1922] Fishes in Relation to Mosquito Control 163 



may be determined by the inspection and eomi)arison of quiet waters 

 which support top minnows with those in the same locality which do 

 not support top minnows, in order to determine which waters produce 

 the most mosquitoes. Ponds heavily infested with mosquito larvae 

 and pupae may be stocked with Gamhusia and results awaited and 

 observed. One of two similar, neighboring bodies of water both 

 infested with immature mosquito, may be stocked with Gamhusia. 

 The water which is not stocked with top minnows will serve as 

 a ''control" which will furnish a fair comparison. Occasionally 

 it is possible to demonstrate the destruction of mosquito larvae 

 which Gamhusia accomplishes by connecting by means of a ditch one 

 pond or pool heavily infested with mosquito larvffi with another sup- 

 porting top minncws. All of the foregoing experiments and obser- 

 vations have been made within recent years and in every instance 

 either a great reduction in mosquito production resulted or, as in 

 man,y cases, complete control followed. Wherever mosquito produc- 

 tion was not eliminated, if sufficient minnows were used, complete con- 

 trol was prevented by the presence of places of protection for the 

 immature mosquito against fish. 



Places of refuge for mosquito larvae and pupae in the presence 

 of fish are furnished by such types of plants and floatage which are 

 slightl}' or partly submerged, causing a mere film of water to stand 

 above them. Anopheles larvae are particularly keen in seeking these 

 places of refuge where they cannot be reached by the minnows. 

 The low aquatic grasses, particularly Hijdrochloa carolinensis, are 

 a source cf much difficulty in securing mosquito control by the use of 

 fish. Mats of floating algffi with a film of water over them also form 

 excellent hiding places for the immature mosquito. ]\Iany other 

 plants, some of them strictly aquatic, others of the shore variety, often 

 cause imperfect control when they are partly or slightly submerged. 

 Plants which are wholly submerged do not appear to hinder mosquito 

 control by the use of fish. Certain plants, such as the floating prim- 

 rose willows and the tall wire grasses, sometimes develop dense masses 

 of roots near the surface of the water wliich provide a limited amount 

 of protection. The tall plants with straight stalks and with few or no 

 leaves, and without dense roots near the surface, like the cat-tails, the 

 rushes, etc., provide little or no protection for mosciuito larvae and 

 pupte. The duck weeds not only furnish no refuge but whenever 



