Intra- and inter-crown distribution of the eastern spruce gall adelgid, Adelges abietis (L.), on young white spruce.
The intra- and inter-crown distribution of Adelges abietis [Sacchiphantes abietis] on young Picea glauca was studied at 3 sites in New Brunswick. Within a branch, the majority of galls were found on lateral shoots. A similar distribution of galls within trees was observed regardless of whether 20 or 30 lateral shoots, or all shoots on a branch were sampled. Gall densities were highest on mid-crown branches of open-grown trees 1-4 m tall, and were not influenced by cardinal direction. After crown closure, most galls were found in the upper crown, above the point of branch overlap. In all circumstances, gall distributions were strongly clumped within trees. The inter-tree distribution of S. abietis galls differed significantly from a Poisson but not a negative binomial distribution, indicating a high degree of aggregation among trees. Therefore, a stratified random sampling plan using 20 lateral shoots of a mid-crown branch as a sampling unit would be adequate for monitoring A. abietis populations. If the mid-crown branches were undergoing crown closure, it was suggested that samples should be taken in the lowest open-grown branch above the point of crown overlap.