INFLUENCE OF CULTIVATION, PRUNING, AND CLIMATE. 



With the orange and lemon as with other plants, negligent cultiva- 

 tion and improper care, or any unfavorable conditions of climate 

 which weaken the vitalitj" and vigor of the tree, encourage the pres- 

 ence and multiplication of the insect enemies. On the other hand, 

 vigor of growth is repellent to insect attack; and it will be almost 

 invariably found that the unhealth}^ tree is the one first severely 

 infested with scale insects or mites. This does not mean that vigor- 

 ous healthy trees will not be attacked, but such trees are less apt to 

 be completely invaded. As a means of protection against scale insects, 

 a proper system of cultivation and pruning is therefore highly 

 important. 



The value of pruning as a means of preventing scale-insect injury 

 can not be too strongly urged. Scale insects thrive best where they 

 are protected from direct sunlight and free movement of the air, hence 

 trees of dense growth, unpruned, are almost certain to have their cen- 

 ters, at least, scaly. A well-pruned tree, in w^hich free access is given 

 to light and air, is much less apt to be badl}^ attacked than a thick- 

 headed tree, the interior of which is entirely shaded, thus furnishing 

 the conditions most favorable for the well-being of scale insects. 



The abundance or scarcity of scale-insect pests is ver}' much influ- 

 enced by climatic conditions. A moderate amount of moisture and 

 warmth are the favoring conditions. On the other hand, a very dry 

 climate accompanied during the summer season b}^ excessive heat, will 

 frequently destroy most scale pests, as will also a high degree of 

 humidity with high temperature such as characterizes many areas 

 within the Tropics, the latter condition developing fungous diseases 

 which often keep most scale species well nigh exterminated. The 

 favoring intermediate climate is illustrated by the citrus districts of 

 Florida, Jamaica, and the West Indies, where scale enemies are more 

 troublesome than they are in the drier climate of California. On the 

 Pacific coast, also, the moister ocean districts are worse infested than 

 the drier regions farther inland with greater elevation. Under the 

 latter conditions the black scale, for example, has been almost entirely 

 exterminated by a temperature holding for several da3^s above a 

 hundred degrees, and similar results have been noted with other 

 species. 



PEEIODICITY IN SCALE INSECTS. 



With most insects injui-ious to cultivated plants a periodicity is 

 noted in their occurrence in injurious numbers. In the case of sub- 

 tropical species, like the scale insects affecting citrus plants, this 

 periodicity is not so marked as it is with insects in temperate latitudes. 

 That there may be more or less well-defined periods of destructive 

 abundance separated by periods of comparative scarcity is illustrated 



172 



