10 N. S. ENTOMOLOGICAL SOCIETY 



THE MEANING OF NATURAL CONTROL. 



by 

 John D. Tothill, 



(In Charge of Natural Control Investigations, Entomological 



Branch, Ottawa.) 



Introduction. 



The adult of the forest tent caterpillar (Malacosoma sp.) 

 has an egg mass of approximately 200 eggs. If each egg came 

 to maturity as a moth there would be an annual increase of the 

 insect of 100 fold. No such increase ever takes place, however, 

 in nature and under average conditions only a pair of moths 

 result from each egg mass. In other words 198 of the 200 eggs 

 in each mass usually perish before reaching the moth stage. 

 This astonishing mortality is brought about by natural causes 

 and is the result of what has been called natural control. All 

 living things of the earth are of course subject to this natural 

 control and my own remarks will be confined to pointing out 

 how it operates in the case of one or two common insects. These 

 factors of control may be conveniently grouped into two 

 kinds, first those that restrict an animal or plant to certain 

 geographical ranges, such as oceans, mountain ranges, deserts, 

 climate and so on. On account of these factors the red 

 spruce has a geographical range of boreal north eastern Amer- 

 ica; the Monterey cypress a range of only a few square miles on 

 the California coast; and the white-marked tussock moth is 

 confined to eastern North America. Secondly, there are those 

 factors that effect the increase or decrease of an organism within 

 its natural range and it is this second group of factors to which 

 my remarks will be confined. 



Table 1 shows the chief of these factors so far as lepidop- 

 terous insects are concerned, and may serve to emphasize their 

 variety. 



Climate is, of course, one of the obviously potent factors in 

 control. This is particularly true of unusual weather condi- 

 tions, such as warm days in February; a light frost in June; 

 hailstorms in July; floods in July and so on. 



The food supply is also of first rate importance. Without 

 potatoes we should have few or no potato beetles. The cut- 

 ting out of white pine from our forests is automatically reducing 

 the fauna that feeds upon it. Southern British Columbia does 

 not have outbreaks of forest tent caterpillar because the pop- 

 lar supply is too limited. 



