202 



PROTP:CTION AC4AINST INSECTS. 



a fine evening in May, especially after a shower of warm rain. 



In mountainous countries, the cockchafer appears somewhat 



later than in the milder plains. 



The generation differs in duration according to latitude and 



climate, as a rule lasting 4 years in Great Britain and Europe 

 north of the river Main. This forms an 

 approximate boundary, south of which 

 the generation is triennial. In East 

 Prussia, swarms of cockchafers have 

 been observed at intervals of 5 years. 

 It follows therefore that the time of 

 development of the cockchafer is not the 

 same everywhere, but depends, within 

 certain limits, on the latitude and longi- 

 tude of a place, and on its corresponding 

 climate. If there is an unusually large 

 swarm in any given year the same thing 

 will occur at stated intervals every few 

 years according to the locality. These 

 critical years are known as swarm-years; 

 in the intermediate years at least a few 

 cockchafers appear, either descended from 

 stragglers, which have since continued to 

 produce regular generations, or from 

 irregular individuals in an existing 

 generation. In Switzerland, there are 

 three districts, in each of which there 

 is a swarm of cockchafers every three 

 years, but in different years for each 

 district. They are termed the Bernese, 



Fig. 75. Three-year- old 

 beech gnawed by chafer- 

 grubs, with loss of roots. 

 (Natural size.) 



Urne and Basel swarm - years, and 

 there is every year a swarm-year in one 

 This fact has been observed for more than 



of these districts. 

 160 years. 



In Chorin, it has been observed for 30 years, that when in 

 one part of the forest there is a swarm-year, there are no 

 chafers in an adjoining part, and vice versa. This is found to 

 be due to the fact, that the larvae in a swarm-year eat all the 

 larvae of the next year in their burrows. The strong larvae 



