Q36 MICROBIAL DISEASES OF INSECTS 



METHODS OF INFECTION. Infection naturally takes place through 

 the mouth by means of the food. Predisposition to the disease is se- 

 cured by giving the caterpillars food which has been placed in water 

 and renewed only every three or four days. This causes an increase in 

 the acidity of the leaves which in turn decreases the alkalinity of the 

 caterpillar's digestive fluid. Before the visible outbreak of flacherie, as 

 an early symptom, a characteristic sweet odor is recognized in the 

 breeding cages which resembles that of withered lilac blossoms some- 

 what. Whenever this odor is noticeable, flacherie soon makes its 

 appearance, and as it progresses the odor increases proportionately 

 (Fischer). 



Lack of food, which is necessarily brought about by the cater- 

 pillars themselves, causes them to lose their vitality, thus producing a 

 greater susceptibility to the disease. Defoliation also exposes them 

 to the sun's rays which have the effect of converting the chronic into 

 the acute form of wilt. In lightly infested woodland this does not hap- 

 pen as the caterpillars can always find shade. Flacherie, however, 

 seems to be influenced by climate and weather conditions 

 less than any other caterpillar disease. 



Wilt is always prevalent among the older caterpillars; young cater- 

 pillars often live several days before succumbing to the disease. Female 

 caterpillars always succumb more readily to the wilt disease than the 

 male; this may perhaps be due to the fact that they require a longer 

 time to mature than the male. Diseased females deposit egg clusters 

 reduced in size, which contain usually, embryos, incompletely or not 

 at all developed. In this case there are always found undeposited 

 eggs within the body of the female, which never occurs with healthy 

 moths. Genetic immunity of certain individuals is probable. Sub- 

 kthal doses of the virulent filtrate may produce active immunization. 

 Although probable, there is yet no definite evidence that wilt is trans- 

 mitted from one generation to another. 



PATHOLOGY OF WILT. When a caterpillar dies of wilt, all of its 

 tissues are in a state of disintegration. The intestine is the last in- 

 ternal organ to disintegrate. A smear of the brown liquid from a dead 

 caterpillar examined microscopically with a high power lens will be 

 found to contain, besides the elements of disorganized tissues, myriads 

 of highly refractive polyhedral bodies of various sizes. The average 

 polyhedron measures from i/* to 6/z in diameter and is never regular as 



