74 



MISC. PUBLICATION 626, U. S. DEPT. OF AGRICULTURE 



attacked by the pitted ambrosia beetle 

 (Corthylus punctatissimus (Zimm.)). 

 Other shrubs and young trees may be 

 similarly attacked, including hazel, 

 huckleberry, ironwood, sassafras, sugar 

 maple, and water birch. The beetle 

 makes a number of horizontal galleries 

 in the wood at the base of the stem, caus- 

 ing it to wilt, die, and later break off 

 near the surface of the ground. Heavily 

 infested plantings of rhododendrons soon 

 become ragged and unsightly in appear- 

 ance, owing to the weakened, stunted, or 

 dying plants. The beetle is dark brown 

 or black, stout, and about y 8 -inch long. 

 The grub is small, whitish, and provided 

 with strong jaws for boring. 



Treatment. — Cut out and burn the 

 wilted stems. Take care not to break 

 the shoots at the base where the beetles 

 enter the plant ; otherwise some of them 

 may escape and infest nearby plants. 

 Remove the surface mulch from around 

 the base of the plants. 



Rhododendron Whitefly 



The rhododendron whitefly (Dialeuro- 

 des chittendeni (Laing)) has only recently 

 been discovered in this country. It is a 

 serious pest in the State of Washington, 

 although infestations have also been 

 found in Long Island, eastern Tennessee, 

 West Virginia, and Pennsylvania. The 

 sucking of plant sap by the whiteflies 

 causes a yellow mottling on the upper 

 surface of the leaves, sometimes accom- 

 panied by a curling of the leaf margins. 

 As with other whiteflies, the formation 

 of sooty mold fungus on the honeydew 

 which these insects excrete soon ruins 

 the ornamental value of plants attacked. 

 One generation occurs annually in 

 Washington, and the insects overwinter 

 chiefly as second- and third-stage larvae. 



In general appearance the adults (fig. 

 121, A) and the several immature stages 

 (fig. 121, B) are not unlike those of the 

 greenhouse whitefly and other species of 

 whiteflies. 



Treatment. — In the Pacific North- 

 west spraying w T ith a 2-percent white-oil 

 emulsion, preferably in the fall before 

 frost occurs, has proved most effective. 

 It is essential to wet thoroughly the 

 under sides of the leaves where the 

 insects are located. Applications should 

 be made on cloudy days and under con- 

 siderable pressure. Nicotine sulfate 

 applied as a spray or dust is effective in 

 reducing the number of adults, but is of 

 less value against the immature stages. 



Figure 121. — Rhododendron whitefly: 

 A, Adults ; B, larvae (two smaller forms) 

 and pupae (larger forms). About 8 

 times natural size. 



Other Pests of Rhododendron 



Page 



Azalea bark scale 22 



Red spiders 11 



Azalea stem borer 24 



Black vine weevil 93 



Fuller's rose beetle 44 



Oleander scale 54 



Giant hornet 57 



ROSE 



Flower Thrips 



Garden roses are attacked by several 

 species of thrips, of which the flower 

 thrips (Frankliniella tritici (Fitch)) is 

 the most important. The thrips enter 

 the developing flower buds and feed on 

 the tender flower parts, causing the 

 petals to become flecked and discolored 

 (fig. 122) and the flowers deformed. 

 Often the flowers fail to open. The 



