— 20 — 



3. — "The infections secured by artificial means of introducing fungi, while suc- 

 cessful in introducing the fungi, have thus far proved of little or no avail in 

 increasing their efficacy after they have once become generally established in 

 a grove. 

 4. — "Experiments by the authors, and by citrus growers in cooperation with the 

 authors, involving the treatment of thousands of trees with suitable "checks" 

 or "controls" have shown that when fungus (red or yellow Aschersonia) 

 even in small quantities is present in a grove, there is no certainty that from 

 three to six applications of fungus spores in water solution will result in an 

 increased abundance of the infection on the treated blocks of trees by the end 

 of the season. In some of the most important and carefully planned and 

 executed experiments, the fungus has increased more rapidly in sections of 

 the groves which were not sprayed with spore solutions than in the exper- 

 imental blocks". 



The Brown-tail caterpillar is attacked by the fungus Entotnophthora aulicae, 

 and an attempt was made by Speare and Colley in 1912 to use it in the control of 

 the pest. They state that considerable success has attended their efforts, not 

 that the fungus is a "cure-all" but it is a powerful check. Under proper condi- 

 tions of introduction from 63 to 100 percent of the caterpillars can be destroyed. 

 Reference has already been made to the "Wilt disease" of Gypsy-moth cater- 

 pillars as a factor in the control of these pests in New England. It made its 

 appearance about 1900 and is now distributed over the territory infested with the 

 Gypsy-moth, according to Glaser. It is an infectious disease but epidemics occur 

 only in localities infested heavily with the Gypsy-moth. Infected caterpillars 

 become flaccid and later their tissues disintegrate completely, due to the fermen- 

 tative and toxic nature of the virus. The brown liquid of a dead caterpillar shows 

 under the high power of the microscope large numbers of polyhedral bodies of 

 various sizes, but the exact nature of the causal organism has not been determined. 

 The virus is filterable with difficulty. 



The success of wilt infection experiments is absolutely dependent upon atten- 

 tion to seemingly insignificant details, but this much is known that infection takes 

 place through the mouth by means of the food. 



It is probable that further studies of the disease will evolve some practical 

 method of using the virus for the destruction of larger numbers of the cater- 

 pillars. Already, however, it has been ascertained that climatic conditions appear 

 to bear an important relation to wilt in the field, and that temperature has an 

 important relation to the period of incubation of wilt. 



The Complexity of the Problem. 



The problem of the control of insects by the natural agencies I have just 

 mentioned is in most cases a very complex one, on account of the large number 

 of factors operating, many being for the present beyond man's control. Under 

 one set of conditions a certain control is observed but under another set of con- 



