I'O 



SCIENTIFIC A G R I C U L T U R J] 



January, 1921. 



2. Insects in Relation to Other Animals. 



Nature has evolved not as iiide|iendeiit but often as 

 closely dependent organisms. The well-being of one 

 -set is frequently related in many ways to other sets, 

 aud no creature can be said to live unto itself. Insects 

 and plants, for example, have been for long ages 

 mutually adapting themselves one to the other, the 

 plant to the insect and the insect to the plant. We all 

 know how birds keep down insects and many rodents; 

 in fact, an approximate equilibrium has been establish- 

 ed between them. Any disturbance or sudden reduction 

 in the numbers of the birds of a region is sure to dis- 

 turb the equilibrium or balance in the insect world 

 and cause much loss to the crops upon which the in- 

 sects feed. 



Moreover, the number of carnivorous animals bears 

 a definite proportion to the herbivorous animals upon 

 which they feed, the herbivorous animals to the plants, 

 the plants to insect visitors, etc. In this web of Nature 

 we may note that the animals preyed upon are more 



.\. K. c, 



A. — Fhiromalus puparlum, a chalcid parasite of chrysaUds of the 



cabbagre caterpUlar. 

 B. — Pliupla conqnisitor depositing on cocoon of tent caterpiUar 



(after Fiske). 

 C. — A parasitic tacliina-fly and its puparium (after Weed). 



prolific than the predaceous forms. "Small rodents 

 lend to be much more prolific than carnivores. The 

 primary reason for this is probably that less individual- 

 ed t.vpes tend to be more/ prolific. "" — (Thomson). 



Sometimes man interferes with the balance of Nature 

 and serious consequences follow. Rats became a great 

 plague in Jamaica and to offset them the mongoose, a 

 weasel-like animal, was introdiiced. The mongoose 

 made short work of the rats, but it turned its attention 

 to useful animals t-iuch as poultry^ ground-birds and 

 insect-eating lizards and snakes. As a result injurious 

 insects and ticks have increased greatly, and both 

 plants and animals have suffered much injury. 



Facts which convince even the most sceptical are 

 accumulating regarding the valuable role played by 

 liirds in the control of noxious insects. Studies of bird 

 diets prove conclusively that the majorit.v of our com- 

 mon birds feed mainly upon insects. P\)rbes of Illinois 

 states after a careful study of the contents of the 

 stomachs of birds that about two-thirds of the food 

 of birds consists of insects. Well-informed writers tell 

 us that without birds the earth would be uninhabitable 

 after six yeai's, and yet man in his ignorance is con- 

 stantly destroying these valuable friends, simply be- 

 cause he finds that they levy an insignificant toll on 

 his fruits and grains. It is possible that we would be 

 better off if certain birds were greatly reduced in 

 numbers, but of this we are not absolutely certain, 

 for the web of life is most complex, and no person 

 knows how far-reaching the results would be. 



Aside from the fact that birds aid very materially 

 in reducing the numbers of insects when they come as 

 scourges, it is very important to remember that birds 

 nip many incipient scourges in the bud Their mobil- 



ity and varied character and habits enable them to 

 move rapidly from place to place and thus maintain 

 the balance of nature which man is always tending to 

 upset. Even in wild nature the balance is never quite 

 complete ; at best the equilibrium is unstable. 



"It is very interesting that the two great classes of 

 successful fliers should be in the wide economics of 

 Nature fitted against one another, wiugs against wings, 

 freeman against freeman, invertebrate against invert- 

 ebrate, "little brain" against "big brain", "instinct" 

 against "intelligence". Practically this is the most 

 important conflict of classes that the world knows." 

 — (Thomson). 



It is worthy of note that the Italian entomologists 

 do not share the opinion of American and British 

 fellow workers as to the great value of birds in the 

 control of insect life. 



From 1850 to 1873, Rondani, an Italian, made most 

 valuable contributions t-o the study of parasitology. He 

 was of the opinion, however, that parasites were far 

 more important than birds as control factors. He 

 said: "The policing of the fields cannot be entrusted 

 to birds because the.v are unreliable and kill the guilty 

 with the innocent ; they are rolibers as well as guard- 

 ians of the field products, and therefore do not yield 

 the most, and sometimes any calculable advantage. In 

 the latter case they often do even more harm than good 

 in the very things which were sought to be saved by 

 their means."' 



Ferris and Decaux of France in the seventies and 

 eighties advocated strongl.v the use of parasites, and 

 Berlese and Del Guercio of Italy later recognized and 

 emphasized the value of entomophagous insects rather 

 than insectivorous birds. 



In this connection it is interesting to note Silvestri's 

 own opinion. He says: "I, for my jiart, believe that 



Syrphus flies: 1 and 2. adults; 3, larvae eating plant lice; lower 

 figure contracted larva; 5 and 6, view of larva, enlarged, 

 and pupa. 



the usefulness aud the harm of insectivorous birds . 

 balance each other and that more frequently the former 

 may be superior to the latter, considering things only 

 from the viewpoint of immediate agricultural interest". 



Insects and Animal Diseases. 



The relation of insects to active diseases is now well 

 known. The Anopheles mosquito the yellow-fever 

 organism, the Stegomyia mosquito the yellow-fever 

 organism ; the house-fly tubercular, t.vphoid fever and 

 other germs; the stable-fly infantile paralysis; t.setse 

 flies the trypanosomes of tlic terrilde "sleeping sick- 



