Some Consequences of the 1975–1976 Drought for Dutch Elm Disease in Southern England

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Abstract
During the exceptionally hot, dry summer of 1976 the large elm bark beetle, Scolytus scolytus , was able to invade and successfully breed in apparently healthy, undiseased elms. A conspicuous feature was the clustering of wasps, bees and ladybirds on the stems, attracted by sap exudation. The trees were probably suffering from drought, although many showed no symptoms. The phenomenon occurred mainly on shallow soils over limestone; trees on chalk were not generally affected. In 1976, and to some extent in 1975, the downward development of Ceratocystis ulmi through the tree was less than in previous years, and only a low proportion of the numerous maturation feeding wounds became infected. However the drought does not appear to have significantly influenced the course of the epidemic. The attacks by S. scolytus on undiseased trees are discussed in respect of 19th Century reports of attacks on elms, and it is concluded that the ability of S. scolytus to assume a more primary role may have been underestimated.

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