DISEASES AND INSECTS. S9 



heated houses, The principal species that have been grown in such frames 

 are E. chloranthus, E. Fendleri, E. gonacanthua, Opuntia oamanchioa, 

 O. hystricina, and O. Whipplei ; but, as was observed in the earlier 

 chapters, a large number of Caoteaa are benefited by being so treated 

 during the summer months. 



DISEASES AND INSECTS. 



Cactaceous plants are subject to few diseases, and the only one which 

 gives any trouble to the cultivator is a decay of the stem, which is some- 

 times very rapid. It is oooasioned by excessive moisture in the soil, or 

 more frequently perhaps by a drip from the roof of the house, and can be 

 guarded against by the exercise of a little care. If a decay spot is ob- 

 gerved it is a good plan to rub a little lime on it or cut the piece out, and 

 fill the hole with lime ; but very often when the injury is first noticed it is 

 too late to effect a cure, so that preventive measures are in this, as in 

 many other matters, much the best. Numerous insects attaclt these 

 plants — green fly, red spider, mealy bug, and various isinds of scale, but 

 the two last-named are the most troublesome and difficiUt to eradicate. 

 The mealy bug in particular harbours in the clusters of spines, and is not 

 easily removed by syringing, a small stiff brush being the most useful and 

 ready means of cleaning the plants. Scale can be removed in a similar 

 way, or with a small pointed stick, and green fly can be easily destroyed 

 by moderate fumigation with tobacco. An excessively dry hot atmosphere 

 causes the appearance and rapid increase of red spider, which soon 

 eifectuaUy injures the plants. Cooler quarters, with frequent syringing, 

 or sponging with softsoap and water, will soon diminish their numbers, 

 while diluted petroleum is strongly recommended by some porsoas for 

 the same purpose, Mr. Wright, who has charge of the Cromwell House 

 collection, relying exclusively upon this as an effective insecticide. 



