THE MOSQUITOES OF THE SOUTHEASTERN STATES 21 



Soap emulsions of pyrethrum extract in kerosene oil have been 

 developed by workers in New Jersey (47) and are employed suc- 

 cessfully as mosquito larvicides. The cost is low, and the use of 

 pyrethrum greatly reduces the quantity of oil required, which is a 

 very desirable feature under some conditions. Two formulas for the 

 preparation of the emulsion, adapted from those given by Ginsburg 

 (47), are as follows: 



Against fresh-water larvae. — Two gallons of kerosene containing pyrethrum 

 extract equivalent to 1 pound of pyrthrum flowers per gallon, and 1 gallon of 

 water containing 8 to 10 ounces of liquid 40-percent potash soap. 



Against salt-water or fresh-ivater larvae. — Two gallons of kerosene containing 

 pyrethrum extract as above plus 2 ounces of defoamer, and 1 gallon of water 

 containing 2 ounces of sodium lauryl sulphate. 



With both formulas the emulsifying agent is dissolved in the water 

 and the oil containing pyrethrum extract slowly added with constant 

 mixing (in a container with agitators or by pumping the mixture 

 vigorously back into itself) until a creamy emulsion is obtained. 

 After the foam has settled, 1 part of this stock solution is mixed 

 with 9 parts of water, and the diluted mixture is sprayed onto the 

 breeding places at the rate of about 50 gallons per acre. The stock 

 emulsion can be prepared in large quantities by mixing in the tank 

 of a power sprayer. The pyrethrum extract is usually purchased 

 in a concentrated form, 20 or 40 pounds of the flowers per gallon, and 

 diluted with oil as necessary. In making the stock solution accord- 

 ing to the above formula, use 6.4 fluid ounces (189 cc) of the 20:1 

 extract for each gallon of stock solution or half that quantity of the 

 40 : 1 extract. 



The sodium lauryl sulphate and the defoamer can be obtained on 

 the market, or the defoamer can be prepared by mixing equal parts 

 of fuel oil and wool grease. The prepared stock emulsion can also 

 be purchased. 



NATURAL ENEMIES OF LARVAE 



Various kinds of insects and other animals prey upon mosquito 

 larvae and undoubtedly destroy large numbers. Of the many nat- 

 ural enemies, however, in most cases only the small larva-eating fishes 

 have been found practical for use in control. In the Southern States 

 the most important of these is the top-water minnow (Gambusia 

 patruelis (affirm)), which occurs in both fresh and brackish water. 

 These fish are most effective against subsurface-feeding larvae and in 

 places where the larvae are not protected by aquatic vegetation. They 

 have been used to stock ornamental pools and other isolated bodies of 

 water. They are highly useful in permanent ponds and in the salt 

 marshes. The International Health Board of the Rockefeller Foun- 

 dation (108) has prepared a review of the literature on the use of 

 fish for mosquito control, and Hinman (59, 60) has given numerous 

 references on other predators of mosquitoes. 



Different aquatic plants have been claimed to be of value in elim- 

 inating mosquito breeding. One species of Char a (G. fragilis) ap- 

 pears to exert a deterrent effect, although certain other species of 

 this genus have been found to be innocuous. The bladderworts 

 (Utricularia) capture and destroy small aquatic animals, including 

 mosquito larvae. Duckweed (Lemna) and similar floating plants 

 (Azolla and Wolfia) may form such dense mats on the water sur- 



