THE CONTROL OF INJURIOUS INSECTS 403 



to New Zealand, Portugal, Cape Colony, Florida, Hawaiian Islands, 

 Italy, Syria and Egypt, and equally good results were secured. Dr. 

 Howard gives the following reasons for this success: (i) Novius pro- 

 duces in one year double the number of generations that Icerya does; 

 (2) Novius feeds preferably on eggs of Icerya; (3) the absence of para- 

 sites of Novius; and (4) Novius is an active insect, while Icerya is fixed 

 to the plant. 



Another interesting example of successful parasitism through 

 importation in 1901 is that of Scutellista cyanea, a chalcid insect from 

 South Africa to control the Black Scale of the Olive (Lecanium oleoB) of 

 California. 



The West Indian Peach Scale {Diaspis pentagona) occurs in the 

 Southern States but is not very injurious on account of the presence of 

 a parasite, Prospaltella herlesei. Specimens of the parasite were sent 

 to Italy where mulberry plantations were seriously injured, with the 

 result that it is now well established and holding the scale in check. 

 The recent attempt to control the Gypsy and Brown-tail Moths 

 in New England has been of great value to entomologists in demon- 

 strating the many complicated factors that exist when foreign parasites 

 are introduced. From 1905 to 1913 more than thirty species of para- 

 sites were imported from Europe and Japan and a number of the most 

 important ones have been acclimatized and are rapidly spreading. 



The present improvement in Massachusetts "is due to at* least 

 four main causes: (i) The perfection and standardization of the methods 

 for artificial repression; (2) the death of a large proportion of the 

 more susceptible trees or their removal from the infested woodlands; 

 (3) the importation of parasitic and predatory insect enemies; (4) 

 the development of the 'wilt' disease" (Fiske). 



The problem of the control of insects such as the Gypsy Moth and 

 the Brown-tail Moth by parasites is a more difficult one than appears 

 on the surface. From extensive studies of the life-history of the Gypsy 

 Moth it has been determined that the probable potential rate of in- 

 crease is 250-fold annually. On account, however, of the heavy death- 

 rate from various causes the actual rate of increase is only 6- to lo-fold. 

 The problem was then to secure sufficient parasites to keep the insect 

 in control. In other words, if the increase annually be 6-fold, five out 

 of every six insects, either egg, caterpillar, or pupa, or 83.3 per cent, 

 would require to be parasitized. If the increase be lo-fold, nine out of 



