August, '21] smith: black scale control 349 



and Scutellista cyanea, a parasite which feeds on the eggs beneath the 

 parent scale, at times did good wjrk. The introduction into Califor- 

 nia in 1919 of Aphycus lounsburyi How., an effective internal parasite of 

 the immature rtages of the scale, fills a conspicuous gap and completes 

 the sequence. At about the same time the writer demonstrated 

 that black scale, like mealybugs, could be grown under laboratory- 

 conditions on potato vt'prouts, so that during the summer of 1919 con- 

 ditions seemed to warrant an attempt to bring about a control Df the 

 pest by the biological method. 



Accordingly, in September, 1919, a second insectary was established 

 at Alhambra for the purpose of propagating and distributing natural 

 enemies of the black scale, and two plots of trees selected for the pur- 

 pose of demonstration, one at Alhambra and the other at Santa Paula, 

 Ventura County, on the Limoneira ranch, where the management had 

 volunteered to maintain an insectary imder supervision of the State 

 Department of Agricultiire. A block of 54 infested trees on the Li- 

 moneira ranch was set aside, imfumigated, for this work. Aphycus 

 lounsburyi was colonized on these trees, began breeding with rapidity, 

 and by May of the following year had rendered them commercially 

 free of scale. The Limoneira company were so encouraged by this 

 demonstration that they then set aside a block of 10,000 infested trees. 

 The same performance was repeated, together with colonizatons of 

 Rhizobius centralis and Scutellista cyanea, and at the present writing 

 75% of the scale have been destroyed and there is no doubt but that 

 by spring this orchard, too, will be commercially clean. The Aphycus 

 particularly has done wonderful work, the adults Hterally swarming 

 on the trees on warm sunny days. 



It was soon foimd, however, that while the biological control was 

 apparently a simple matter imder certain conditions, under others it 

 was a more difficult problem. At Santa Paula and various other citrus 

 districts along the sea coast, the black scale has an uneven hatch, that 

 is to say, almost any stage of scale may be found at almost any season, 

 although many entomologists insist that there is only a single genera- 

 tion. On the other hand, in the interior districts, the hatch is very 

 even, the scale being largely of one stage at any given time. This has: 

 a very important bearing on the propagation of Aphycus lounsburyi, 

 the most effective natural enemy, in the orchards. Aphycus, breeding 

 on certain stages only, is unable to maintain itself in numbers in those 

 localities where, for a period of several months, those stages of the 

 scale favorable to its development are not present or are very scarce 

 in the orchards. On the other hand, in districts where the scale has 



