OUR FRIENDS THE INSECTS BALDUF 435 



the latter may be subjugated by a third parasite, which, better than 

 anywhere else in the animal world, illustrates well the poem of the 

 fleas that have lesser fleas, and so ad infinitum. We see, then, by 

 these examples that parasitic insects are by no means limited to any 

 particular place, host, or host stage, and still they are so bound to 

 their habits by heredity that they select their hosts within certain 

 group limits and die without progeny if certain hosts, or sometimes 

 a single species of host, are not available. It is this relative uni- 

 formity and, furthermore, their limitation to a parasitic life that 

 makes them dependable for use in biological control. 



METHODS OF BIOLOGICAL CONTROL 



It is necessary to admit at once that the use of parasites to combat 

 their injurious relatives has limitations. Where a native parasite 

 of an indigenous pest already exists and fails to hold its host suffi- 

 ciently in check, there is not much that has been done to increase 

 the numbers of the benefactor, but new methods may possibly be 

 originated in the future. Their rate of growth is much controlled 

 by weather and the available numbers of the host, and men can 

 scarcely hope to regulate these influences. However, even in the 

 instance of native parasites various means of utilizing them are 

 known or may be developed. It is a well known fact that winters 

 reduce the numbers of parasites considerably below their status of 

 the previous year. The host is likewise reduced, oftentimes, but the 

 parasite can not reproduce extensively until its host is first plentiful. 

 Consequently, the host is free to do more or less damage in the first 

 months of the growing season, whereas the parasite requires a month 

 or two to " catch up " or reach effective numbers. At present, proj- 

 ects begun in California are under way in several States of this 

 country to develop an abundance of the egg parasite {Tricho gramma 

 minutum) of certain moths in laboratories during the early spring. 

 They are then released in orchards for the control of the codling 

 moth, or in southern fields to hold the cane stalk borer or celery 

 leaf tyer in check, while the outdoor parasites are building up a 

 controlling number. The method of securing the parasite in plenty 

 is to use the Angumois grain moth which reproduces in stored grains 

 indoors under warm conditions during the early spring and whose 

 eggs can, therefore, be secured in large numbers. These eggs are 

 exposed to the adult parasites which deposit their eggs into those 

 of the moth. The life cycle of the parasite is short, hence a good 

 number of generations is produced annually and many thousand 

 individuals are reared quickly with proper mechanism and man- 

 agement. 



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