184 AGGREGATION, COMPETITION AND CYCLES 



on insects. In 1890, Snow (1891) distributed a chinchbug fungus to 

 the farmers and the first reports were thought to indicate that it was 

 effective. It was found later that these so-called disease fungi were 

 present not only on chinchbugs but also on other insects and objects 

 and always in the cultivated field, becoming conspicuous only under 

 favorable moisture conditions. Headlee (1913) found that this fungus 

 played the role of a pathogenic organism only under a mean tempera- 

 ture of 75° F. and a humidity near saturation, but that in ordinary 

 conditions it is of no importance whatsoever. 



A bacterium {C occobacillus acridiorum) has proved to be patho- 

 genic in locusts in Yucatan. It was introduced into North Africa by 

 d'Herelle, but the bacterium gave none of the expected results, and 

 further investigation showed that it was a normal symbiont in all 

 locusts. The bacterial action liquefied the internal organs of the 

 locusts, but only when the temperature was low and the humidity 

 very high, a condition harmful to locust but favorable to bacterium 

 (Pospelov, 1926). This and other experiences have caused economic 

 entomologists to turn to other fields for explanations of the decline of 

 insect pests. 



Workers on birds and mammals have paid no attention to these 

 findings and have partially duplicated all the experiences of the 

 entomologists. Piper (1908), in his report on mouse plagues, refers to 

 Loeffler's experiment with the supposed positive results in destroying 

 field mice in Thessaly in 1892-93 by means of Bacillus typhimurium. 

 Piper failed to find any definite bacterial disease in the rodents of 

 the declining phase of the Nevada mouse plague, and referred to the 

 apparent success attained with Bacillus typhimurium as probably 

 merely the synchronizing of the experiment with the natural abate- 

 ment of the plague. In 1926, Elton was inclined to refer to the decline 

 in abundant vertebrates as a result of epidemic diseases, but his long 

 and careful studies of dying mice reported in 1931 gave no actual evi- 

 dence of pathological conditions, and the mice may have died of old 

 age. They might readily represent a group of vigorous individuals 

 resulting from a favorable period of a few years before. 



Recently, MacLulich (1937) has given a succinct account of the 

 bacteriology of the varying hare and its external parasites (71-86), 

 and of the correlation of numbers with disease (91-99), reaching 

 the conclusion that decrease was due to wholesale dying-off (125). 

 However, even though Horn discovered bacterial causes for typical 

 lesions in the Norway lemming, the significance of this in reducing 

 the numbers of lemming has not been shown. The reduction of rabbits 

 and grouse by tularemia in Minnesota (Green, 1932) appears probable, 



