Jaimarv, 1921. 



SCIENTIFIC AGRICULTURE 



19 



Inter-Relations in Nature 



By Prof. W. LOCHHEAD, Maudonald College. 

 (Paper read before the Outario Entomological Society, Guelpli, November, 1920). 



To the eeouomie eutomologist the investigation of 

 the "Inter-relations in Nature" should be one of the 

 most important fields of studj', for abundant evidence 

 has been collected to show that all nature is a; vast 

 system of linkages, one part dependent upon another 

 in an intricate web of life, and that disturbances in 

 one portion of the system are followed by disturbances 

 in another. To Darwin more than any other person 

 science is indebted for the elaboration of the idea and 

 for the clear demonstration of its practical importance. 



Since Darwin's time the number of examples of inter- 

 relations has been greatly extended through the 

 observations of thousands of investigators. In our 

 boyhood days we were accustomed to rhj-me the chain 

 of events in the House that Jack Built which ends with 

 "This is the cat that killed the rat that ate the malt 

 that lay in the house that Jack built." In Nature many 

 such chains have been unravelled binding animal with 

 animal and animals with plants and these again with 

 the inorganic world. Man eats the fishes that eat 

 Crustacea that eat infusoria that eat bacteria that feed 

 on decaying organic matter in some pond. 



Diagram showing the sciences bearing most directly upon 

 Economic Entomology. 



The purpose of this paper is to discuss briefly, or 

 rather to point out, those inter-relations in nature that 

 have to do more particularly with insect life. The sub- 

 .iect is/ a big one, so that much moi-e must necessaril.y 

 be left unsaid than can possibly be said in a paper on 

 this occasion. 



The following relationships will be discussed : — 



1. Insects in relation to insects; 



2. Insects in relation to other animals; 



:?. Insects in relation to plants, including bacteria 



and fungi ; 

 4. Insects in relation to inorganic nature. 

 1. Insects in Relation to Insects. 

 The part played by predatory and parasitic insects 

 in the regulation of insect life has been frequently dis- 

 cussed at these meetings. The topic is a very important 



one and studies are being carried on at many stations 

 and none better than those by Mr. Tothill of the 

 Dominion Entomological Laboratory at Fredericton, 

 on the factors operating on the Forest Caterpillar and 

 the Fall Web Worm. 



The elaborate studies of the parasites of the Gj'psy 

 Moth in New England and their part in the control of 

 the pest have been summarized by H. S. Smith in the 

 Journal of Economic Entomology, August 1919, as 

 follows : — 



1. — The re|)rodnctive capacity of available entomo- 

 phagous in.sects must be much higher than that of the 

 host. This proposition is self-evident, and needs no 

 amplification. 



2. — A complete sequence of parasites affecting the 

 egg, larva and pupa of the pest. The importance of 

 this factor was fully shown in the eampaigni against 

 the Gypsy moth. 



3. — The entomophagous forms must be capable of 

 being reared or obtained in sufficient quantities to 

 overcome the pest in the field. This factor is one of 

 the most difficult to bring into operation. Lady- 

 birds are readily reared and collected. 



4. — The cost of producing natural enemies must re- 

 main well within the bounds of profitable crop produc- 

 tion. 



5. — Secondar}^ parasites in the local fauna should 

 limit as little as possible, much less entirely prevent, 

 the action of the primary parasites. This factor is also 

 one that is most difficult to control. 



fi. — Agricultural practices such as spraying and 

 fumigation which affect adversely the breeding of nat- 

 ural enemies should be prevented. It is probable that 

 the extermination of the imported Chinese Lady-bird 

 in southern orchards was due mainly to spraying oper- 

 ations with lime sulphur carried on as a practice. 



7. — The relative ability of the pest and its enemies 

 to spread is an important factor. If both be good 

 fliers the power of spreading is increased but the 

 likelihood of extermination of the pest is lessened. 



The citrus mealy-bug has been controlled in South- 

 ern California by the Australian lady-bird Crtjptola- 

 rnus montrouzieri wihich was reared in large numbers 

 at the State Insectai'v and collected in orchards where 

 they had become abundant late in the season. In this 

 instance, as in that of the cottony-cushion scale, the 

 lady-bird is an active insect while the scale insect is 

 fixed to the plant. Moreover, the lad.v-bird has more 

 generations than the scale insect and is practically 

 free from parasites. 



Dr. P. Marehal calls the Gypsy and brown-tail exper- 

 iments in America "a gigantic biological analysis and 

 synthesis bearing upon all the elements which constit- 

 ute the harmonic groupings of plant-feeding insects, 

 their predators, parasites and h^-piparasites. the taking 

 apart piece by piece of the whole system, and its partial 

 reconstruction in a new environment, .forcing it to give 

 the greatest possible stress to the elements most favor- 

 able to man and reducing to the minimum those which 

 oppose their action". 



