HORNET CLEARWING MOTH (SESIA APIFORMIS [CLERCK]) AND DIEBACK OF POPLARS IN EASTERN ENGLAND

Abstract

Summary Poplar trees showing severe dieback, or defoliation and death within a growing season, have been reported during recent years from the region around Bedford, Milton Keynes and Northampton. Many of the affected trees have also been found to be attacked by the hornet clearwing moth, Sesia apiformis (Cl.). Larvae of this moth tunnel beneath the bark of the lower trunk and roots, and it has been suggested that the moth might be the cause of the decline in tree health. However, a survey of 801 poplar trees in 1999 confirmed that, although there was a correlation between the amount of dieback and the number of moth exit holes visible in the trunk (r2 = 0.38), there was considerable variation in the relationship and a significant number of trees showed severe dieback but no evidence of infestation. The data demonstrate that dieback in many trees cannot be attributed to S. apiformis and that the moth most likely acts as a secondary agent in tree decline. Up to 53 per cent of trees with severe dieback occurred in shelterbelts, and many of these sites were situated on reclaimed land or other man-made substrates. The area where dieback has been most prevalent is one of the hottest and driest parts of the UK, and the reports of dieback in poplar and the apparent increase in S. apiformis activity, follow particular dry springs and early summers in 1995 and 1996. The available evidence suggests, therefore, that the deterioration in the health of poplars in this region is not caused directly by S. apiformis, but by a combination of climate and perhaps human influences.