332 



[April, 1012. 



PLANT SANITATION. 



FORMALIN AND HOUSE FLIES. 



By E. Ernest Green, 

 Government Entomologist. 



The use of Formaldehyde (or Formalin), 

 as a deterrent against House Flies, has 

 been recommended — from time to time — 

 for come years. The simple exposure of 

 dishes containing dilute formalin was 

 said to drive away every house fly from 

 the premises, and the success of this treat- 

 ment has been vouched for by various 

 persons. I must confess, however, that 

 my own experiments with dilute formalin 

 have been uniformly disappointing. 

 Whether our Ceylon house fly is a more 

 hardy race, or whether our warmer 

 climate has a weakening effect upon the 

 acton of formalin, the fact remains that 

 I have never been able to record the 

 slightest success with this simple treat- 

 ment. 



But if the house fly can be induced to 

 swallow even a weak dose of formalin, it 

 is certainly fatal to that insect. It does 

 not find plain formalin and water suffi- 

 ciently attractive : but formalin mixed 

 with sugar or milk, in judicious propor- 

 tion, is readily imbibed by flies— with 

 fatal results. One method is to fill a soup 

 plate with damp sand, place a disc of 

 blotting paper on the sand, spread the 

 paper with sugar, and sprinkle the sugar 

 with dilute formalin— in the proportion 

 of one part to twenty of water. As 

 commercial formalin is of the strength of 

 only 40 %, this dilution represents a mix- 

 ture of about 2 %. 



The Journal of Economic Entomology 

 for October, 1911, (Vol. 4, No. 5) publishes 

 an article on "Formalin for Poisoning 

 House Flies," by Ii. I. Smith, of the North 

 Carolina Experiment Station. Mr. Smith 

 set himself the task of freeing a college 

 dairy from a plague of flies. Of the 

 condition of affairs before the treatment 

 he remarks :— " In the milk room the flies 

 covered the walls aud ceiling, and the 

 straining cloth at milking time was actu- 

 ally black with flies." He continues :— 

 "My first experiment proved successful. 



This was the addition of one ounce of 40 

 per cent, formalin to sixteen ounces 

 of fresh milk. This mixture was placed 

 in four shallow tin plates and set on the 

 floor of the milk room about 3 o'clock one 

 afternoon. The flies commenced to feed 

 and die within a few minutes, and conti- 

 nued to die rapidly even while the even- 

 ing's milk was being brought in and 

 strained. These plates of poison were 

 left over night, and the milkers advised 

 me that the flies were feeding greedily 

 the next morning soon after daylight. 

 The dead flies, swept up about 8 a.m., 

 measured about one pint, representing 

 fully 5,000 flies." 



"This experiment was repeated for 

 three successive days, and about one pint 

 of dead flies were swept up every morn- 

 ing. In addition to the flies actually 

 secured, many dropped dead outside the 

 windows." 



" My next experiment was to use a 

 mixture of half milk and half water 

 instead of whole milk. Formalin was 

 added in the same proportion, one ounce 

 to sixteen ounces of diluted milk. This 

 proved to attract the flies as well as 

 the whole milk." 



"Several variations in the proportion 

 of formalin and milk were tested, but 

 my conclusion is that the use of one 

 ounce to sixteen is most effective. The 

 following method of stating the formula 

 has been used for newspaper articles, in 

 order that every housekeeper can pre- 

 pare it easily :— One ounce (two table- 

 spoonfuls) of 40 per cent, formalin ; six- 

 teen ounces (one pint) of equal parts milk 

 and water. This mixture should be ex- 

 posed in shallow plates and by putting a , 

 piece of bread in the middle of each plate, 

 it furnishes more space for the flies to 

 alight and feed, and in that way serves 

 to attract a greater number of them. 

 Whole milk can be used, but the diluted 

 milk seems to be just as successful." 



" A very conclusive test of the effici- 

 ency of the above formalin mixture was 

 made in a large calf barn where flies were 

 extremely numerous, Six plates of the 



