Eucalyptus trees, native to Australia, are widely valued around the globe for their rapid growth and distinctive aesthetic appeal in landscaping and forestry. Their resilience and ability to thrive in poor soils and dry conditions have led to their widespread planting outside of their native range. However, when relocated to new environments, these species often lose the natural balance of predators that keep their specialized insect pests in check. This imbalance leaves them vulnerable to a few specific insect threats that can cause significant decline if not managed effectively.
Identifying the Primary Insect Threats
Psyllids, commonly known as jumping plant lice, are a major group of pests that feed on eucalyptus foliage, with the Red Gum Lerp Psyllid (Glycaspis brimblecombei) being a widespread threat. These sap-sucking insects target the phloem within the leaves, draining the tree of sugars and nutrients. The nymphs protect themselves by constructing a white, conical, sugary covering called a “lerp,” which is an accumulation of crystallized honeydew and wax.
Borers, especially the Eucalyptus Longhorned Borer (Phoracantha species), primarily attack the tree’s trunk and larger limbs. The adult beetles, recognized by their long antennae, are attracted to stressed or freshly cut wood to lay their eggs. Once hatched, the larvae tunnel aggressively beneath the bark, feeding on the tree’s critical vascular tissue.
Leaf-feeding beetles, such as the Eucalyptus Snout Beetle (Gonipterus scutellatus), are serious defoliators worldwide. Both the adult weevils and their larvae consume the foliage, with adults leaving characteristic scalloped notches along the leaf edges. The larvae are soft-bodied, slug-like creatures that feed voraciously on the leaf surface, which can quickly lead to severe stripping of the canopy.
Recognizing Damage Signs and Secondary Issues
Psyllid presence is visible through the accumulation of white, waxy lerps encrusting the leaves, often causing premature leaf drop. They also excrete sticky honeydew, which coats the leaves and surfaces below. This sugary residue fosters the growth of sooty mold, a black fungus that reduces photosynthesis.
Borer activity is identified by exit holes left by emerging adults and oozing sap or “gummy resin” flowing from the trunk and limbs. Beneath the bark, the larvae create extensive, winding galleries packed with frass, a mixture of sawdust and excrement. When these tunnels completely encircle the trunk, the borer has effectively “girdled” the tree, stopping the flow of nutrients and often resulting in rapid death.
Damage caused by primary pests invites secondary issues, as stressed trees become susceptible to fungal pathogens. Canker diseases (Cryphonectria species) frequently enter through wounds created by borers or mechanical injury. These infections create sunken, necrotic areas on the bark that can eventually girdle the tree or cause sections of the wood to split and die back. Other stress-related diseases, such as root rots caused by Phytophthora, can develop when trees are weakened by repeated defoliation or drought stress, further compromising tree stability and health.
Non-Chemical Strategies for Pest Management
The most effective initial defense against eucalyptus pests is maintaining the tree’s natural vigor through cultural control, as stressed trees are highly attractive to borers and psyllids. This includes providing deep, infrequent watering during dry periods, which mimics the natural environment and reduces drought stress. Proper irrigation ensures the tree can produce protective chemicals and resin to defend against larvae.
Sanitation is important for managing borers, which breed in fallen or freshly cut wood. Infested wood, including dead or dying limbs, should be removed, burned, or buried immediately to prevent new adult beetles from emerging and colonizing living trees. Pruning should be timed carefully, ideally during the winter months when adult borers are not actively flying and laying eggs, to avoid attracting them to fresh wound sites.
Biological control utilizes natural enemies to keep pest populations at bay, a successful long-term strategy for introduced pests. For instance, the parasitic wasp Psyllaephagus bliteus was introduced to control the Red Gum Lerp Psyllid and has provided substantial control, especially in moderate climates. These tiny wasps lay their eggs inside the psyllid nymphs, which kills the pest before it can mature and reproduce.
Targeted Chemical and Professional Interventions
When infestations are severe or localized on smaller trees, targeted chemical applications should start with the least toxic options. Horticultural oils and insecticidal soaps are contact insecticides sprayed directly onto psyllid-infested leaves. These products smother insects or disrupt cell membranes but require thorough coverage and are ineffective against pests protected by thick lerps or bark.
For larger trees or deep-boring pests, systemic insecticides are the most practical solution, as they are absorbed by the tree’s vascular system to kill feeding insects. Neonicotinoid compounds like imidacloprid can be applied as a soil drench or trunk injection, offering protection for growing seasons. Trunk injections are particularly advantageous in urban settings because they place the chemical directly into the tree, minimizing off-target exposure to beneficial insects and the surrounding environment.
Professional intervention by a certified arborist is necessary when infestations involve large trees, reach the trunk, or compromise tree structure. Arborists can accurately diagnose the specific pest and disease combination, which is often complex, and safely apply restricted-use systemic insecticides through injection methods. They can also assess the tree’s stability, which is vital if the trunk has been significantly damaged by borer galleries or canker.