INSECT CONTROL 319 



parasite is only able to exist on a single species, and that 

 species fail temporarily, the newcomer will be rapidly 

 exterminated through lack of food, and the trouble and 

 expense incurred over its importation be brought to naught. 



The adult Trichogramma lays its eggs in the eggs of 

 lepidoptera, and from the eggs the new parasite generation 

 arises; not so, however, with all parasitic hymenoptera, 

 for another insect, Chelonus texanus, which, like Tricho- 

 gramma, belongs to the family Chalcididou, also deposits its 

 eggs in the eggs of butterflies and moths, but the new 

 parasite emerges from the lepidopterous larvae which hatch 

 from the parasitised eggs. This second Chalcidid is also 

 an enemy of Ckloridea obsoleta. 



It will be recognised that unless a parasite shows 

 greater fertility than its host, its power of controlling the 

 latter must be very greatly diminished. In this connection 

 nature has come to the rescue in a marvellous manner, by 

 endowing certain of the parasitic Chalcididce with the 

 power of " polyembryony," a phenomenon that is unique in 

 the animal kingdom. Parasites so endowed lay eggs, 

 similar in external form to those of their less fortunate 

 relations, but from each egg emerges in due course not 

 one larva but a number of larvae, and by this means their 

 fertility is much increased. 



This science of insect control by the aid of other insects, 

 though one of the most important branches of economic 

 entomology, can be but lightly reviewed in these pages. 

 Despite the fact that it is as yet in its infancy, a large 

 volume would have to be filled to do it justice. Mention 

 should be made of many other important predaceous and 

 parasitic insects that have been pressed into service in 

 America; the aphis-eating ladybird, Megilla maculata, 

 that has done such yeoman service in the tobacco planta- 

 tions, to mention only one. 



Before passing to another theme, let us see what the Old 

 World has done in this connection. In many European 



