INSECT ENEMIES OF INSECTS CLAUSEN 355 



uberance of development and increase greatly beyond anything 

 known in its native home, and a species previously considered to be 

 rather harmless looms up as a major crop pest. 



There are wide differences in the habits of these insect parasites 

 and predators. Some moths, for instance, are parasitic in ihe larval 

 stage on adult cicadas and upon other insects of that order, while 

 other species in the same stage may feed upon aphids or various scale 

 insects. The two groups which dominate in the parasitic role are 

 the wasps and flies. To most people, the fly is essentially a house- 

 hold pest, and they are not at all familiar with the very large num- 

 ber of species which live at the expense of other insects and are 

 seldom seen. 



There are many ways in which these parasites and predators live 

 at the expense of other insects. Some feed exclusively upon the 

 eggs, some feed upon the larvae or the pupae, or even upon the 

 adult insects, while still others are parasitic, either externally or 

 internally, upon the various stages. Those species which are true 

 parasites live at the expense of a single individual throughout their 

 period of development and consequently do not cause its death 

 until their growth is completed. This may occur in different stages, 

 dependent upon the species concerned. From the point of view of 

 the parasite, this is of little consequence. From the viewpoint of 

 the agriculturist, however, it is essential that the host be killed 

 before it is able to reproduce itself, otherwise very little benefit will 

 be derived by way of reduction in numbers of the following genera- 

 tions. It is for this reason that a 50 per cent parasitization, or 

 even less, may result in the practical control of one pest, whereas 

 100 per cent may be of little value in the case of another. An in- 

 stance of this is shown in the case of a small wasp, Scutellista cyanea^ 

 which attacks the black scale of citrus in California. The egg is 

 deposited beneath the scale and the larva feeds upon the host eggs. 

 The individual scale deposits from 1,000 to 2,000 eggs, and the 

 parasite larva in the course of its development is able to consume 

 only a portion of them. Those which remain are sufficient to main- 

 tain the infestation upon the trees at a maximum figure, and the 

 presence of the parasite is consequently of little practical value. 



Some species of insects are attacked by a very large series of 

 parasites, in some instances numbering several dozen species. \VTiere 

 this occurs there may be extensive competition among them for the 

 possession of the host. ^^Hiere several species attack the same host 

 individual, only one normally survives. This may be determined 

 by an earlier time of the attack by the one species, or by a greater 

 aggressiveness of its larva. In some species the newly hatched 

 larvae are decidedly aggressive and have very pronounced cannibal- 

 149571—33 24 



