300 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 11 
tory explanation of the principles underlying its use. The public 
gradually drifted into the impression that the oil killed the larvae by 
suffocation brought about by the formation of an impenetrable layer 
of oil on the surface through which the larvae were unable to thrust 
their breathing tubes to get oxygen. With this idea in view most of 
the recommendations were based on the purely mechanical problem of 
getting an oil that would spread easily and be fairly permanent. It 
has been the practice of the entomological division at the University 
of California to recommend a half and half mixture of crude oil and 
kerosene which would form an oil having a Baum6 reading of 28° to 
32° and which was found to answer the mechanical requirements men¬ 
tioned above. It was our idea to check this recommendation with 
laboratory methods to determine if possible if any other oils would be 
better suited for the purpose. 
In order to determine an ideal oil it was necessary to know first how 
the larvae were actually killed and this paper takes up that phase of 
the problem. Various suggestions have been advanced, and in order 
to satisfy ourselves and reach a conclusion of our own we attempted 
to check up these different theories. 
In all our experiments the mosquitoes used were the full grown 
larvae of Culiseta incidens, a common culicine mosquito of California 
that is widespread throughout the state and to be found breeding in 
situations of varied character. 
The oils used were a series of standard commercial oils of California 
origin. Their trade names and Baume readings were as follows: 
Crude, 15.4°; low grade stove distillate, 29°; high grade stove distil¬ 
late, 33.3°; commercial engine oil distillate, 38.5°; kerosene, 39.7°; 
gasoline, 55.5°; and a by-product with a Baume reading of 20.3°, 
called “ still bottoms,” a residuum obtained from the stills after dis¬ 
tillation. Besides these so-called toxic petroleum oils we used a 
standard brand of the non-toxic liquid petrolatum of 27° B. 
The several theories, including our own, which have been advanced 
in explanation of the lethal action of the petroleum oils upon the 
mosquito larvae may be summarized as follows: 
(1) The physical properties of the oil layer are such that the surface 
tension is “ annulled” to the extent that the larvae are unable to hold 
themselves to the surface for the necessary breathing period. 
(2) The layer of oil acts as a definite mechanical barrier between 
the larvae and the outside air and thus leads to their ultimate suffoca¬ 
tion. 
(3) A portion of the oil or some of its dissolved or suspended material 
goes into solution in the water and poisons the larvae. 
