As its name suggests, the Eucalyptus Longhorn Borer (Phoracantha semipunctata) is an Australian endemic beetle that is easily recognised by its long antennae and the distinctive holes it bores into suitable host species.
In severe cases, stressed trees hosting this pest can decline with astounding speed and can die within a matter of months.
In this article we will answer:
- How do I identify the Eucalyptus Longhorn Borer
- What is the life cycle of the Eucalyptus Longhorn Borer?
- How do I know if my tree is infected?
- How is the Eucalyptus Longhorn Borer managed?
- How can we help?
In this article we will answer:
Adult beetles:
Most obviously, adult beetles can be identified by their large antennae, spanning the length of their body. Beyond this distinctive feature, adult beetles can be identified by their reddish to dark brown colour, with beige or cream coloured markings on their wings. Fully grown adult beetles can reach anywhere between 15-23 mm in length, with female beetles typically growing to be larger than their male counterparts.
Adult beetles can be spotted flying around host trees during dusk when temperatures are above 15 degrees celsius.
Eggs:
Longhorn Borer eggs typically measure 2.5 – 3 mm in length, and appear yellow in colour. Shaped like elongated ovals, these eggs can be found under loose or detached bark, clumped together in groups of 10-100.
Larvae:
Longhorn Borer larvae grow in a cylindrical shape advantageous to chewing holes in soft wood, and measure roughly 30-40 mm in length and 10 mm in diameter. Similar to their egg form, they are typically yellowish-white in colour. Unfortunately, Longhorn Borer larvae are particularly difficult to spot as they live inside tree branches and trunks greater than 5 cm in diameter and remain there, hidden, until they emerge as adult beetles.
Figure 1. Longhorn Borer eggs
Figure 2. Damage caused to tree by a Longhorn Borer
What is the life cycle of the Eucalyptus Longhorn Borer?
Longhorn Borer eggs laid under loose or detached bark can take 1-2 weeks to mature, depending on the weather conditions at the time. Precisely when and how many eggs are laid depend on the surrounding environmental conditions, and can occur numerous times during the summer in temperate climates or all year round in warmer climates.
After hatching, larvae development can take 6–180 days, again, depending on the environmental conditions.
As an adult, the beetle may be found throughout the summer season (or all year round in warmer climates) mating, feeding and ovipositing.
What damage do they cause?
During their time as larvae, the longhorn beetle chews through the soft wood underneath the host tree’s exterior bark, leaving hollowed out galleries within the tree. In addition to cutting off the nutrient transportation system of the tree, these hollow galleries can impact its structural integrity leading to limb failure.
Infested trees can also attract birds that will strip bark and dig at the tree in order to access the grubs, causing further damage to the tree in the process.
Figure 3. An adult Longhorn Borer
Figure 4. Longhorn Borer under tree bark
How do I know if my tree is infected?
The good news is, the longhorn beetle tends to attack damaged or stressed trees. Healthy, well-watered trees are rarely attacked though it does occur.
The bad news is, identifying infection is often difficult, as the telltale holes bored by larvae appear predominantly underneath the tree’s bark.
An infected tree may also experience die-back in its canopy and wilting leaves. Severe infestations can kill the tree, beginning with the death of infected limbs.
What species of trees host the beetle?
| Liable | Resilient |
| Eucalyptus globulus | Eucalyptus trabutii |
| Eucalyptus grandis | Eucalyptus citriodora ‘Lemon Gum’ |
| Eucalyptus diversicolor | Eucalyptus dairympleana |
| Eucalyptus vimanilis | Eucalyptus camaldulensis |
| Eucalyptus platypus | Eucalyptus sideroxylon |
| Eucalyptus nitens | Eucalyptus cladocalyx |
| Eucalyptus saligna | Eucalyptus robusta |
How is the Eucalyptus Longhorn Borer managed?
The best prevention against this pest is good tree health. Improving your tree’s health and reducing its environmental stressors can involve laying mulch, providing irrigation during droughts, refraining from using herbicides, and improving the soil condition.
In the case where an infected tree has been removed, there are a number of steps you can take to discourage the beetle from reestablishing. One such precaution involves stripping the bark from felled trees, eliminating the protected habitat for new eggs. Additionally, splitting and drying the wood removes the soft fresh wood larvae feed on.
In Tasmania and Australia broadly, there are sufficient natural predators to impact the survivorship of exposed larvae. These include parasitic wasps that eat the beetle and larvae, as well as ants that will carry away exposed eggs.
Figure 5. Longhorn Borer damage taken by Tasmanian Tree Care
Figure 6. Longhorn Borer damage taken by Tasmanian Tree Care
How can we help?
Tasmanian Tree Care provides professional solutions to myrtle rust for any commercial or residential property. If you’re unsure about the health of your natural assets, please contact us to organise an assessment. It’s true what they say: prevention is the better than a cure.
Images courtesy of Brisbane Insects and Spiders Home Page and the European Institute Of Planted Forest.
Content collected by Corbin Tattersall
Corbin is responsible for our operational systems and crew training. He is also one of the top tree climbers in Australia and regularly travels to compete. He has excellent communication skills and is extremely efficient in the tree, which makes him an asset to the business and a great mentor to our other climbers. Pull up a chair and watch a master at work, knowing you are being looked after by one of the most respected arborists in Tasmania.
Eucalyptus Longhorn Borer FACT SHEET
Want to learn more about Eucalyptus Longhorn Borer? Check out our detailed Fact Sheet below.
View Eucalyptus Longhorn Borer Fact Sheet View All Fact Sheets