FOREST INSECTS 79 



very heavily parasitized, and on examining the pupae of the 

 first brood during 1896, it was found that from 624 cocoons 

 collected only 12 moths emerged, while from those remain- 

 ing (excluding a few which had died of disease) no less than 

 916 specimens of parasitic insects emerged, indicating a 

 mortality of over 98 per cent. This explains very satis- 

 factorily the great fluctuation in numbers noticed with this 

 species, as also with many other pests which gradually be- 

 come more and more abundant from season to season for a 

 number of years, and then suddenly suffer a set-back, are 

 greatly decimated, and become relatively unimportant for a 

 number of years, until they can regain their lost ground. 

 When again very abundant, their parasites enjoy unusual 

 opportunities for multiplication, and there is another abrupt 

 decrease in the numbers of the host species. 



The condition just described is similar in a way to the out- 

 breaks of other forest insects discussed earlier in the present 

 chapter, but although the depredations terminate suddenly 

 in both cases, their appearance is gradual in one instance, and 

 quite without warning in the other. Possibly entomologists 

 have only failed to read the signs aright, but it seems more 

 likely that most extensive and unexpected invasions are due 

 to other causes. 



One interesting case of an insect enemy of deciduous trees, 

 which closely parallels the condition seen with agricultural 

 insects, is that of the locust-borer (Cyllene robinice), an 

 enemy of the yellow locust (Robinia pseudacacia) . The larvae 

 of the locust-borer feed within the sapwood of living trees 

 where they tunnel in the trunk and branches. This tree is 

 native over a very restricted range along the Appalachian 

 Mountains from Pennsylvania to northern Georgia, but has 

 been planted over wide areas throughout the eastern states, 

 on account of its rapid growth and valuable timber. The 

 locust-borer has spread with its host tree, and the thickly 

 planted groves of locusts have offered such opportunities for 

 the multiplication of the insect that the propagation of these 

 trees has been abandoned in many localities where they 

 might otherwise have been grown very profitably. 



