320 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 7 



the amount of radiating surface approaches the minimum. Oven 

 tests can only be taken as indicative of the general results that should 

 be obtained in practical work in mills. Definite statements based on 

 a specific laboratory test cannot be relied upon as general and the 

 conclusion must be reached as a whole, after allowing for normal 

 variations. The fact that a moist atmosphere at a high temperature 

 is very effective for the control of a few cereal pests, does not make it 

 preferable to dry heated air for practical use in a commercial way. 

 The excessive amount of radiating required to produce a killing 

 temperature under moist conditions makes economy of equipment a 

 much more important factor. It also means the choosing of a dry, 

 warm day, as the time to heat up the average flour mill. Leaks in the 

 heating system also mean almost sure failure unless they are extremely 

 small. Very few of the species of insects treated were more seriously 

 affected by the moist heated atmosphere than by a dry one, and in 

 those cases where the former was effective at a lower temperature, the 

 penetration or conductivity did not aj^jDear to be as rapid as in tests 

 of the latter kind. A further deterrent to the practical use of a moist 

 heated atmosphere is that the least injurious of the flour mill pests are 

 the only ones advantageously controlled by this treatment. The rice- 

 weevil is an exception, as it succumbs in a moist heated atmosphere at 

 3°C. to 4°C. lower temperature than in a dry heated atmosphere. 



Rice mills could readily be treated by the heat method and the rice 

 weevil thus destroyed. In case of reinfestation, the treatment could 

 be repeated and much injury preA^ented. The cost of treating once 

 every month would not be prohibitive. If the flour, or cereal mill, 

 is so located that it is very moist during the summer, special provi- 

 sion should be made for abundant radiation so that the mill can be 

 readily heated to the required temperature. 



Summarizing our conclusions, oven experiments demonstrate that 

 50°C. to 55°C. kills all stages of cereal insect pests if they are actually 

 subjected to this temperature for one to two hours. Further than 

 this, in practical work, moisture conditions are extremely important 

 and must not be overlooked, as failure to obtain the necessary temper- 

 ature is often due to lack of sufficient radiating surface to overcome 

 the excessive moisture conditions. Because of the more rapid radia- 

 tion obtained, the use of steam at 50 to 60 pounds pressure will give 

 results superior to those gotten with 8 pounds pressure with one fourth 

 more of radiating surface. 



A steam pressure of 100 pounds can be used if the heating system 

 is constructed of new piping. However, considering the danger of a 

 break in the average heating sj'stem with cast radiators, or second- 

 hand pipe coils, 50 to 60 pounds is the maximum steam pressure that 



