DUCK-WEEDS 419 
“ While most forms of aquatic vegetation promote the breeding of mosquitoes, 
the Lemnacea, 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 larve 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 larve wander in from adjacent waters, they would be unable to 
reach the surface for air, and would thus soon become asphyxiated. Larve 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, larve 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 larve would obviously be much more easily deterred by Lemna than 
Culex larvee, 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. 
Recent 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 larve. 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 larve. 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 larve. 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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