Season and drought stress mediate growth and weight of the green spruce aphid on Sitka spruce

The most important defoliating pest of Sitka spruce Picea sitchensis in the U.K. is the green spruce aphid Elatobium abietinum. This aphid species is expected to respond to a predicted increase in the incidence of spring-summer drought events, with serious implications for growth of infested spruce plants and control of the pest. Growth rate, adult weight and nymph weight of E. abietinum were assessed under controlled laboratory conditions, using five levels of spring-summer drought of differing intensities and frequencies. Trials were time-staggered to investigate changes through season. Growth rate, adult weight and nymph weight were consistently increased under low amplitude intermittent drought stress, suggesting improved host nutritional quality through the year and increased fecundity. A complex response under severe and low amplitude intermittent drought stress was also observed, mediated by season. Although an overall reduction in host quality was typically observed, continuous severe stress improved host quality during the autumn. Despite this, the onset of dormancy was unaffected. The findings of the present study have implications for E. abietinum population development, Sitka spruce growth and biological control of the species under climate change and increased drought frequency.