INSECTS 217 



of much practical importance, as it could only be applied to 

 cuttings or very young bushes, which, as has just been remarked, 

 should never require treatment : but there were points of interest 

 attaching to it, independent of its practical application to black 

 currants. 



As a result of thirty experiments with immersions for different 

 lengths of time at twelve different temperatures, the conditions 

 which proved fatal to the mites were 



10 seconds at 212 F. 



30 175 p, 

 30 ,,.160 



30 150 

 i minute ,, 130 ,, 



5 "5 ,', 



Subsequent experiments were made (XIV, 69) to ascertain 

 whether such heating, as well as heating at still lower tempera- 

 tures, had killed the eggs as well as the mites, and also, what 

 the effect of the heating had been on the bushes themselves. 

 Infested cuttings were used, with the big buds removed from 

 them in some cases, and not removed in others ; after treatment, 

 they were planted, and their condition noted eighteen months 

 later, by recording the number of big buds then found on them. 

 The results may be summarised as follows 



Mortality 



Big buds per cent. 



Treatment. developed. of cuttings. 



5 min. at 135 . . 100 



5-10 125 . 0-5 * 95 



5-15 115 . 20 55 



10-20 ,, 105 . ' 34 53 



15-30 95 - 30 44 



It is clear from these results that, though a temperature of 

 115 may have killed the mites, it had not killed the eggs, for 

 the infection reappeared after the treatment, and even a tem- 

 perature of 125 did not seem to have killed them entirely, 

 whilst such a temperature had proved almost completely fatal 

 to the plants ; even temperatures down to 95 killed nearly half 

 the plants. Immersion in hot water is, therefore, useless as a 

 remedy. 



1 Three big buds in six sets. 



