Defoliation and growth loss in young Sitka spruce following repeated attack by the green spruce aphid, Elatobium abietinum (Walker)

Green spruce aphid, Elatobium abietinum (Walker), is a serious defoliator of Sitka spruce (Picea sitchensis (Bong.) Carr.). In a field experiment in Hafren Forest, mid-Wales, different population densities of E. abietinum were created amongst plots of P91 Sitka spruce by applying insecticides and artificially infesting trees with aphids. Trees subjected to high aphid populations (HP treatment) lost 51, 29 and 15% of their older needles because of infestation in 1994, 1997 and 1999, respectively, whereas trees with low aphid populations (LP treatments) suffered only 15% defoliation in 1999. Aphid numbers, defoliation rates and tree growth were monitored from 1993 to 2000, to determine: (1) relationships between defoliation and aphid density, (2) the impact of repeated infestations on spruce growth, and (3) whether previous defoliation influenced subsequent infestation and damage. Defoliation rate showed an asymptotic relationship with E. abietinum density in each year, and was higher relative to aphid density for older age-classes of shoots. Relationships obtained in 1997 and 1999 confirmed previous observations that low E. abietinum densities cause relatively high rates of needle loss. Repeated defoliation in the HP treatment reduced height and volume growth by 8 and 6%, respectively, by the end of the experiment. Reductions in volume increment were related to changes in total needle DW per tree. In 1999, all trees in the experiment were colonised by E. abietinum because of a natural increase in aphid populations, but significantly fewer aphids were recorded on the previously defoliated trees in the HP treatment (P = 0.007). However, despite lower aphid densities, defoliation rates were similar and growth losses were higher in the HP treatment, suggesting that the differences in aphid density were primarily a reflection of changes in general tree vigour, rather than a result of induced changes in host quality.