Scale insects consistently affect roots more than shoots: The impact of infestation size on growth of eucalypt seedlings

1 Different infestation levels of the phloem-feeding scale insect, Eriococcus coriaceus Maskell (Hemiptera: Eriococcidae), consistently reduced root growth more than shoot growth in seedlings of Eucalyptus blakelyi Maiden (Myrtaceae). 2 Low levels of scale insect infestation significantly decreased root and lignotuber biomass but not shoot biomass. High levels of infestation , however, adversely affected all plant parts. Root and lignotuber biomass declined linearly, while the responses of stem and leaf biomass were nonlinear, with respect to increasing total insect load. 3 Plant responses to scale insect infestations may be explained by considering sap-sucking insects as additional sinks that compete against plant sinks and both redirect and drain resources from the plant. 4 Scale insect populations exhibited strong density dependent effects, suggesting they were severely depleting resources within their hosts. Extensive shoot death at high levels of infestation may have contributed to insect population decline by causing scale insects to die prematurely or by reducing the availability of suitable feeding sites.