FIG. 17-1 Onset and subsidence of an outbreak of chinch bugs 

 in Illinois during the 1930'$. Areas supporting the densest popu- 



lations are indicated by the darker pattern (Shelford and Flint 

 1943). 



come an important crop or fruit pest in the United 

 States, investigators are sent to its place of origin, 

 to discover its natural parasitoid or predator enemies 

 and introduce them into the area of infestation. On 

 the whole, this procedure has been successful (Sweet- 

 man 1958). 



About the turn of the century several attempts 

 were made in Europe, especially in France, to sup- 

 press plagues of meadow voles by starting epizootics 

 of typhoid. This bacterium was found present in 

 dying voles, was cultured and distributed through the 

 fields on food that the voles would eat. The success 

 of the various attempts was always controversial, and 

 when it was appreciated that the disease was also 

 dangerous to man, these procedures were generally 

 abandoned (Elton 1942). Considerable study is be- 

 ing made at the present time of the use of bacteria 

 and other micro-organisms in the biological control 

 of insects (Steinhaus 1960). 



Myxomatosis has been used for the suppression of 

 the rabbit population in Australia. Caused by a filter- 

 able virus, it is highly contagious among the intro- 

 duced European rabbits, but apparently not trans- 

 missible to man or other animals. The virus is carried 

 between rabbits principally by mosquitoes in Aus- 

 tralia, and by fleas in England. Death occurs about 

 15 days after exposure. In 1950, extensive field trials 

 with the myxoma virus were undertaken in eastern 

 Australia, and by the end of the year mortalities 



locally as high as 99.8 per cent occurred. Epizootics 

 have continued in later years but with somewhat 

 lower virulence. The virus was introduced into 

 France in 1952, where it spread rapidly; it reached 

 England in 1953. The prognosis of the disease is un- 

 certain. In some regions of Australia, rabbits have 

 recovered from less virulent strains of the virus, or 

 there has been selection of genetically more resistant 

 individuals, so it is possible that some degree of im- 

 munization may arise. The disease, however, may be 

 successful in keeping the population at a low level 

 (Bourlicre 1956). 



Irruptions may occur with almost any kind of 

 animal in any habitat. Irruptions of the bean clam 

 occurred several times between 1894 and 1955 in the 

 intertidal zone at La Jolla, California. The abrupt 

 decline of the last outbreak in 1951-52 was the result 

 of an epizootic associated with a minute unicellular 

 organism of uncertain identity, found in the tissues 

 of the clam (Coe 1955). 



CATASTROPHES 



Catastrophes occur at more or less widely 

 spaced intervals and bring marked depressions in the 

 population level of a species. Figure 17.2 shows an- 

 nual populations of the house wren over 41 years, 

 first in Ohio, then in Illinois. Decidedly low points 



Irruptions, catastrophes, and cycles 235 



