EATE OF INCREASE OF GIPSY MOTH. 



Ill 



in successive years, the unhatched or latest clusters being easily distinguished from 

 the hatched or "old" clusters, and the age of these latter, whether one, two, three, 

 or more years, being indicated by their state of preservation. The ratio of the aver- 

 age annual increase of 10 such colonies was found to be 6.42; that is, six or seven 

 egg clusters on an average may be found in the second season to one of the first 

 season. 



If the number of eggs deposited by the average female moth be 

 set at 500, and if the sexes of her progeny are equally divided, a 

 potential increase of 250-fold for each annual generation is provided 

 for. Under complete control only one pair of moths would average 

 to be produced from each mass of eggs deposited, and since each egg 

 represents an individual embryo, all but 2 of each 500 must fail to 

 reach full maturity. Reduced to percentage this is equivalent to 

 the survival of 0.4 per cent and the destruction of 99.6 per cent of 

 the gipsy moths in one stage or another every year. Since the total 

 number of gipsy moths in any locality can not possibly be computed, 

 the only method by which mortality through any cause may be 

 expressed is on this basis. 



It will surprise many who have not given the matter considera- 

 tion to learn what an extraordinary apparent mortality it requires to 

 offset a potential increase of 250-fold. The gipsy-moth caterpillars 

 molt five or six times after they hatch and before they change to pupae, 

 making the number of caterpillar stages six or seven. If through 

 natural controlling agencies 50 per cent of the young caterpillars 

 were destroyed in the first stage before they had molted, and this 

 was followed by similar destruction of another 50 per cent in the 

 second stage, and so on through the third, fourth, fifth, sixth, and 

 seventh stages, respectively, and in addition 25 per cent of the pupae 

 and 25 per cent of the adults before depositing their eggs were simi- 

 larly destroyed, it would still permit of a slight annual increase. 



The following table (if the incongruity of fractions as applied to 

 insects may be overlooked) indicates the number of survivors of each 

 stage resulting from the hatching of a mass of 500 eggs : 





Stage. 



Number. 



Loss. 



Number 

 remain- 

 ing. 



Potential increase. 



Eggs 





500 

 500 

 250 

 125 

 62 

 31 

 15.5 

 7.75 

 3.875 

 2.906 



Per cent. 

 

 50 

 50 

 50 

 50 

 50 

 50 

 50 

 25 

 25 



500 

 250 

 . 125 

 62 

 31 

 15.5 

 7.75 

 3.875 

 2.906 

 2.179 



250 fold. 



Caterpillars 



First. . . . 



125 fold. 



Do 



Second 



Third. 



62 fold. 



Do 



31 fold. 



Do 



Fourth 



Fifth 



Sixth 



15.5 fold. 



Do 



7.75 fold. 



Do 



3 875 fold. 



Do 



Seventh 



2 906 fold. 



Pupae 



2 179 fold. 



Adults 





1.634 fold. 









To give another illustration : The life of the gipsy-moth caterpillar 

 is approximately seven weeks. If beginning on the first day after 



