INSECTS INJURIOUS TO AGRICULTURE IN JAPAN 47 



between the stalk and leaf sheath. Ishikawa (1^3) records the de- 

 struction of 28 per cent of the eggs in Niigata Prefecture by a 

 hymenopteron. One of the methods of control advocated is the col- 

 lection of adults by lantern trap, and in experiments at the Hok- 

 kaido Agricultural Experiment Station on an area of 1 cho (2% 

 acres) in which two lamps were used, the following numbers were 

 taken during the period from the end of June to the middle of 

 August : 



Year 



Females 



Males 



1911 



163 

 452 

 361 



144 



1912 



361 



1913 



334 









Total 



976 



839 







It is thus seen that a slightly greater number of females than males 

 were caught at the traps, but whether this indicates a greater response 

 on the part of females to light is uncertain, as the normal sex ratio 

 in the field is not known. 



Various experiments have been conducted to determine the effect 

 upon this pest of submergence during the growing period of the 

 host plant and the possibility of utilizing submergence as a means 

 of control. The more extensive tests along this line have been con- 

 ducted by Harukawa (18, 20, 26) in Okayama Prefecture. The em- 

 bankments surrounding the rice fields are often such that flooding 

 to a depth of from 6 to 8 inches is the maximum possible, and con- 

 sequently a portion of the leaves and the tip of the stalk may remain 

 above the water level. At the middle of July such partial submer- 

 gence for 24 hours resulted in a mortality of first-brood larvae of 

 from 50 to 60 per cent. If the plants were completely submerged, 

 however, the proportion killed increased from 80 to 90 per cent. 

 One of the most important factors entering into the problem is the 

 temperature of the water, the higher temperatures being the most 

 effective. The check plots showed a reduction in yield of from 6 to 

 13 per cent due to injury by complete submergence (26, f. 183). 



The second generation of larvae can not be dealt with in this way, 

 as at the time they are present the plants are higher than the em- 

 bankments inclosing the fields. The general practice at this stage 

 is to have coolies go through the paddy fields during the period in 

 which the eggs are hatching and remove all plants upon which colo- 

 nies of young larvae are feeding. This must be done before they 

 scatter to adjacent plants and bore into the stems. 



Schoenobius incertellus (51, 170) in Japan is primarily a pest of 

 rice, but is doubtfully said to occur also upon millet and other 

 gramineous plants. It is the most important pest of rice in China 

 and India. Three broods are produced each year in Kyushu, the 

 first brood of adults appearing in late April and May, the second 

 in late June and July, and the third in August and September. 

 Hibernation takes place in the mature larval stage in the stubble. 

 The adult females are nocturnal in habit, and the eggs are laid late 

 in the evening, those of the first generation in the nursery bed upon 

 the larger leaves near the tip, whereas those of the following two 



