Beech Resistance to the Beech Scale: A Variety of Defenses

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Abstract

The scale insect,Cryptococcus facrisuqa, is monophagous onFacrus spp. It is indigenous to Europe, principally onF.sylvatica but has been accidentally introduced into North America where it attacks the nativeF.cyrandifolia. Other important forest species are also susceptible, for example,F.orientalis in Iran (Adeli 1975). Although we will be concerned exclusively with host resistance and with the ecology of the insect, we should stress that its reputation as a pest of beech is very largely due to its association with a weakly pathogenic fungus,Nectria coccinea. These two organisms together are responsible for the disease syndrome known as beech bark disease. Typically, the bark of trees heavily infested withCryptococcus becomes infected with Nectria. The subsequent death of the bark may be sufficient to kill the tree. The fungus occasionally attacks trees stressed by factors other than scale infestation, as occurred on mature trees following the extreme drought over much of Europe in 1976 (Lonsdale 1983a). Further details of the complex etiology of this disease can be found in a number of reviews (Ehrlich 1934, Houston et al. 1979b, Lonsdale and Wainhouse 1987).