McMillin conclude that C11SO4 cannot be recommended for controlling oyster 

 drills in the field where other commercial species have to be considered. 



These studies indicate that Q1SO4 may have some application only as a 

 dipping agent to destroy prehatching drill stages. The relatively small quantities 

 of CuSO^ which would be returned to estuarine waters in this method would very 

 quickly be dissipated below toxic concentrations . On the other hand the high cost 

 of handling may well negate its use in this way. 



Mercuric chloride 



According to Lindsay and his associates (Anon., 1948; Lindsay & McMillin, 

 1950; Lindsay, pers. com.) mercuric chloride (HgC^) is moderately effective in 

 the control of the Japanese oyster drill. These investigators at first utilized the 

 chemical in an attempt to eradicate drills among oysters in water in diked beds . 

 Final concentrations ranging from one part of the poison in 10, 000 to 100, 000 parts 

 of sea water proved effective in destroying drills and the young in egg cases in a 

 period of three to four hours . However some mortalities occurred among the oysters, 

 and tests made by the US. Food and Drug Administration of oyster meats from 

 oysters exposed to the poison 7 months previously demonstrated the presence of 

 small but abnormal amounts of mercury. 



These results led to the application of HgCl 2 as a spray over intertidal 

 grounds at low water. Eight pounds of HgC^ and 8 pounds of a wetting agent (such 

 as Ultrawet) dissolved in 50 gallons of either fresh or sea water was sprayed over 

 infested exposed oyster grounds on hot sunny days at a rate of 50 gallons per acre. 

 This method proved quite effective in destroying unhatched drills, but killed only 

 those adults submerged in small tidal pools. For a time several oyster companies 

 used the method for destroying unhatched drills on the west coast of the United States. 

 Because of a possible accumulation of mercuric chloride in oysters, its extreme 

 danger to careless persons applying it, and its high cost, its use has been largely 

 discontinued. Lindsay believes that on drill infested grounds which become exposed 

 at low tide HgCL properly applied, as to the outside of dike walls and to oyster 

 grounds from which oysters have been harvested, could be quite effective in extermin- 

 ating drills . 



On the basis of these investigations, Korringa (1949) developed a similar 

 method for the control of Crepidula which he says has since been adopted by Dutch 

 oystermen with considerable success. Oyster collectors covered with Crepidula are 

 dredged, brought ashore, washed thoroughly, placed in large concrete tanks, and 

 immersed in sea water containing one part of HgCl, to 15, 000 parts of water for two 

 hours . Young oysters are said to close and to remain unharmed, but Crepidula and 

 early stages of shell disease are destroyed. 



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