94 ANNUAL KEI»ORTS OF DEPARTMENT OF AGBI< [JLTUBE, 194<> 
Planl Quarantine. Studies were conducted of the effect on wildlife 
and midlife food of the standard methods of malaria mosquito con- 
trol, including fluctuation <»f water Levels and the use of paris green 
and oils. Asa result of this cooperat ive M udv a better understanding 
was obtained of many of the problems of mutual interest to malaria 
control workers and those concerned with li>h and <rame conservation. 
Certain adjustments of the mosquito-control procedure were made 
and data were secured which will be useful in similar situations in 
other parts of the country. This cooperative study is being con- 
t inued. 
Investigations of salt-marsh mosquitoes were intensified along the 
east coast of Florida. Their purpose is the development of methods 
of control of salt-marsh mosquitoes which are more effective and less 
expensive, and at the same time will not adversely affect wildlife on 
the marshes. As a basis for this work, studies were begun to deter- 
mine the little-known biologies of the species and their ecological rela- 
tions. The more important preliminary results indicate that marshes 
where the grass Distichlis predominated produced the greatest num- 
ber of mosquitoes, about 4C> million larvae per acre: and where the 
plant Juncw occurs, about 30 million per acre. Observations showed 
that the greater the proportion of these plants in the marsh flora, the 
greater was the mosquito population. 
Comprehensive studies on the development of more effective mos- 
quito larvicides were begun at the Orlando, Fla.. laboratory. During 
the year 76 chemicals were tested for their relative toxicity to mos- 
quito larvae; '20 of these were toxic in concentrations of 20 parts per 
million or less and 6 in concentrations of from *2 to 5 part- per 
million. 
The more important results of studies on the biologies of mosqui- 
toes in the Pacific Northwest showed that e££S of Aedes vexans 
(Mei<r.) and .1. hit oralis (Mcig.) would remain viable in the soil for 
at least 6 years. A study of the resistance of the various instars of 
mosquitoes to pyrethrum-oil emulsion showed that differences exist 
in mortality between instars of the same species and between those 
of different species. Larvae in the first and second stages were more 
quickly killed, and with less emulsion, than were those of the third 
and fourth stages. Calex and Theobaldia larvae showed greater 
resistance than did Anhs larvae. 
CLEAB LAKi: GNATS 
As a result of investigations of the Clear Lake gnat in California an 
electric-light suction trap was developed that captures Large quantities 
of gnats under certain weather conditions. Two such traps at the 
laboratory, 75 yards apart, took 1,750 pounds of gnats during the 
summer of L039. Another trap, operated at Lucerne, took 800 pounds 
in a 4B day period. The daily catches ranged from to v ^ j pounds. 
The latter catch was estimated to contain about 88 million gnats. 
From 50 to 90 percent of the larger catches were of females heavilv 
laden with eggs. 
Over 700 tests of larvicides, in w hich many different materials were 
Used) were conducted on the gnat larvae. In the laboratory the lar- 
vae are killed by high dilutions of certain Insecticides. Pyrethrum. 
phenot biazine, carbon tetrachloride, trichlorethylene. and certain mix- 
