Soil application of systemic insecticides for control of woolly apple aphid (Eriosoma lanigerum (Hsm.)) in Rhodesia

Abstract

The Woolly Apple Aphid (Eriosoma lanigerum (Hsm.)) is a pest of apple trees in Rhodesia where it attacks both the more superficial roots and the foliar portion of the tree. It is at present restricted to Inyanga, Odzani, Melsetter and Salisbury, and in most cases appears to have been introduced into the orchard on young stock. With the fruit industry rapidly expanding, it could easily become very much more widespread, and a considerable amount of work has been done in an attempt to find a suitable control measure.Carnegie (1963, 1965) studied methods of reducing the foliar population, and succeeded in obtaining good control. He also introduced into the country the parasite Aphelinus mali (Hald.), which has since become widespread, due to both natural spread and that assisted by man. It has not been found to be entirely effective in controlling the aphid and most growers consider that at least one spray a season is necessary. This is mainly due to the presence of the root population, which appears to be unavailable to the parasite and furnishes migrants that can infest the upper part of trees under suitable conditions.No insecticide known to the author is capable of controlling the root population when applied to the foliage, and for effective control it is necessary to reduce the root population so as to prevent reinfestation from this source. Myburgh & van Nierkerk (1964) obtained good control of both the foliar population and the root population with a soil application of phorate either in granular form or as an emulsion. Carnegie (1962) tested two nematicides (DBCP (l,2-dibromo-3-chloropropene) and V-C 13 (0,0-diethyl 0-2,4-dichlorophenyl phosphorothioate)), for the same purpose, and with V-C 13 obtained control of the root populations but not of the foliar populations. The DBCP was ineffective. Later (1965) he tested applications of dimethoate at the rate of \ fl. oz. per tree: in one treatment diluted in four gallons of water and applied in a circular six-inch furrow surrounding the stem, and in another diluted in four parts of water and injected into the soil at 25 points with a nematieide injector. Neither appeared to be effective.The experiments described below were carried out in an attempt to find a suitable chemical that would give good control of both the foliar and root populations in one application.The considerations which had to be borne in mind when designing these experiments were that the application should be easy, relatively cheap, needed as infrequently as possible and should control simultaneously both the root and foliar populations and that it should affect the wasp parasite population as little as possible.It was decided that these requirements would best be met by the application of a systemic insecticide to the drip area of the tree, as this was the method most calculated to control the foliar and root populations, to have little effect on the parasite, and to be easy to use. The question of cost and persistency would depend on the efficiency of the insecticide.