INSECTS OF THE CITRUS FRUITS 315 



of adults have disappeared. Several other sprayings are 

 usually necessary and are determined by the location. The 

 exact time to spray should be determined by the flies and 

 should be about two weeks after a great number of the 

 adults have disappeared. Usually spraying must be done in 

 late August and early September. This spray is considered 

 the most important of the year. 



The whiteflies are also attacked by certain fungi. These 

 fungi have proved very effective in holding the insects in 

 check. When the s umm er rains begin, which is usually in 

 June or July, an attempt should be made to introduce these 

 fungi in the citrus grove which will operate as natural enemies 

 for. the control of the whitefly. 



Control of Insects. — The control measures for the citrus 

 insects are different from the control measures for most 

 other insects. This is made necessary because the trees 

 have their leaves the entire season, and in the case of the 

 scale insects, the insecticide effective in killing the scale 

 would also destroy the foliage. In Florida where the white- 

 flies are very destructive spraying with miscible oils has 

 proved very effective in controlling these insects. However, 

 owing to the different classes of insects as well as the fact 

 that the trees are in foliage at the time control measures 

 have to be practised, has led to another method which is 

 largely practised in certain citrus regions, namely that of 

 fumigation. 



About 1886 California first seriously considered fumiga- 

 tion as a means of destroying injurious insects. As time 

 passed the methods of fumigation were greatly improved 

 but the fundamental principles remain the same. 



Fumigation is practised by covering the tree with a tent 

 made of heavy duck. Under the tent the fumigating material 

 is placed. Hydrocyanic acid gas is the material commonly 

 used, and is made by depositing sodium or potassium cyanide 

 in an earthen jar and covering it with sulphuric acid. The 

 jars should be at least 2 gallons in capacity to prevent the 

 acid from foaming up and spilling out. The amount of 

 material which is used depends upon the size of the tree and 

 the insect which is doing the damage. There are dosage 



