INSECTS AFFECTING PARK AND WOODLAND TREES 



615 



PLANT GALLS AND GALL MAKERS 



The abnormal growths frequently found on plants and known as insect 

 galls, excite considerable interest and have led to much speculation as to 

 their origin and manner of development. The species producing these 

 peculiar structures do not compose a natural group, but are found in six 

 orders and two classes of the animal kingdom. Certain of the insects 

 present most remarkable features in their life histories. and all have attained 

 the position sought by many of a higher race, in that they secure both food 

 and shelter with little or no labor. 



Development of galls. Galls produced by insects or by the somewhat 

 closely allied mites, may be found on practically every portion of the plant. 

 The roots are affected by a number of species of plant lice and mites ; 

 trunks or stems are attacked by certain 4-winged gall flies, beetles or Cole- 

 optera, 2-winged gall flies and plant lice, while branches, foliage and iruit 

 or seeds are infested by various species producing most remarkable and 

 diverse growths. These abnormal structures may vary from the compara- 

 tively simple galls of certain plant lice, which consist of little more than a 

 folded leaf to the nearly solid, hard cynipid galls on s^ems. The many 

 celled bud galls may be contrasted with the beautiful fuzzy swellings adorn- 

 ing the stems or leaves of oaks and various plants. These structures, as 

 previously stated, are caused by several insects belonging to widely sepa- 

 rated groups, and it is probable that no general law can account for their 

 production. Certain galls like those of the Tenthredinidae or sawflies, are 

 probably produced by the irritation incident to oviposition or the presence 

 of the egg, since, according to several observers, the gall develops before 

 the young hatches from the egg. The larvae of the 4-winged gall flies or 

 Cynipidae, are probably responsible in most cases for the development of 

 the abnormal growths caused by this family, as their activity results in 

 abnormal stimulation followed by excessive cell formation and the develop- 

 ment of a mass of unhealthy tissue. The plant lice illustrate another and 

 in certain ways a more remarkable method of development, in that the 

 tissues of the affected plant grow around and inclose the insect. The plant 



