1920 ENTOMOLOGICAL SOCIETY. 31 



INSECT OUTBREAKS AND THEIR CAUSES. 

 John D. Tothill, Fkedericton, N.B. 



The Standard Dictionary defines an outbreak as "a sudden and violent 

 breaking forth as of something that has been pent up or restrained." This defi- 

 nition seems peculiarly apt for describing the biological meaning of the word 

 because it implies that all nature is in a condition of restraint and that an out- 

 break is something abnormal due to the breaking of one or more restraining bonds. 



Outbreaks are not confined to species of the insect world and neither are they 

 confined to the animal kingdom. In the vegetable kingdom for instance, there are the 

 familiar cases of the Russian thistle in Western Canada and the California Prickly 

 Pear in Australia. There is also in our own country the case of the Northern 

 Scrub Pine that so often comes up in pure stands after a fire has swept away 

 the original soft wood forest. In the animal kingdom we have among insects 

 such familar cases as the European Gipsy Moth in the New England States, the 

 Forest Tent Caterpillars that greased the tracks and stopped some trains in Canada 

 in 1914; the Army-Worms that at times have spoiled the Western wheat crop; 

 and periodical outbreaks of short-horned grasshoppers. Examples of outbreaks 

 of various species of vertebrates are also quite plentiful; there is the historical 

 case of the European Cotton-tail Rabbit in Australia and there is the present case 

 of the little prairie dog in Alberta. Even man himself has been known to be in 

 a condition of biological outbreak; Caucasian Man in the 17th and 18th centuries 

 doubled his population every twenty-five years on the North American Continent. 

 So that outbreaks of general occurrence may be met with almost anywhere in 

 the realm of living things. 



To what causes are these outbreaks due? 



As each species is held in equilibrium by the pressures of its environment it 

 is obvious that an outbreak is due to a relaxing of one or more of these pressures. 



Let us examine the cases of a few insect outbreaks the causes of which have 

 been studied. 



During the first twenty-five years of the Oyster Shell Scale's regime on this 

 continent it increased so abundantly that men like Fitch held fears for the develop- 

 ment of an apple industry. With the passage of the time, however, the menace 

 of this scale insect has subsided. In the light of studies made on the present 

 environmental conditions of this insect in Canada it seems probable that the early 

 outbreak was due to an absence of its most effective enemy, a predaeeous mite. 



Turning to the Gipsy Moth I think we arc more or less agreed that the 

 New England outbreak was due more especially to an absence of natural enemies, 

 such as the handsome Calosoma of Europe and the eflieient little two-winged fly 

 Compsilura ; and also perhaps to a partial release of the food pressure. 



In some of our Maritime Province cities there was last year an outbreak of 

 the White-Marked Tussock. Mr. Dustan, who was detailed to make a study of 

 these outbreaks, found that they were due largely to an abundant food supply; 

 to an absence in cities of chickadees and the larger species of woodland ants; and 

 to a relative scarcity of parasitic insects. 



There is, at the present time, an outbreak of the Forest Tent insect in Alberta. 

 Studies by Mr. Baird and myself have shown that the outbreak is due at least 

 partially to an almost total absence of its usual insect parasites. It is also in- 

 fluenced perhaps by a relaxing of the food pressure or, in other words, to an 

 increased proportion of trembling poplar. 



