348 JOURNAL OF EONOMIC EMTOMOLOGY [VOL. 1 4 



BIOLOGICAL CONTROL OF THE BLACK SCALE (Saissetia oleae) 

 Bern. IN CALIFORNIA 



By Harry Scott Smith, ChieJ, Pest Control Service, California Depattment oj Agri 



culture 



For many ^^ears the black ^^cale, Saissetia oleae Bern., has held the 

 foremost place as a pest both of citrus fruits and olives in California. 

 It is wide-spread in its distribution and capable of serious injury to 

 either fruit. In fact it would be an impossibility in most parts of the 

 state to grow either citrus fruits or olives without carrying on control 

 measures against this pest. Los Angeles Count}'- alone spent S850,000 

 during the season of 1919 for fiunigation of citrus trees, most of which 

 was directed against the black scale, and the total sum expended for 

 this purpose in California exceeds two millions of dollars per annum. 

 This does not take into consideration the expense of washing the fruit, 

 necessitated by the secretion of honey dew, or the reduction in quality 

 or quantity of fruit due to the iafestation. 



Obviously, in the face of such a staggering tax on production, there 

 is a strong incentive to students of the problem to seek a reduction in 

 the cost of suppression. Biological control has seemed to hold such 

 a possibility. 



Following the successful outcome of the biological control work a- 

 gainst the citrus mealybug, a study of the applicability of this method 

 to black scale control has been taken up. This method is based on 

 the proposition that, given a host insect of certain type, which is nor- 

 mally attacked b}^ an effective sequence of parasites, fluctuation in num- 

 bers between host and natural enemies can be prevented by the con- 

 tinued artificial propagation and liberation of the natural enemies in 

 the infested orchards. The writer has previously pointed out^ that 

 success with this method is dependent upon certain factors, among 

 which were "vSequence of available entomophagous insects" and "Pos- 

 sibility of rearing or obtaining the entomophagous insects in suffijient 

 quantities." Until recently there was not available for the black scale 

 in California a satisfacton-^ sequence of natural enemies, there being 

 no effective parasite of the intermediate stages of the host in our fauna. 

 Rhizohius ventralis, the ladybird introduced by Koebele from Aus- 

 tralia, was of considerable importance as a predator on the young scale. 



'Jour. Econ. Ent. Vol. 12, 1919, p. 288. 



