Foliar nitrogen: a basis for host suitability for elongate hemlock scale, Fiorinia externa (Homoptera: Diaspididae) [Tsuga canadensis]

The success of elongate hemlock scale, Fiorinia externa Ferris (Homoptera: Diaspididae) varied significantly on 14 of its coniferous host species and on Eastern hemlocks supporting different scale densities. An important factor determining host suitability was the concentration of nitrogen in the young foliage available to nymphs during their development. Nymphs residing on hosts whose foliage provided a relatively rich source of nitrogen suffered less mortality, developed at a faster rate, and ultimately produced more progeny than did nymphs on hosts with relatively low nitrogen foliage. Nymphs responded positively to increases in foliar nitrogen associated with fertilization, host phenology, and levels of herbivore-induced stress. Studies at two arboretums in southwestern Connecticut, USA revealed that the suitability of 14 host species to attack by F. externa was positively related to the nitrogen concentration and water content of the young foilage in mid-June, during peak colonization by nymphs. Interspecific differences in the concentrations of foliar nitrogen and water were attributable to differences in the degree to which the young foliage had matured by that time. Data from experiments on greenhouse, farm, and forest scale populations on hemlock indicated that feeding by nymphs significantly reduced the quantity (biomass) and quality (nitrogen concentration) of food available for subsequent consumption. Scales were more successful on trees which previously had supported lower densities. The availability of nitrogen to feeding nymphs underlies this mechanism of host suitability for F. externa.