CONTEOL OF ARGE:S^TINE ANT IN CALIFORNIA. 



to adjacent trees. The chief damage done by the Argentine ant in 

 Cahfornia citrus orchards has been through its relation to mealybug 

 infestations, particularly the common mealj^bug, Pseudococcus citri 

 (Risso) . Prior to the reported presence of this ant in the Southwest, 

 one species of mealybug was recorded as a serious citrus pest in only 

 three or four limited districts, where it was characterized by sporadic 

 outbreaks which would quickly disappear. In the last decade, 

 which is contemporaneous 

 with the Argentine ant inva- 

 sion, the severe infestation 

 of the conomon mealybug has 

 increased many hundred per 

 cent and districts have been 

 invaded from which mealy- 

 bugs were never before re- 

 corded. 



In the course of investiga- 

 tions under the direction of 

 the senior writer to establish 

 effective control of the com- 

 mon mealybug, Pseudococcus 

 citri (Risso) , it was discovered 

 that the Argentine ant not 

 only brings about increased 

 severity of infestation but 

 causes the mealybug to per- 

 sist j^ear after year. It was 

 furthermore discovered that 

 the common mealybug was 

 attacked by many predatory 

 insects, some of such high 

 efficiency that in ant-freed 

 territory the}' were capable 

 of keeping the mealybug un- 

 der commercial control ex- 

 cept at those infrequent periods normal in the " ups and downs" of 

 biologically controlled pests. Demonstrations carried out in several 

 mealybug-ant infested orchards in the San Gabriel Valley during the 

 years 1915 and 1916, in which alternate rows were kept free of ants, 

 proved that trees rid of ants quickly become freed of mealybugs, 

 although on adjacent ant-attended trees the mealybug infestation 

 persists. The results of these demonstrations are verified by the 

 fact that in every experiment ])erformcd by the writers during the 

 period of 1915-1920 equally good results have been obtained. The 



Fig. 1.— Ants attending a group of mealybugs. Their 

 almost constant presence protects the mealybug from 

 its natural enemies. 



