GYPSY AND BROWN-TAIL MOTHS. 49 



Increase in the average abundance of the insect from year to 

 year over the area included in its range would be comparatively 

 slow at first, but would later become much more rapid, on ac- 

 count of the rate of increase being. so much greater than that 

 of dispersion. An attempt has been made to illustrate it in 

 Diagram III,, in which each square represents an area of 200 

 acres. The number of the insects (Monodontomerus in this 

 instance) which are to be found within a territory of this extent 

 is indicated by the black dots. It becomes impossible to crowd 

 a sufficiently large number into the limited space available to 

 indicate accurately the proportionate abundance which will re- 

 sult in 1910 and 1911, if the rate of increase continues without 

 diminution. 



The parasites are generally very inconspicuous, and when not 

 common are difficult to find. Monodontomerus, as has been 

 stated, is particularly easy to find, owing to its hibernating 

 habitsj and it is partly on this account that so much is known of 

 its progress. The only methods which may be employed for the 

 recovery of the most of them is the collection of a quantity of the 

 caterpillars or pupsB of the gypsy moth, which are confined in 

 the proper form of cage in the laboratory until the parasites issue 

 from the affected individuals after destroying them. To collect 

 all of the gypsy caterpillars or pupse over a territory as large as 

 1 acre is out of the question when the insect is abundant. It is 

 therefore impracticable to follow systematically the progress of 

 an insect which would be so uncommon as to be represented by 

 only a single individual in an area of this extent. It is small 

 wonder that no trace of certain of the parasites which have been 

 liberated has been found during the first few years following. 



It was not until the summer of 1909, the fourth after its estab- 

 lishment, that Monodontomerus was first recovered as a parasite 

 of the gypsy moth in the field; and if a parasite can increase 

 at such a rate and remain unnoticed for three years, there is good 

 foundation for hoping that other species may be doing as well. 



The Calosoma beetles, which were also liberated for the first 

 time in 1906, in time to attack the gypsy moth caterpillars that 

 year have ever since increased at a less rapid but at the same 

 time a very satisfactory rate. Their rate of dispersion is also 

 much less, and probably does not exceed a mile or two per year. 



