Frequency and intensity of drought stress alters the population size and dynamics of Elatobium abietinum on Sitka spruce

The green spruce aphid, Elatobium abietinum, is the most important defoliating pest of Sitka spruce, Picea sitchensis, in the UK. Populations are expected to increase in response to predicted climate change, therefore placing Sitka spruce under increased risk of widespread and severe defoliation. The effect of spring–summer drought stress on E. abietinum population dynamics and development over multiple years was assessed in a field experiment under five different drought treatments with differing intensities and frequencies. The impact on host tolerance, in terms of needle retention, was also investigated. No differences in the length of time taken to reach peak population size were observed. Despite this, E. abietinum populations were found to respond positively to low amplitude intermittent stress, with high densities maintained for longer after the peak. Spring peak densities of aphids did not differ among drought levels, although an autumn peak was observed consistently over 2 years on trees subjected to continuous severe drought. This suggested advancement in the onset of Sitka spruce dormancy. Aphid infestation significantly increased percentage needle loss under all drought treatments, although no differences were observed between drought levels. An interaction between aphid presence and drought treatment was, however, observed during a second year of drought where no aphids were present. The study herein presented has contributed to the understanding of E. abietinum population responses under a changing climate. The implications for damage to host Sitka spruce are of relevance to forest management strategies, as an increase in drought events are predicted in the UK.