CHAP, xiv.] Insect Enemies 321 



insects. Experience of past calamities, collated with the recent 

 investigations made in Bavaria owing to the devastations of the 

 Spruce moth, have shown that the chief fungoid diseases, by 

 means of which nature reasserts the normal balance obtaining 

 before any plague begins, are produced by species of Botrytis 

 (which occasions Muscardine^ B. bassiana^ in silkworms) Micro- 

 coccus, Bacterium, Isaria, Cordiceps, Saccharomyces^ and Torula. 



Preventive Measures. 



As a numerical increase of injurious insects is most of all to 

 be feared in crops of backward development or sickly growth, 

 all prophylactic measures for warding off any predisposition to 

 disease or to interference with the normal transpiratory and 

 assimilative processes must tend to their repression, and obviate 

 any inducement to abnormal reproduction favoured by increase 

 in the quantity of material offered as suitable feeding-grounds 

 and breeding-places. The careful clearing, weeding, thinning, 

 and general tending of all kinds of woodland crops at the 

 different stages of their development are therefore necessary in 

 order that all suppressed, unnecessary, diseased, or damaged 

 individuals may be eliminated before they offer attractions 

 to insects which may ultimately be fatal to the more vigorous 

 and healthy portions of the crop. Such measures of tending 

 will, of course, be least needed wherever, at the time of the 

 formation of the woods, due consideration has been given to 

 the choice of the species of trees best suited for the varying 

 conditions of soil and situation throughout the area to be 

 brought under timber, and to the invaluable advantages to be 

 gained by the formation of mixed woods 1 of broad-leaved species 

 and conifers (wherever possible with any given conditions 

 of soil and situation) in place of pure crops, or of mixed crops 

 of conifers only. The crops should be inspected frequently ; 



1 See lecture On the Advantages of Mixed Woods over Pure Forests, 

 para. 7, p. 135. 



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