CHEMOTEOPIC TESTS WITH THE SCREW-WOEM FLY 6 
quarter-inch diamond mesh. This netting was tied over the opening 
of cylindrical glass dishes containing food samples. Oil of star anise 
was effective in preventing the food from being blown for 24 hours; 
samples protected with eucalyptus oil, formic acid, and sometimes 
clove oil remained untouched for 12 hours, while samples protected 
by oil of almonds, oil of citronella, oil of cinnamon, boracic acid, picric 
acid, or nitrobenzene were blown within 6 hours. 
MATERIALS NOW USED AS SCREW-WORM FLY REPELLENTS 
Pine tar, tannic acid, turpentine, kerosene, gasoline, various sheep 
and cattle dips, hydrated lime, calomel, and other materials have been 
used in the past with more or less success. 5 Many home remedies, 
such as axle grease and lampblack are used by ranchmen, but prob- 
ably proprietary " screw-worm killers" of one sort or another are now 
most prevalently used. These consist largely of crude carbolic acid, 
which, though efficacious in killing all fly larvae with which it comes into 
contact, is also very poisonous to animals. As a result, many animals 
are killed by the treatment. In addition, many ranchmen hesitate 
to use these carbolic preparations upon their fine stock and confine 
themselves to the use of chloroform or other larvicides. Although 
chloroform is in extensive use for killing fly larvae in wounds, it has 
no repellent value and does not prevent reinfestation. 
The cost of treating an animal for screw-worm infestation has been 
estimated by several ranchmen to be from 25 to 50 cents for each 
treatment. A conservative estimate would be 25 cents for each 
treatment, or $25 per 100 infested animals per day when treatments 
are required from once to twice daily. Repeated treatments by 
improper methods and successive worm infestations occasionally 
necessitate the treatment of the cases for months. 
PURPOSE OF CHEMOTROPIC TESTS 
The purpose of this study has been to find a material that will 
prevent reinfestation for 48 hours or longer. This would relieve the 
situation to a great extent, as it is not uncommon to find as many as 
400 to 500 cases of worms on a single ranch in seasons favorable for 
the screw worm, and a considerable number of cases on most ranches 
every season. 
The problem involves two objects to be accomplished through the 
treatment of wounds: (1) The destruction of the larvae if present, 
and (2) the protection of the wound from infestation for a reasonable 
length of time. It is obvious that any treatment which will injure the 
tissues so as to delay healing or which will act as a local or systemic 
poison, will defeat the ends in view. 
The experience of the writers indicates that there is considerable 
difficulty in successfully combining a larvicide and a repellent to be 
used as a single treatment, as the killing properties of the larvicides 
are too much reduced by the admixture of the repellent material. 
Hence it is logical to attempt to develop a strong and lasting repellent 
without larvicidal action. 
In this bulletin, therefore, the data given deal essentially with the 
question of the chemotropic responses of the screw-worm fly to various 
» Bishopp, F. C, Mitchell, J. D., and Parman, D. C. screw-worms and other maggots affect- 
ing animals. IT. S. Dept. Agr. Farmers' Bui. 857, 19 p., illus. 1919. (Revised, 1922.) 
