57 



Varying Abundance of the Species in Different Years 



An examination of my data of the relative numbers of specimens 

 of the several species in successive years gives little reason to suppose 

 that periods of unusual abundance in any locality are commonly sep- 

 arated from one another by an interval equal to that between succes- 

 sive generations of a species. Times of abundance and scarcity are 

 much too irregular, and either one or the other is often too long con- 

 tinued, to make this a plausible explanation of the facts. In view of 

 the number and effectiveness of the animal and vegetable parasites of 

 white-grubs and of the insect parasites of the beetles, it seems much 

 more likely that parasitism, possibly more or less modified by the 

 weather of the period, is a principal cause of these frequently enor- 

 mous fluctuations in numbers; and even if a periodicity appears 

 corresponding to the length of the life cycle of an abundant species, 

 it is likely soon to be broken up or set aside by a consequent rapid 

 multiplication of parasitic insects and annelids and the spread of 

 contagious diseases due to parasitic fungi and Protozoa, The practical 

 importance of this conclusion is evident. In the absence of such para- 

 sitic disturbances of the normal course of events, a season of injurious 

 abundance would be always followed by another such season after a 

 period of years sufficient to bring the next generation of the abundant 

 species to the same stage of larval activity; and if the species con- 

 cerned and their life history for the latitude were generally and ac- 

 curately known, measures of precaution might be taken, especially 

 with crop rotations, of a kind to reduce to a minimum the injuries to 

 be expected. This is indeed the case in Europe, with the Old World 

 representatives of our American white-grubs — the vers blancs of the 

 French and the engerling of the Germans. There the so-called ' ' flight 

 year ' ' of the beetles or a season of serious injuries by the grubs, may 

 be accurately foretold for any locality and measures taken accord- 

 ingly ; but in Europe no parasites of these insects are known, and in 

 their absence there is comparatively little to interfere with the peri- 

 odical recurrence of these seasons of their destructive abundance, es- 

 pecially as the species are but two as compared with the thirty-four 

 species in Illinois alone. To understand the probabilities with respect 

 to our American white-grubs it is, in my judgment, at least as im- 

 portant to know the status, at the time, of their most effective parasites 

 as to know the life histories of the May-beetles for all our latitudes 

 and climates. This is especially true because of a latent possibility 

 that the grub and beetle parasites, especially the fungi of disease, may 

 be so cultivated and distributed as to assist materially in the control 

 of the insects — an undertaking in which there have been many failures, 

 but one the possibilities of which have been by no means exhausted. 

 We have lately found, for example, that a new annelid parasite of 

 the grubs is the cause of epidemic destruction of them, and that it 



