448 FUNGUS-INSECT INTERRELATIONSHIPS 



are species of Septobasidium, a genus monographically treated by 

 Couch (1938). Its members live in mutualistic association with 

 colonies of scale insects, using some individuals for food and 

 giving shelter and protection to others. 



BIOLOGICAL CONTROL OF INSECTS 



That competition between organisms exists everywhere 

 throughout nature is clearly appreciated by biologists. This con- 

 cept was crystallized from observations and incisive analyses by 

 Darwin, and he expressed it by the connotation "the struggle for 

 existence." The discussion that follows is intended merely to 

 direct attention to man's efforts to intervene in a struggle between 

 fungi and insects in order to suppress epidemics of insect pests, 

 at least to the extent of bringing them under control. 



The basic principles of biological control of noxious insects by 

 microorganisms (fungi, bacteria, viruses, and protozoa) have been 

 given consideration by Sweetman (1936). He indicates that the 

 following factors should be given particular attention: (1) the 

 differences in receptivity or susceptibility of the insect at different 

 stages of development; (2) the environment most favorable to the 

 pathogenic agent; (3) the virulence of the pathogen as modified 

 by environment; and (4) the necessity of having the optimum 

 conditions for attack by the pathogen coincide with the occur- 

 rence of favorable abundance and developmental stage of the 

 insect to be controlled. 



Upon contemplation of these factors it will become apparent 

 that little hope of success should be expected in controlling a par- 

 ticular insect pest by use of a given entomogenous fungus unless 

 and until an understanding has been gained of the aggressiveness 

 or virulence of the fungus. For example, some fun^i, such as 

 species of Penicillium, Alternaria, and Cladosporium, whose mem- 

 bers rarely attack living organisms, may be presumed when pres- 

 ent to have invaded the bodies of insects after they have died. At 

 the opposite extreme in intergradation of parasitism are such obli- 

 gate parasites as Entomophthora and Sorosporella, which thrive 

 only while the insect remains alive. Such fungi produce spores 

 during a brief period before the death of their victim or immedi- 

 ately thereafter, and the spores remain dormant or fail to germi- 

 nate unless they come in contact with another living insect. 



