Februaky 17, 1899.] 



SCIENCE. 



239 



single caterpillar of Plusia hramcw have been 

 reared 2,528 individuals of a little hj'men- 

 opterous parasite, Copidosoma truneatellum.-'' 



Outbreaks of injurious insects are fre- 

 quently stopped as though by magic by the 

 work of insect enemies of the species. 

 Hubbard found, in 1880, that a minute par- 

 asite, Triclwcjramma pret'iosa, alone and un- 

 aided, almost annihilated the fifth brood of 

 the cotton worm in Florida, fully ninety 

 per cent, of the eggs of this prolific crop 

 enemy being infested by the parasite. I^Tot 

 longer ago than 1895, in the city of Wash- 

 ington, more than ninetj'-seven per cent, of 

 the caterpillars of one of our most important 

 shade- tree pests were destroyed by para- 

 sitic insects, to the complete relief of the city 

 the following year. The Hessian fly, that 

 destructive enemy to wheat crops in the 

 United States, is practically unconsidered 

 by the wheat growers of certain States, for 

 the reason that whenever its numbers be- 

 gin to be injuriously great its parasites in- 

 crease to such a degree as to prevent ap- 

 preciable damage. 



The control of a plant-feeding insect by 

 its insect enemies is an extremely compli- 

 cated matter, since, as we have already 

 hinted, the parasites of the parasites play 

 an important part. The undue multiplica- 

 tion of a vegetable feeder is followed by 

 the undue multiplication of parasites, and 

 their increase is followed by the increase of 

 hyperparasites. Following the very in- 

 stance of the multiplication of the shade- 

 tree caterpillar just mentioned, the writer 

 was able to determine this parasitic chain 

 during the nest season down to quaternary 

 parasitism. Beyond this point, true internal 

 parasitism probably did not exist, but even 



* This observation, which for some years ' held the 

 record, ' as the expression is, was made by Mr. Per- 

 gande, of the U. S. Department of Agriculture. Re. 

 cently, however. Professor A. Giard, of Paris, has 

 more than 3,000 specimens of the same parasite reared 

 from a Plusia caterpillar. 



these quaternary parasites were subject to 

 bacterial or fungus disease and to the at- 

 tacks of predatory insects. 



The prime cause of the abundance or 

 scarcity of a leaf-feeding species is, there- 

 fore, obscure, since it is hindered by an 

 abundance of primary parasites, favored by 

 an abundance of secondarjr parasites (since 

 these will destroy the primary parasites) , 

 hindered again by an abundance of tertiary 

 parasites, and favored again by an abun- 

 dance of quaternary parasites. 



The subject of practical handling of in- 

 sect enemies of insects has come into great 

 prominence during the past ten years. The 

 suggestion by the Rev. Dr. Bethune, of 

 Canada, many years ago, of the desirability 

 of importing the European parasite of the 

 wheat midge into America was probably 

 the first published international suggestion 

 of this nature, and, although some subse- 

 quent correspondence between English and 

 American entomologists ensued, no parasites 

 were actually sent over. Later, attempts 

 were made by LeBaron in the case of a 

 parasite of the oyster-shell bark-louse of 

 the apple, and by Professor Riley in the 

 case of a parasite of the plum curculio, to 

 transport parasites from one section of the 

 United States to another, both attempts 

 meeting with some slight success. 



In 1873 Planchon and Riley introduced 

 an American predatory mite, which feeds 

 in this country on the grape vine Phylloxera, 

 into France, where it became established, 

 but where it accomplished no appreciable 

 results in the way of checking the spread of 

 this famous vine pest. 



In 1874 efforts were made to send certain 

 parasites of plant-lice from England to New 

 Zealand, without recorded results of value. 



In 1880, in an article upon the para- 

 sites of American scale insects, the writer 

 showed that international transportation is 

 especially easy, and especially desirable in 

 the case of these insects. 



