The performance of the green spruce aphid (Elatobium abietinum Walker) on provenances of Sitka spruce (Picea sitchensis (Bong.) Carr.)

There is accumulating evidence that the origins of Sitka spruce Picea sitchensis (provenance or genetic family) play an important role in determining the population dynamics of the green spruce aphid Elatobium abietinum. Genetic variability in P.sitchensis makes it a good candidate for tree breeding, although recent genetic gains in 'super-Sitka' have not accounted for pest resistance explicitly. This paper reviews two studies, 11 years apart, on the aphid populations of P.sitchensis provenances in the same experimental field demonstration. The trends in susceptibility of the provenances were consistent, with the exception of the slowest growing and most northerly provenance. There were relationships between the latitudinal origins of provenances and their aphid loads; in general it was the most vigorously growing provenances that proved to be the most resistant to aphids, There are recorded differences between P.sitchensis provenances in the time of shoot growth cessation and budset in late summer and autumn. An hypothesis that earlier dormancy was linked with greater nutrient availability for aphids, was not supported by data on the levels of amino acids in needles in October. In fact total amino nitrogen was inversely related to the summer aphid population hosted by each provenance and most essential amino acids were at very low levels. Together with the similarity in budburst phenology among provenances, this suggested that factors other than nutrients are responsible for the differences in provenance susceptibility. The green spruce aphid is thought to avoid some species of spruce and also new foliage by responding to volatile compounds in epicuticular wax at the needle leaf surface. A resistance mechanism based on secondary metabolites would correspond with observations, since more-vigorous trees probably sequester more effective surface deterrents. In support of this, it was found that relative provenance susceptibility to aphids was correlated with feeding damage by a completely different insect, the large pine weevil Hylobius abietis. In a separate series of experiments, shoot material from P.sitchensis provenances was challenged with weevils of known age and cultural background, and the amount of bark removed in a feeding trial was recorded. The greatest levels of bark removal tended to be found on provenances which had hosted larger peak aphid populations. The most likely explanation for a fairly universal resistance mechanism probably derives from secondary plant metabolites rather than the nutrient levels available to both insects.