DISEASES OF INSECTS 369 



sometimes these, though very rarely, may possibly be attacked 

 by quarternary parasites. This is called hyper parasitism. This 

 condition of affairs is sometimes humorously described thus: 



Great fleas have little fleas 



Upon their backs to bite 'em, 

 And little fleas have lesser fleas, 



And so ad infinitum. 



Examples. The Hessian fly, which is one of our worst wheat 

 pests, is kept within bounds almost entirely through the agency 

 of native parasites. In 1895 a severe outbreak of the white- 

 marked tussock moth injured a large number of shade trees in 

 Washington, but more than 97 per cent of these caterpillars were 

 destroyed by parasites, and the pest did not appear the next year. 

 L. O. Howard, of the United States Bureau of Entomology, 

 states that in 1880 Hubbard found "that a minute parasite, 

 Trichogramma pretiosa, alone and unaided, almost annihilated 

 the fifth brood of the cotton worm in Florida, fully 90 per cent 

 of the eggs of this crop enemy being infested by the parasite." 



Summarizing, we can classify beneficial insects under the 

 following heads and sub-heads : 



1. Directly Beneficial. 



A. Insects themselves. 



a. Food for Indians, such as Grubs, Grasshoppers, or "Locusts," 



and some Maggots. 



b. Food for Animals; for Birds, Poultry, Gophers, Skunks. 



c. Medicine: "Spanish Fly." 



B. Products of Insects. 



a. Honey from Honey-bees. 



b. Shellac from Lac Insect. 



c. Silk from Silkworm Moth. 



d. Ink from Galls. (In former days.) 



e. Cochineal. 



2. Indirectly Beneficial. 



a. Pollination of Fruit Trees, Berries, Clover, and other plants. 

 6. Removers of Carrion and Waste Matter: Flies, Carrion Beetle, 

 Burying Beetle. 



c. Predaceous: Ground Beetles, Digger Wasps, Ladybird Beetles, 



Tiger Beetles, Syrphus Flies, Lace-winged Flies. 



d. Parasitic: Large 2- and 4-winged; Small 4- winged; Tiny Egg 



Parasites. 



Diseases of Insects. We can hardly dismiss this subject 

 without consideration of two other valuable allies of man in his 

 fight against injurious insects, namely, bacterial and fungous 

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