PLANTS AND MOSQUITO CONTROL MATHESON 417 



that the entire surface is covered, causin<^ the larvae to die from 

 Buffocation, or the plants in some way inhibit the female mosquitoes 

 from laying their eggs in such situations. Smith (11)10) investi- 

 gated the reported effect of a species of Azolla growing on the water 

 in the canals of Holland. Here he found it in certain regions, the 

 peat and turf areas, growing in such profusion as to cover the 

 canals completely and prevent mosquito breeding. This plant seems 

 to have restricted breeding areas, and though introduced into Amer- 

 ica its growth was not very successful. Conflicting reports as to its 

 value have since been published, but as far as I know no experi- 

 mental work has been undertaken to test its reported value. Mac- 

 Gregor (1920) thinks that Azallo fliculoides is a deterrent to ano- 

 pheline breeding. He found no larvae or egg deposition in experi- 

 mental tanks covered by this plant, whereas oviposition and breed- 

 ing took place in near-by tanks containing other kinds of aquatic 

 plants. Eugling (1921) states that the planting of Azolla in Alba- 

 nia seems to favor the breeding of anophelines. Miihlens (1924, 

 1925) reports that in parts of Argentina he found no mosquito 

 larvae in pools and lagoons covered with Azolla sp. and Azolla fllcu- 

 lokles^ whereas in near-by pools where these plants were absent 

 mosquitoes bred in abundance. 



Various species of the Letrvnaceae (duckweeds) have been reported 

 as effective water coverings (pi. 3). In New Jersey, Johnson (1902) 

 found that no mosquito breeding took place wdiere the Lemna 

 formed a complete mantle but where open spaces occurred Gulex and 

 Anopheles bred in small numbers. Furthermore, Ze^^na-covered 

 ponds harbor numerous predacious insects which attack mosquito 

 larvae. Howard, Dyar, and Knab (1913) state that one of the most 

 abundant breeding grounds of Culex saUnarius was a large marsh 

 completely covered by Lemna. Bentley (1910) found that species 

 of Lemna were of no value but that a related plant, Wollfia arhiza, 

 was quite effective in the tanks in and around Bombay. In Sar- 

 dinia, Fermi (1917) recommends the planting of Lemna 'palustris 

 where oiling can not be employed. In Corsica, Kegnault (1919) 

 reports that L.emna was successfully grown and prevented mosquito 

 breeding. Whenever the Lemna disappeared breeding took place. 

 In Russia, Vasilev (1925) found that wherever Lemna minor and 

 Lemna polyrrhiza covered the surface no mosquito larvae could be 

 found. Other workers present very conflicting reports, but the 

 evidence of general observations would seem to indicate that the 

 various species of the Lemnacae may prove of value in reducing the 

 abundance of mosquitoes. Plate 3 shows an enlarged (magnified 

 about seven times) view of Wollfia punctata and Lemna minor cov- 



