Eucalyptus trees, prized for their aromatic foliage and unique peeling bark, are widely grown globally. Although these Australian natives are generally robust, growing them outside their native habitat often exposes them to specialized pests that cause significant damage. Environmental stress weakens the trees’ natural defenses, making them vulnerable to insect colonization. Understanding the signs of infestation and appropriate management strategies is the first step in maintaining the health and longevity of these trees.
Identifying the Most Common Eucalyptus Pests
The Eucalyptus Lerp Psyllid (Glycaspis brimblecombei) is a sap-sucking insect signaled by a sugary excretion called honeydew. The insect is small, but the most obvious sign is the protective white, waxy cap, or “lerp,” which the nymphs construct over themselves. These hemispherical lerps resemble small white scales on the leaf surface. Heavy infestations cause the leaves to yellow and curl before dropping prematurely. The sticky honeydew provides a medium for the growth of sooty mold, a black coating that reduces the tree’s ability to photosynthesize.
A more serious threat is the Eucalyptus Longhorned Borer (Phoracantha species), a beetle that attacks the structural integrity of the tree. Adult beetles are recognizable by their long antennae and their dark brown and yellow markings. The most damaging stage is the cream-colored, legless larva, which bores into the tree immediately after hatching. Larval feeding occurs beneath the bark in the cambial layer, which transports water and nutrients. This internal feeding creates extensive galleries that can effectively girdle a branch or the entire trunk. External signs of infestation include branch dieback, wilting foliage, and circular or oval exit holes left by emerging adult beetles. Infested areas may also exude a dark, sticky sap or resin as the tree attempts to repel the invader.
A third common group of pests is the Scale Insects, such as the Gumtree Scale, which appear as small, immobile, waxy capsules clustered on leaves and stems. Like the psyllid, scale insects feed by sucking sap from the vascular system, leading to leaf yellowing and eventual defoliation. The presence of honeydew and the resulting black sooty mold growth are reliable indicators of a scale infestation.
Cultural and Physical Control Techniques
Maintaining optimal tree health is the most effective preventative measure against most eucalyptus pests, as stressed trees are highly attractive targets. Proper irrigation requires deep, infrequent watering to encourage a robust root system. Drought stress, particularly during hot summer months, makes a tree highly susceptible to attack by borers and psyllids. Watering should be applied beneath the outer canopy, allowing the water to penetrate the soil deeply without creating surface moisture that promotes disease.
Avoid excessive use of high-nitrogen fertilizers, which increase pest susceptibility. High nitrogen levels stimulate the soft, succulent new growth that psyllids and other sap-suckers prefer for feeding and egg-laying. Necessary pruning should be scheduled for late winter or very early spring, typically between February and March. This timing allows pruning wounds to heal quickly as the tree exits dormancy, avoiding summer months when longhorned borers are most active and attracted to fresh cuts.
Sanitation is a practical defense, especially against the longhorned borer. Cut or fallen eucalyptus wood, including stored firewood, can serve as a breeding ground for the beetles. Infested wood should be promptly chipped, burned, or covered tightly to prevent the emergence of adult borers. For small trees affected by lerp psyllids or soft scale, a strong jet of water from a garden hose can physically dislodge the insects and their protective coverings. Repeated applications can significantly reduce pest populations.
Targeted Treatment Options
When cultural controls are insufficient, targeted intervention often begins with biological methods. Biological control involves introducing a pest’s natural enemy to regulate its population, a method highly effective against several eucalyptus pests. For the redgum lerp psyllid, the parasitic wasp Psyllaephagus bliteus lays its eggs inside the psyllid nymph, killing the host. Evidence of this activity is a small, round exit hole chewed into the protective lerp cap. The parasitic wasp Avetianella longoi targets the longhorned borer’s eggs. Because these wasps are host-specific, they pose no threat to humans or other insects and are best conserved by avoiding broad-spectrum insecticides.
Chemical treatments are typically reserved for heavy infestations and primarily involve horticultural oils or systemic insecticides. Horticultural oils, which include mineral and neem-based products, smother soft-bodied pests like scale insects and psyllid nymphs. The oil works by clogging the spiracles, or breathing pores, causing death by suffocation. These oils also dissolve sticky honeydew and sooty mold, allowing them to be washed off the foliage. Application requires complete coverage, but oils must be applied during moderate temperatures to prevent leaf injury. For large trees or persistent problems, systemic insecticides containing active ingredients like imidacloprid can be used. These chemicals are applied as a soil drench or trunk injection, absorbed by the tree’s vascular system, and circulated throughout the plant tissue. Systemic treatments should be applied in the fall or early spring for best uptake, and timing must be managed to occur outside of the tree’s flowering period to minimize risk to beneficial pollinators.