28 FARMERS' BULLKTIN n33. 



peratures follow the application of oil sprays, an excessive shedding 

 of foliage usually occurs. Spraying operations that are being car- 

 ried on during the winter should be discontinued as soon as a cold 

 wave is predicted. The sulphur sprays, either alone or in combina- 

 tion with the tobacco solutions, usually will cause no damage to the 

 blossoms if used at any recommended strength. Neither will the 

 tobacco solutions cause any damage to the blossoms even if used 

 at several times the recommended strength. If it is the intention 

 to spray during the winter, it will be found much more convenient 

 to spray after the fruit has been removed. The insecticide usually 

 will do no harm to the fruit, but the spraying operations may cause 

 more or less mechanical injury when the branches are heavily laden 

 with fruit. It is also difficult to do as thorough work when the 

 branches are weighted with fruit. 



So far as the effectiveness of the various insecticides on the dif- 

 ferent stages of the insects and mites is concerned, it is about as 

 great at one time as at another. The white flies and scale insects can 

 be killed both in the summer and during the winter, either in the 

 mature or in the immature stages. The mites also can be killed at 

 any time they may be present. 



The life history and habits of nearly all pests on citrus trees are, 

 fortunately, such that good and satisfactory results can be obtained 

 at any time the spray is applied, but to obtain the maximum benefit 

 with the minimum expense the spraying must be opportune. Much 

 of the spraying done at present brings no returns to the grower, 

 because it is done after the damage has taken place. The writer 

 has seen many groves being sprayed when the purple scale had al- 

 ready done $2 worth of damage per tree. Men also have been seen 

 spraying where the pests were so few that practically no beneficial 

 results would ever follow the treatment. The spraying for rust mites 

 is often done when not more than two or three mites can ~be found on 

 a fruit, or after they have become so numerous that the russeting 

 has already appeared and could not ~be prevented by any ainounl 

 of spraying. 



As a general rule, the time to spray for the control of pests on 

 citrus trees is when they are present in such numbers that if left to 

 reproduce without artificial hindrance they would soon become in- 

 jurious. The pests should not be allowed to increase to .a point 

 where they attract noticeable attention, as in figure 19. In case the 

 various pests of citrus trees are permitted to become so abundant as 

 to cause damage, the profits which might be derived from artificial 

 treatment, such as spraying, are to a certain extent lost. 



Specifically then, what should guide the citrus grower in deciding 

 just when spraying should be done or if it should be done at all ? 

 It must be confessed that it is most difficult to answer such a ques- 



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