164 Journal of the Mitchell Society [March 



a pond is largely covered with them they appear to prevent mosquito 

 production. The leaves of these plants are wholly above the surface 

 of the water, they do not have a dense root system, and furthermore 

 it is possible that the more or less constant shifting of the weed masses 

 with the winds or currents may drown the eggs. 



Since there are many conditions under which plants furnish pro- 

 tection for the immature mosquito against fish, it is very essential in 

 the conduct of anti-malaria campaigns to make frequent and careful 

 inspections of all waters in the area under control. If it is found that 

 certain plants are preventing mosquito control, then it is time to aid 

 the minnows. The indiscriminate cutting and clearing of vegetation 

 from water deposits certainly is not an economical practice, as such 

 work frequently is not necessary. The biological condition existing in 

 each pond must determine whether or not top minnows will require 

 assistance from man to furnish control of mosquito production. The 

 abundance of top minnows must be carefully considered, and certainly 

 the amount of food available for them in each body of water is of 

 great importance. The writer has shown by experiment that when 

 food is scarce better mosquito control will result than when it is plen- 

 tiful even though much vegetatioji of the "protective tj^pe" is present, 

 and he knows of no other way to explain why mosquito production in 

 certain ponds is much more nearly eliminated than in others when 

 minnows and plants appear to be identical and present in equal abun- 

 dance. 



It is evident then that top minnows are of much value in control- 

 ling mosquito production in permanent standing bodies of water and 

 that they also render excellent service in sluggish ditches. Their value 

 as eradicators of mosquito larvae has been recognized by health officers, 

 and minnows are used very widely in the South where anti-malaria 

 campaigns are conducted, having in a large measure replaced the use 

 of oil. A definite use for oil in anti-malaria work remains; for places 

 exist in nearl}' every locality where mosquitoes breed and where fish 

 cannot be used, as, for example, in a film of seepage water, iix lioof 

 prints, in temporary pools, etc. It is well known that in order to 

 secure mosquito control by the use of oil a continuous film must cover 

 the entire surface of the water and that during hot weather a new 

 coat of oil must be applied about once a week. It is quite impossible 

 to obtain a continuous film on ponds and lakes, because the oil is 



