402 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 15 



one ftunigation are more resistant to a second fumigation than individuals 

 which have not been previously fumigated. Also, that the greatest 

 resistance is shown by scales on trees that have been fumigated regular- 

 ly, once, or even twice, a year. More satisfactory fumigation results 

 have often been secured on trees that have not been fumigated for two 

 or three years than on trees that have been regularly fumigated where 

 a certain number of the scale escaped being killed. 



The variable factors of time and place affecting fumigation results 

 have to do chiefly with meteorological conditions. There is also a 

 variation in the condition of the tree itself, but this may be independent 

 of the time or locality. Differences in the amount of foliage on the tree 

 may influence fumigation results. Foliage absorbs hydrocyanic acid, 

 and the more foliage there is present the more HCN will be absorbed and 

 the less, apparently, will be left in the atmosphere to kill the insects on 

 the exterior of the tree. 



A series of tests were made to determine the relation of the foliage to 

 the dosage. An ordinary orange tree with a medium amount of foliage 

 was covered with a tent, and alongside this tree a wooden framework 

 was constructed to support a tent, which enclosed a space identical in 

 form and size with the orange tree. Lemons infested with red scale 

 were placed in the same relative position under each tent. The tents 

 were frequently interchanged to overcome tent variation. The dosage 

 was varied, but was always the same under both tents in any given test. 

 These tests showed that the proportion of scales killed was approximate- 

 ly byi per cent greater under the tent that contained no tree or foliage. 



It has been noted in the field that on the heavy foliage-Lisbon type of 

 lemon tree, scales are more likely to survive a fumigation than scales on 

 trees with sparse foliage. Scales are much more difficult to kill on the 

 fruit than on the twigs or leaves. Likewise, they are more difficult 

 to kill on vigorous and thrifty leaves, and on vigorous shoots such as 

 suckers, than on less thrifty leaves and twigs. This difference in resist- 

 ance on different parts of the tree, or between two trees of dift'erent 

 vigor, seems to be related to the food supply of the scale. If a particu- 

 lar locality had a monoply on the vigorous trees, the resistance of the 

 insects might be explained on this basis, but the red scale is more difficult 

 to kill on unhealthy trees in the areas where it shows exceptional re- 

 sistance, than on healthy trees in other localities. The tree may con- 

 stitute one of the variable factors in fumigation results, but that scale 

 resistance is not entirely determined by the tree, is shown by the tests 

 given. Here the resistance persisted after the scale-infested fruit was 

 removed from the tree and fumigated in a different locality. 



