452 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



leaves and fruit, and the fungus growing in it forms a thin 

 covering that seriously interferes with the function of the leaf 

 and very materially damages the fruit. 



Certain fungus diseases that attack the white-flies often 

 control them quite effectually but it is sometimes necessary to 

 resort to spraying. The resin wash has been much used. A 

 spray made by thoroughly emulsifying two gallons of paraffine 

 oil with one gallon of whale-oil soap and one gallon of water is 

 strongly recommended. One gallon of the stock emulsion 

 should be used in fifty gallons of water. December, January, 

 and February are the best months to spray for these insects, as 

 practically all of them are then in the larval stage. As it is 

 very difficult to reach thoroughly all of the foliage on an orange 

 or lemon tree with any spray mixture, it is much more satis- 

 factory, where possible, to fumigate with hydrocyanic acid gas. 

 If properly done at the season suggested for spraying, it will 

 effectively control the white-fly. 



There are two other species of white-fly, A. nubifera, and A. 

 howardi, affecting citrus trees, but they are similar in appear- 

 ance and habits to A. citri. 



Black Scale (Saissetia oka). This is perhaps the most 

 important scale-insect on the orange trees of the Pacific 

 Coast, not so much on account of the direct injury caused by 

 the loss of sap, which the myriads of insects are con- 

 stantly drawing from the leaves and branches, but because, 

 like the white-fly, it secretes a great deal of honey-dew in which 

 a sooty mould grows. This is an insect widely spread over the 

 world and long known as a pest of olives and oleanders. The 

 adult female is almost hemispherical in shape, a little longer 

 than broad, hard or leathery in texture and dark brown or 

 black in color. The younger stages are flatter and on the 

 dorsal side are marked by one longitudinal and two transverse 

 ridges, forming the letter "H." The adult also shows this H, 

 but less plainly. The black scale is not nearly as serious a pest 

 in the Gulf region as it is on the Pacific Coast. 



In 1900 there was introduced into California a small Hymen- 

 opterous parasite, Scutellista cyanea, of the black scale that 

 has proved to be, in some localities at least, an important factor 



