Under plantation conditions characterized by uniform irrigation, uniform tree age, and uniform tree spacing, treatment of Eucalyptus rudis trees by soil injection of imidacloprid was highly significant for reducing red gum lerp populations. Previous studies have demonstrated that bark moisture content of Eucalyptus trees is critical resistance to colonization by the eucalyptus longhorned borer. If bark moisture is maintained above approximately 55%, then the young beetle larvae are incapable of penetrating through the bark tissue. Thus, irrigation of trees to maintain critical levels of bark moisture has been an important cultural management too. However, moisture and fertilization could make the trees more susceptible to the psyllids. Our results demonstrate that trees under low irrigation treatments had significantly higher densities of psyllids than trees maintained under high levels of irrigation. Trees with high levels of nitrogen fertilization had significantly higher densities of psyllids than trees maintained under low levels of fertilization. Not surprisingly, analysis of the combination of treatments demonstrated that trees maintained under conditions of high irrigation and low fertilization were least colonized by psyllids.