Three Insect Pests of Rhododendrons
P. H. Brydon
ILLUSTRATION No. 1: The scalloped margins of the leaf have been caused by the nocturnal visitations of the adult form of Black Vine Weevil (Brachyrhinus sulcatus). Control is effected by scattering, a commercial product known as "Go-West Applebait" on the ground around the plants. If this is applied according to directions and kept up at intervals of from ten days to two weeks for eight or nine applications, reasonable success will be assured. Certainly one or even two applications will not eliminate this pest and constant vigilance must be maintained if full control is de, sired. This particular pest has been discussed quite fully in the 1945 and 1918 Year Books of this Society and readers are referred to them for further information.
ILLUSTRATION No. 2: The mottled discoloration is caused by the adults and nymphs of the Lace Wing Fly (Stephanitis rhododendri) feeding on the lower leaf surfaces. The insects pierce the underside of the leaf and -.suck out the sap leaving the blanched markings seen in the illustration. The adult insect is about 1/8 inch long and 1/16 inch wide and has transparent lace like wings which are held fiat against the body. Eggs are laid along the veins on the leaf undersurface and hatch out in May. Control is obtained by spraying with a one percent solution of Volck to which has been added Black Leaf "40" at the rate of two teaspoonfuls to one gallon of the solution. It is important that the spray is directed towards the undersurface of the foliage and that it is applied during the last week in May or early June. One application usually suffices but in more severe infestations another spray may be necessary in ten days or two weeks.
ILLUSTRATION No. 3: This is a similar injury to No. 2 except that the mottled area is less well defined and usually not so evenly scattered over the leaf surface. This injury is caused by Thrip and their presence can be readily ascertained by the silvery appearance of the leaf undersurface and the small dark spots caused by deposits of excrement. The adult insect is flat bodied, about 1/20 inch long and narrow in proportion to its length. Under glass they may be found on both leaf surfaces but out of doors they usually occur on the under side where they feed by tearing the epidermis with their mouth parts and lapping up the plant juices. A forceful spray consisting of two teaspoonfuls of tincture of Green Soap to one gallon of water applied to the leaf undersurfaces and continued at ten day intervals will effectively control this pest.