To date after preliminary trials they have found that at 14.3 milliamperes 

 per square inch of direct current drills immediately withdraw into their shells and 

 remain there while the current is on. In many cases they do not resume active 

 crawling for several hours after the experiment has been terminated , At 5 . 9 to 

 14 o3 milliamperes drills become very active It is between these intensities that 

 Lindsay's group seek a guiding or repelling effect. Attempts to kill drills with 

 electricity have been unsuccessful , Preliminary experiments with electrical 

 fences have also failed, No adverse effects on oysters have been noted 



Ultrasonics 



Although no research has been performed on the utilization of ultrasonics 

 in drill control (Henry, 1954), investigations in this direction may reveal a fre- 

 quency, possibly of high intensity, to which Urosalpinx is specifically sensitive, 

 and which may be exploited in repelling or attracting or even permanently inactivat- 

 ing the drill in situ On the bottom . 



Fresh, brackish, and brine water 



Federighi (1931c) and Galtsoff et al. (1937) recommend floating drill in- 

 fested oysters in brackish water as an unusually efficient method for killing all 

 stages of the drill, particularly during the transplantation of oysters. In practice 

 drill infested dredged oysters are placed in large floats and anchored for about 10 

 days in brackish water which is fresh enough to kill the drills but salty enough to 

 cause no damage to the oysters „ Inasmuch as destruction of drills m brackish 

 water depends on a specific low salinity > the salinity of the water in which the drills 

 have lived, and the temperature of the water, these environmental factors should 

 be determined for each region , It can be seen that this method will prove practic- 

 able only where brackish water is available in the near vicinity of drill infested 

 oysters. J. R Nelson (1931) states that the transplantation of drill infested oysters 

 to grounds overlain by relatively fresh water provides a good method of drill control 

 where it can be utilized . Lindsay and McMillm (1950) report that the technic of 

 flooding diked oyster beds with fresh water to kill unhatched drills is in use in one 

 place on the west coast of the United States and is economical and has apparently 

 proved effective against further reproduction. 



In Japan an interesting modification of these methods is suggested by Hori 

 (quoted by Cahn. 1950) in control of Japanese drills . A combination of the following 

 two methods, either of which will work singly but not as efficiently; is recommended: 

 (1) Dissolve table salt at the rate of 6 giams per liter of sea water and immerse drill 

 infested seed oysters in this solution for one to two minutes; after shaking off 'the 

 drills 3 wash the seed in normal sea water; (2) immerse the seed oysters in fresh 

 water for about two minutes and after shaking off the drills wash in normal sea water . 



117 



