30 THE REPORT OF THE No. 33 



THE VALUE OF NATURAL ENEMIES OF INJURIOUS INSECTS 



A. F. Burgess 



A belief has become firmly established in the public mind that insects are 

 held in check by parasites under ordinary conditions. This has been taught in 

 the colleges and institutions of learning and has been given prominence in the 

 published works of many entomologists. The usual formula proposed is that 

 when an insect is abundant its parasites increase enormously and bring about 

 control, after which enormous mortality results to the parasites on account of 

 scarcity of the proper host and that after a period of years the latter increases 

 again and damage results. It is a well known fact that some insects, which 

 are innocuous in certain parts, at least, of the regions which are their native 

 homes, become excessively abundant and cause havoc after becoming established 

 in a new environment. The above statements indicate the reasons for the 

 attempts that have been made in this country and elsewhere to introduce 

 natural enemies to control insect pests. 



We must all admit that this subject has been given scanty consideration 

 when its importance and complexity is considered. It is the purpose of this 

 paper to consider some of the factors involved in the hope that more attention 

 may be directed to some phases of this difficult problem. 



Everyone understands that the food supply of any organism is one of the 

 determining factors of its survival, but it is fair to assert that the food and feeding 

 habits of many injurious insects have been studied only in a casual way. 



Intensive study of the food preferences and what might be called the inci- 

 dental food of each pest is often of great importance, not only as a means of 

 deciding effective methods of control but of weighing the influence of parasites 

 or predatory enemies on the abundance of any pest. 



As a result of a careful study of the food plants of the gipsy moth in New 

 England carried on for several seasons prior to 1900, it was determined that the 

 larvae of this insect fed upon foliage of most of the trees and shrubs in the 

 region where it was then present. The possibility of the food being unsuitable 

 to newly hatched caterpillars, although entirely satisfactory to those of a later 

 stage, was not considered. Later experiments conducted by Mr. F. H. Mosher 

 and his assistants at the Melrose Highlands Laboratory, supplemented by 

 careful records made of the feeding habits of these caterpillars in the field, have 

 demonstrated that a very decided preference is exhibited by different larval 

 stages of this insect. This has given valuable suggestions as to control methods 

 that are feasible and will be useful when an attempt is made to determine the 

 proper role that parasites will play in gipsy-moth control. 



These experiments have made possible the division of the food plants into 

 various classes based on the food preference of the larvae in different stages 

 and indicate clearly that some plants are merely incidental hosts of this insect. 

 It is easy to accumulate a long list of food plants of an insect if all plants that 

 may be nibbled a little here and there are recorded. Such information is fre- 

 quently inaccurate, and may be misleading unless the details are carefully 

 worked out. Because a few individuals feed on a given plant when the circum- 

 stances are such that no other food is available, it does not necessarily follow 

 that normal reproduction of a pest or continued damage will result. 



One of the essentials of having a basis for determining the normal increase 

 of an insect under field conditions must rest on a thorough knowledge of its 

 food plants and feeding habits. Accurate and detailed information on this 

 phase of insect control is in many cases incomplete. 



