Vertical stratification in the spatial distribution of the beech scale insect (Ultracoelostoma assimile) inNothofagustree canopies in New Zealand

Abstract

Abstract. 1. The degree of infestation by New Zealand sooty beech scale insects (Ultracoelostoma assimile, Homoptera: Margarodidae) varies dramatically among adjacent southern beech trees (Nothofagus spp., Fagaceae), but has previously been assumed to be uniformly or randomly distributed within individual host trees. In this study, a full‐census survey was conducted from ground level to canopy level on 14 naturally occurring, canopy‐dominant red beech (Nothofagus fusca) trees (size range 38.7–107.6 cm diameter at breast height) to determine the degree of within‐tree heterogeneity in herbivore density.2. The within‐tree distribution of the sooty beech scale was vertically stratified and highly heterogeneous, with the greatest densities occurring on bark surfaces in the canopy rather than on the trunk, and on the lower rather than upper sides of the branches. The spatial distribution was strongly negatively correlated with trunk and branch diameter, and increasing bark thickness (as a function of diameter) provides a plausible explanation for differences in the establishment and population density of sooty beech scale insects with trunk and branch size. Furthermore, there was a significant change in the spatial distribution of scale insect populations on trunks and branches of trees of increasing diameter at breast height. This indicates a strong temporal component to the spatial dynamics of the sooty beech scale insect driven by changing host phenology. Future studies on phytophagous insects infesting large host trees need to consider more explicitly changes in population dynamics through space and time.3. Because of the high degree of within‐tree heterogeneity in population density, the total population size of scale insects on an individual tree could not be predicted from any measure of population density low on the trunk. However, the dry weight biomass of sooty mould fungi growing on the ground beneath infested trees was a remarkably accurate predictor of the total population size of scale insects. The use of sooty mould fungi as a relative measure of population size could be incorporated into studies of other honeydew‐producing hemipterans, since the growth of sooty mould is a distinctive feature synonymous with high concentrations of honeydew production worldwide.

The spatial and temporal distributions of arthropods in forest canopies: uniting disparate patterns with hypotheses for specialisation

“…In New Zealand, Wardhaugh et al (2006) showed that the density of the sessile bark-dwelling phloem-feeding scale insect Ultracoelostoma assimile (Margarodidae) was higher on branches in the canopy than low on the trunks of Nothofagus fusca (Fagaceae) trees. The authors suggest that vertical stratification in U. assimile is probably a function of bark thickness, which is vital to locating an appropriate settlement site.…”

“…Thin bark does not provide enough crevices, which are preferentially colonised (McAllum, 1992), and if the bark is too thick, the insect's stylet cannot reach the phloem elements. Indeed, the lower trunks of smaller trees (10-30 cm diameter at breast height), which are approximately the same diameter as the heavily infested branches of large trees, also harboured high densities of U. assimile (Wardhaugh et al, 2006), indicating that vertical stratification was dependent upon resource accessibility/availability.…”

Arthropod vertical stratification in temperate deciduous forests: Implications for conservation-oriented management

“…For example, bees feed on honeydew, sap and other sugar sources in the absence of flowers (Elton, 1966) and the importance of honeydew to the longevity and fitness of hymenopteran parasitoids has been particularly well documented. Honeydew is probably more readily available high in the forest canopy (Moran and Southwood, 1982;Wardhaugh et al, 2006;Nickel, 2008), likely influencing the distributions of associated arthropods. For example, Ulyshen et al (2010b) found bees to be much more abundant high in the canopy than near the forest floor in the United States, especially in mid-summer when flowers were absent from the canopy and scarce near the ground.…”

Vertical stratification in arthropod spatial distribution research

“…Wardhaugh et al, 14 reported greater scale abundance in elevated side branches of Nothofagus forests and briefly discussed possible determinants. However, they did not directly assess the role of biological control.…”