BUREAU OF EXTOMOLOGY AXD PLANT QUARANTINE 59 
to approximately 16 percent in August. When the type of fat was 
correlated with toxicity data, it was found that the percent of linoleic 
acid apparently had a greater inverse relationship to percent kill than 
the total fat or any of the other components. 
Systemic Chemicals Tested Against Cotton Aphid and Spider Mites 
Of 100 chemicals tested, 20 were found to possess systemic in- 
secticidal properties against the cotton aphid and spider mites. Eight 
of these were more effective than OMPA. The most effective systemic 
insecticide tested was Systox. This was about eight times more effec- 
tive against aphids and spider mites than OMPA when applied as a 
seed treatment. Systox also killed these pests both by contact and 
as a fumigant. Systox killed cotton leafworm larvae and thrips by 
contact action, but it did not show any systemic properties against 
these insects. It was not effective against the boll weevil, bollworm, 
yellow-striped armyworm, or the salt-marsh caterpillar. 
INSECTS AFFECTING MAN 
Development of Insecticide Resistance Complicates 
Mosquito Control 
In cooperative investigations with the Bureau of Vector Control, 
California Department of Health, it was found that the difficulty 
in controlling mosquitoes in certain irrigated regions in that State 
was explained bv the appearance of strains of mosquitoes highly 
resistant to DDT. 
Laboratory evaluation of new mosquito larvicides shows that EPN, 
an organic phosphorus compound, is one of the most effective mate- 
rials available. It is equal to or superior to DDT against the common 
species occurring in the Xorthwest. However, the high toxicity of 
the compound to warm blooded animals may limit its usefulness in 
mosquito control. New effective larvicides are now urgently needed 
in the Western States, particularly in California. Culex tarsalis, 
Aedes mgromaculh and A. dorsalis have, during the past 5 years, 
developed tolerances to DDT as much as 10 times that of strains of 
normal susceptibility. Moreover, where toxaphene and aldrin have 
been substituted for DDT, C. tarsalis larvae have within 2 years de- 
veloped a marked resistance to these materials. The resistance prob- 
lem is also acute in Florida and in other places in the United States. 
The problem parallels the difficulties now being experienced in con- 
nection with the control of house flies and body lice. 
Important progress was made in developing practical ways to con- 
trol adult mosquitoes in the Xorthwest. Due to the presence of many 
mosquito breeding areas in mountainous regions, often located in al- 
most inaccessible terrain, it has been impractical to attempt control 
by larviciding. Moreover, the mosquitoes migrate long di>tanees, 
so that destruction of larvae in limited areas may not accomplish satis- 
factory control. During the past year investigations demonstrated 
that the application of 1 or 2 pounds of DDT in residual sprays to all 
vegetation to a height of about 10 feet in the vicinity of homes and 
resort areas provided almost complete relief from mosquito annoyance 
