226 EXPERIMENT STATION. [Jan. 



about twelve days would probably clear an infested house. 

 Hinds's experiments on Thrips (Bulletin No. 67, Hatch Experi- 

 ment Station, Massachusetts, p. 11, 1900), though less complete, 

 indicate that these insects would probably be also controlled by 

 this treatment. 



An examination of the first two experiments given in this 

 paper shows at once that under the conditions stated serious 

 injury or the destruction of the plants would result long before 

 the above-named insects were killed, and a comparison of the 

 data in Experiments II., V. and VI. indicates that daylight 

 treatment was, at least in part, responsible for this. Fumiga- 

 tion in cloudy weather, as shown in Experiment ITT., sustains 

 this A'iew, the injury being less than with plants treated in 

 sunlight. Experiment TV. gives the results of treatment during 

 rain and snowstorms with longer exposure, showing that even 

 in bad weather daylight treatment is unsafe. 



Comparison of the experiments carried on at night is also 

 suggestive. Those treatments which were made on moonlight 

 nights were always more injurious than those made in starlight, 

 while slightly better results were obtained on cloudy nights. 

 Erom the data at hand it would seem probable that the safest 

 treatment for the plants would be on a cloudy night following 

 a dark day ; and the night experiments with plants which had 

 been kept under the benches for a day or so before treatment, 

 thus giving them partial shade, sustain this view. 



The results of variation in temperature of the house during 

 fumigation were by no means as noticeable as had been antici- 

 pated ; indeed, as a result of these tests it would seem to make 

 little difference whether treatment should be given in a warm 

 or a cool house. ]\rueh the same can be said of humidity, 

 though here it would appear probable that with high humidity 

 — Y5 degrees or over — tliere is more chance of injury than 

 would bo the case where the humidity is rather low. 



It may be stated as a general conclusion that prolonged ex- 

 posures to weak strengths of the gas are more liable to cause 

 injury to the plants than are shorter exposures to greater 

 strengths. As this does not entirely meet Morrill's directions 

 for the control of the white fly, which M'ould come under the 

 head of i)rolonged exposure to a rather weak strength of gas, 



