of the rice plant by the rice stem borer, Chilo suppressalis, is severer in the Mizonobe-part of Kahoku-town, Yamagata Prefecture, as compared with that in the Kitayachi-part of the same town, 5km distant. Number of moths attracted by light traps was also more abundant in the former region than in the latter. For the differences observed in both regions, following reasons may be considered:<br>(1) The borers distributed in both regions are different to each other in their biological characters or injuring potencies.<br>(2) Some environmental conditions (including status of the rice plant) of Mizonobe-part differ from those of Kitayachi.<br>(3) (1) and (2) are taken place simultaneously.<br>From observations and experiments, it could not be concluded that the borers of Mizonobe are different from those of Kitayachi in their morphological and biological characters. Thus, reasons (1) and (3) are discarded out from the considerations.<br>Because of a short distance between Mizonobe-part and Kitayachi-part, it seemed that there is no difference in both regions concerning environmental conditions affecting borer's biology, e.g. climatic factors, natural enemies, planting practices, etc. Investigations showed, however, that physical and chemical properties of paddy soils of Mizonobe-part differ from those of Kitayachi soil. The greatest difference among them is the silicon supplying power of soils. Mizonobe soil contained a low level of available (easily soluble) silicon (8.9mg per 100g dry soil), while Kitayachi soil did a relatively high amount of available silicon (17.4mg per 100g soil).<br>Reflecting the silicon supplying power of soil, the rice plant grown on Mizonobe soil has a lower content of silicon as compared with the plant on Kitayachi soil. Silicon contents of the plant at harvest stage were less than 10% in Mizonobe and about 15% in Kitayachi. Consequently the former plant was seen to be more attractive to ovipositing adult moths, more preferable and less antibiotic to boring larvae, and less tolerant to borer infestation. It appears that silicon absorbed by rice plant gives to the host an allround ability on borer resistance. These interrelationships are illustrated as below:<br>As a result of supplying calcium silicate to paddy fields of both regions, rice plant infestation and borer population reduced remarkably in Mizonobe-part where soil was deficient in available silicon; whereas no influence was observed in Kitayachi-part.<br>Thus, the heavier infestation and higher population of the borer found in Mizonobe-part are explained clearly in a view point of low level of available silicon contents in soil of this region.<br>Some geological evidences on formation of Mizonobe soil were discussed in the text, with special reference to origin and translocation of granite sand as a parent material.