58 ENTOMOLOGICAL SOCIETY OF ONTARIO. 



THE ECONOMIC VALUE OF PARASITISM. 



By F. M. Webster. 



In the term parasitism, as here used, is included the preying of one organism upon 

 another, whereby the latter is largely kept within normal, numerical bounds, or is reduced 

 to such conditions when it rises beyond them. Or, in other words, the preying of certain 

 so-called beneficial insects upon others called destructive, and the action of fungoid 

 growths upon such destructive species. Parasitism, in its ^broadest sense, has been aptly 

 termed the balance wheel of nature, because of its similarity in effect to the mechanical 

 contrivance bearing that name, which is instrumental in equalizing the irregularities of 

 motion, in the machicery of which it is a part, and hence dependant upon the same 

 source for its motive power. 



The effect of vegetable devouring insects is to prevent the encroaching of one vege- 

 table upon another, lest the latter should be exterminated : and the insect and fungoid 

 enemies of such vegetable-feeding insects prey upon them in ordsr that they do not them- 

 selves carry their work to such an extent as to exterminate the plant they are only 

 designed to restrict. Thus we have a plant being fed upon by a species of insect, which 

 insect is being kept from exterminating this plant by its own or primary parasites, and 

 these in turn are kept from destroying all of the plant feeding insects by still oLher para- 

 sites, known as secondary parasites, and these also have their parasites, known as tertiary 

 parasites, and besides are more or less influenced by meteorological environment. To 

 make the matter still clearer, it is as if a number of men were sent to prune an orchard, 

 and a superintendent sent with them to see that the task was not over- done, he too, 

 being amenable to still other authority. 



Now, both plants and insects are capable of reproducing far beyond the number of 

 young ordinarily required to keep these elements in equilibrium ; but when, from any 

 cause, one of them becomes abnormally reduced, this reserve reproductive force is brought 

 into play, and the weakened element is thus soon able to regain its normal numerical 

 power, but is restrained from going beyond. We thus have a huge piece of natural 

 mechanism, self regulating and self-adjusting, the balance wheel of which is parasitism. 



Under perfectly natural conditions and uninfluenced by man, all of these natural 

 organisms work in unison, as above indicated, and a temporary disarrangement of any 

 one element, due to outside causes, such as the weather, is soon readjusted with little 

 more disturbance to the others than would result to the Gulf of Mexico from the drop- 

 ping of a pebble into the middle of the Atlantic ocean. In some cases a few plants 

 might be killed throughout the local areas, but these would soon be replaced by others. 

 But the husbandman now appears and upsets this equilibrium by destroying hundreds of 

 species of plants over an area of millions of acres, and in their stead replacing 

 but one. He causes a thousand apple trees to grow where nature intended but ten should 

 exist. He causes the ground to produce a thousand grain plants, where nature intended 

 but one to grow, and to produce seed far in excess of nature's requirements. The result 

 is that the insect enemies of these cultivated plants, or such insects as can feed upon 

 them, are greatly increased in numbers, because more of the young find a sufficient 

 amount of food to develop them, and because they are needed by nature to counteract 

 the influence of man. Later, the parasites, both primary, secondary and tertiary, 

 increase for precisely similar reasons, and in obedience to the same laws, though, of course, 

 they follow more or less distantly the movements of their hosts. Frou the fact that 

 their movements do follow more or less distantly the ebb and flow of their respective 

 hosts, the question of the economic importance of their influence has remained unsettled, 

 and, by some, has even been doubted. When we come to consider that but an exceed- 

 ingly small percentage of the movements of these insects ever reaches the eyes of even 

 the most observing entomologist, and of the interactions of these organisms we really 

 know but very little, it will be observed that to estimate the economic value of their 

 influences is a very difficult task, if one expects to be just and secure the actual facts. A 

 millionaire, in one of our larger cities, may replenish his purse at the bank each morning and 

 go about among the poor, supplying to the needy a coat here, a pair of shoes there, a break- 



