96 [ DISEASES OF CITRUS TREES 



ment according- to Berlese is to give five sprayings from 

 the middle of June to the middle of September, using the 

 i per cent, aqueous solution of this substance. For the 

 Dactylopius or mealy bug an invernal treatment may be 

 undertaken with a 3 per cent, solution. 



The petroleum emulsion is a well-known remedy 

 for scale insects in general, and is still much used in all 

 Citrus-growing countries. Petroleum will not readily 

 unite with water and therefore is associated with soap 

 to form an emulsion, soap itself being also a powerful 

 insecticide. To prepare the petroleum and soap emulsion 

 dissolve i kilog. of common soap in 10 litres of hot water. 

 Add the boiling liquid to 20 litres of petroleum, and mix 

 thoroughly till the whole acquires the appearance and 

 consistence of cream. Mix one part of this creamy 

 emulsion with 10 parts of cold water and the milky 

 mixture which results is the petroleum emulsion ready 

 for use. In spraying Citrus trees with this mixture, as 

 well as with other mixtures, it is advisable to begin from 

 the top, so as to economize the liquid which trickling 

 from the upper twigs goes to disinfect the lower ones. 



The gassing method is used extensively in North 

 America, South Africa and in many other Citrus-growing 

 countries. It is much more expensive than spraying, but 

 gives more satisfactory results, one good gassing being 

 sufficient to keep the tree comparatively clean for two or 

 three years, as the gas will reach all parts of the tree 

 and is sure to penetrate the dense masses of foliage. The 

 tree is covered by a tent of strong and dense material, 

 the texture of the cloth being made almost airtight 

 either when woven, or by the subsequent application of 

 chemicals, and should be propped up above the tree on 

 stout poles, the margin of the tent being in direct 

 contact with the ground, or better earthed up all 

 round the tree, if possible. The fumigation should be 

 carried out preferably in calm weather, and the capacity 

 of the tent in cubic metres should be carefully calculated. 



