Eucalyptus Longhorned Borer – The Garden Pest Extraordinaire

If you have eucalyptus plants in your garden, you need to know all about the eucalyptus longhorned borer, if you want to keep them safe.

Borers are known to be one of the most devastating types of hardwood pests, and rightly so. 

The eucalyptus longhorn borer larva girdles trees (discussed in detail later) and can potentially kill them.

The eucalyptus tree genus falls among the most popular urban forest trees. 

Unlike many other tree species, the risk of fires from dead leaves piling up in loads under trees is lower. 

This also makes eucalyptus longhorned borer infestation a common problem. Let’s find out more about these pests and how you can tackle them.

What Is a Eucalyptus Longhorned Borer?

Eucalyptus longhorned borer refers to two closely related borer beetle species – Phoracantha semipunctata and Phoracantha recurva. 

The former originated in Australia and was eventually spotted in Southern California in 1980. 

The latter was discovered in 1995 in several Californian counties – San Bernardino, Los Angeles, Orange County, and Riverside.  

Adult eucalyptus longhorned borers feed on the pollen of eucalyptus trees and lay eggs in the bark of eucalyptus trees. 

Stressed and freshly cut trees are particularly at risk from these borers.

What Does It Look Like?

Growing to a size of about an inch with brown bodies, adult eucalyptus longhorned borers have segmented antennae. The legs and antennae are of a reddish-brown color.

Like all beetles, they feature a pair of elytra, or hard hind wings.

Yellow-brown in color, these elytra have a thick black band crossing both their posterior ends. Each wing also features two vaguely connected spots.

The larvae resemble white grubs and are often over an inch long. However, you’ll rarely see them as they remain hidden within the tree bark.

What Damage Does It Cause?

Moving on, let’s check out what kind of damage Eucalyptus longhorned borers can cause. 

The discoloration and the wilting of eucalyptus leaves are among the first symptoms of an infestation. 

Eventually, the tree limbs will begin to die back too. As the larvae continue to feed on the tree, oval-shaped holes will start appearing in the bark.

The problem with P. semipunctata/recurva infestation is that the larvae destroy the cambial layer – the layer responsible for the transportation of water and nutrients in trees. 

If left unchecked, the infestation will spread throughout the circumference of the tree and may remove a strip of bark (known as girdling).

You may also notice a liquid oozing out of the trunks and limbs of infested trees. It’s a resin produced by the trees to resist a borer attack.

Different eucalyptus species display varying levels of resistance. It depends on the attributes of eucalyptus trees. 

For instance, a drought-resistant species will also be less vulnerable to borer infestations.

Longhorned borer attacks are particularly common during the hot summer months when the trees are stressed. 

When infesting healthy trees with plenty of moisture content, the larvae may suffer mortality in trees by drowning. 

Opening up a tree infested with these pests will also reveal large networks of tunnels bored by the larvae. These tunnels are usually filled with their excrement.

Lifecycle of the Eucalyptus Longhorned Borer

The lifespan of a eucalyptus longhorned borer may vary a lot, depending on the season. In summer or spring, three to four months are enough for them to complete their life cycle. 

The ones hatching in winter take longer, up to nine months. The four life cycle stages are as follows:

Egg: Female beetles of this species lay up to 300 eggs, distributing them into groups of 3 to 30. These eggs are typically laid in loose bark and take around a week or two to hatch.

Larva: Eucalyptus longhorned borer larvae may take anywhere from 70 to 180 days to develop, depending on the moisture content in the wood. 

The drier the eucalyptus wood, the slower would be their development process. 

These larvae start boring tunnels and mine inward, going through the bark to reach the cambium and sometimes the xylem.

Pupa: Once the larvae are done feeding and ready to pupate, they create pupal chambers in the wood. 

Plugging off the entrances with wood shavings and frass, they remain inside and pupate into adults.

Adult: The adult beetles chew their way out through the wood shaving and frass plugs I mentioned earlier. They eventually lay eggs again, and the cycle continues.

Depending on the egg-hatching timings, there might be two to three generations of eucalyptus longhorned borers per year.

How To Get Rid of Eucalyptus Longhorned Borer?

If you’re dealing with a eucalyptus longhorned borer infestation, here are a few remedies that you may try.

Cultural remedies

• As water-stressed trees are more vulnerable to borer attacks, make sure to irrigate the eucalyptus trees well. Apply the water at a small distance from trees directly below the edge of the canopies. Don’t water too close to the tree – it would promote root disease.
• If you’re keeping eucalyptus logs for use as firewood, try to dry them quickly or solarize them. Any amount of moisture may promote beetle generation.
• Heavily-infested host trees might need to be cut down, burned, or chipped to prevent the spread of the pests.

Biological remedies

• Specialized parasites can get rid of an infestation by destroying the borer eggs. A. Longoi, an encyrtid egg parasitoid, is particularly effective.
• If you’re going to try biological remedies, avoid applying any persistent and broad-spectrum insecticides on the bark.

Chemical remedies

• Managing eucalyptus longhorned beetles requires systematic insecticides, i.e., insecticides that move through trees along with water and nutrients.
• Chemical remedies might be especially necessary against P. recurva due to its resistance to biological remedies.

Apart from these, you can use tree care kits to keep your eucalyptus trees healthy, strong, and resistant to infestations.

Frequently Asked Questions

Wrap Up

As we can see, the two borer variants in question can cause significant damage to eucalyptus trees and may even result in tree mortality. 

Protect your eucalyptus trees from drought stress to keep them safe and take the necessary steps to treat a borer infestation. 

Thanks for reading, and I hope you can control the borer infestation before it grows out of hand.