Interactive effects of host resistance and drought stress on the performance of a gall-making aphid living on Norway spruce

Four full-sib families of Norway spruce (Picea abies), of which two were re sistant to a galling aphid (Adelges abietis) and two were susceptible, were exposed to drought stress for 2 years. The primary aim was to test the hyp othesis that the various life stages of the aphid differ in their response to environmentally induced changes in the host plant. The drought treatment had a significant negative effect on tree growth. This was reflected in ga ll size which responded in a similar way to the drought stress, thus suppor ting the plant vigour hypothesis. Drought affected the survival of aphid st em-mothers negatively in susceptible trees but positively in resistant tree s. This result was matched by the response of an individual phenolic compou nd which, contrary to the total phenolic concentration, tended to increase in susceptible trees exposed to drought and decrease in similarly exposed r esistant trees. Thus it is possible that this single, as yet unidentified, phenolic compound could be used as a marker of resistance. The performance of surviving stem-mothers, evaluated by measuring the diameter of the wax c over they produced (a correlate of fecundity), was not significantly affect ed by drought, but aphids on susceptible trees produced more wax than those on resistant trees. The change in gall density over time (analysed separat ely for each treatment and phenotype) correlated best with patterns of stem -mother survival. This indicates that stem-mother survival, which is closel y linked to host plant quality and resistance, may play a significant role in the population dynamics of this aphid. However, drought-stress-induced c hanges in host plant quality affected survival, and hence gall density, les s than the genetically determined level of resistance. The results also sup port the hypothesis that an insect may respond differently to environmental ly induced changes in the host plant at different stages in its development .