BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE 97 
were determined at intervals in all spray plots containing them to cor- 
relate the amount of residue with insecticidal potency. Distribution 
studies such as were formerly made with lead arsenate were made for 
nicotine-bentonite and phenothiazine, and much the same results ob- 
tained for the variances due to trees, parts of trees, and individual 
apples. An extensive study was made of residues from the use of 
calcium arsenate and of paris green on tomatoes. Immediately after 
the last application of undiluted calcium arsenate the tomatoes showed 
residues exceeding the tolerance, but these were easily removed by 
washing or wiping. 
Articles were published on the removal of residues of phenothiazine 
and of nicotine-bentonite from apples. A very important digest and 
bibliography of the literature concerning insecticidal spray residues 
was issued during the year. 
CHEMICAL INVESTIGATIONS TO DEVELOP INORGANIC INSECTICIDES 
Articles were published concerning a method of making calcium 
arsenate of low solubility ; a review of the uses of fluorine compounds 
as insecticides was issued; and a bibliography of all information on 
magnesium arsenate was published. 
A study was begun of the use of sodium arsenite and white arsenic 
in the control of the Mormon cricket. Present practice calls for dust- 
ing the crickets with a mixture of sodium arsenite and diatomaceous 
earth, but there are some objections to this mixture. By a combina- 
tion of laboratory and field work it was determined that the white, 
pure form of sodium arsenite now being used could be replaced by 
another commercially available gray, impure form with considerable 
saving in cost ; that the poison as now produced is too coarse and the 
diluent too fine for the best results ; and that white arsenic, at least in 
the forms now available, is not toxic enough to the crickets to be sub- 
stituted for the sodium arsenite. The work will be continued to find 
out whether the sodium arsenite is being used at the optimum compo- 
sition and concentration. 
CHEMICAL INVESTIGATIONS OF FUMIGANTS FOR CONTROL OF INSECT PESTS 
The work at Whittier, Calif., continued to deal with controlled 
dosage fumigation of lemons infested with the California red scale, 
in continuation of the effort to develop a set of experimental condi- 
tions so exactly reproducible that the effects due to differences in sus- 
ceptibilities of insects, protective stupefaction, etc., can be more clearly 
demonstrated and measured. In addition, a study was made of the 
possibility of using methyl bromide against this scale. It was found 
that there was little difference in the susceptibility of the early gray 
adult insects from resistant and nonresistant strains, when compared 
either by means of various concentrations for a fixed exposure or at 
various exposures for a fixed concentration. The so-called resistant 
strain (i. e., to hydrocyanic acid) shows more resistance, however, 
when the oldest adults are tested. It seems also as if low tem- 
perature reduces the toxic effect of methyl bromide, which is con- 
trary to the results with hydrocyanic acid. All the tests seem to 
indicate that methyl bromide is not very promising for use against 
188590 — 40 7 
