DUCK-WEEDS 419 



" While most forms of aquatic vegetation promote the breeding of mosquitoes, 

 the Lemnacece, or duckweeds, are unfavorable, and in many waters almost or 

 even wholly prevent it. These tiny plants consist merely of a floating frond, 

 resembling a miniature lily pad. It is circular or more frequently lobated and 

 three to six millimeters in diameter. From the under surface hang one or more 

 roots, which never fasten in the soil, but derive their nourishment from the 

 water. Its reproduction, mainly by division of the frond, is so rapid that in a 

 short time (usually before July 1st) it completely mantles quiet waters, notably 

 sheltered ponds and ditches, without perceptible flow. Its extraordinary abun- 

 dance, often covering whole acres of shallow water, makes it an efficient pro- 

 tection from mosquito breeding. Wherever this plant forms a complete covering 

 no larvge have been found. Such places should never be treated with oil, for 

 nature has provided a far more lasting and equally effective protection. It is 

 probably impossible for a mosquito to lay her eggs on lemna-covered water. 

 Even should larvae wander in from adjacent waters, they would be unable to 

 reach the surface for air, and would thus soon become asphyxiated. Larvae of 

 Culex pungens, injected by means of a pipette beneath the lemna in the jar 

 .... died in less than an hour. Where the lemna mantle is not complete, but 

 presents interspaces of open water, larvae of both Culex and Anopheles will 

 usually be found in small numbers only, for lemna waters are apt to harbor the 

 various predaceous water bugs in great numbers." 



To have any effect, the covering of Lemna must be complete and thick. The 

 most abundant breeding-place of Culex salinarius known to us is a large marsh 

 near Chesapeake Beach, Maryland, which was observed by one of us (Dyar) 

 to be well covered with Lemna in June, but at the time no deterrent effect had 

 been produced upon the breeding of the mosquitoes. Culex abominator, a 

 locally abundant species in the lower Mississippi Valley, according to the obser- 

 vations of J. K. Thibault, Jr., and others, breeds by preference in permanent 

 bodies of water thickly overgrown with duckweed. Recently it has been deter- 

 mined that this mosquito actually deposits its eggs upon the aquatic plants. 

 Anopheles larvae would obviously be much more easily deterred by Lemna than 

 Culex larvae, on account of their different habits of feeding. The Lemna, how- 

 ever, usually only acquires a luxuriant growth late in the summer, when the 

 breeding of mosquitoes is largely over, and also it never occurs abundantly in 

 temporary or semi-temporary pools which are the favorite breeding-places of 

 most of our mosquitoes. 



Eecent observations by Charles A. Bentley, at Bombay, India, show that duck- 

 weeds may be successfully employed under certain conditions, at least in the 

 tropics, but that their value has been very much overestimated. We quote his 

 remarks : 



" The presence of water-weed in a tank, well or cistern is often a source of 

 danger, forming as it does an excellent shelter for mosquito larvae. Some water 

 plants, notably duck-weed or Lemna and Azolla have been suggested as being 

 useful in preventing the breeding of mosquitoes. These suggestions do not 

 appear to have been based upon careful observation and experiment. In my 

 experience these plants are of little or no value in preventing the presence of 

 mosquito larvae. But there is a weed which I have met with in Bombay and else- 

 where which under certain circumstances appears to be a useful preventive of 

 mosquito larvse. This plant is the rootless duck-weed or Wolffia arhiza. It takes 

 the form of small bright green round grains without stem, roots or leaves. These 

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