Eucalyptus Longhorned Borer Management Guidelines--UC IPM

Two closely related species of longhorned borer beetles (family Cerambycidae) attack eucalyptus trees in California. Phoracantha semipunctata, which is native to Australia, was introduced into Southern California in the 1980s. It rapidly became a pest and now appears throughout the state wherever eucalyptus trees grow. Natural enemies were introduced from Australia, and biological control combined with improved cultural care of eucalyptus have dramatically reduced the number of trees this borer kills each year.

In 1995, a second species of longhorned borer, Phoracantha recurva, was discovered in Southern California in Los Angeles, Riverside, Orange, and San Bernardino counties. This beetle has spread throughout much of California wherever eucalyptus grows. However, biological control has been less effective against this new borer. P. recurva, along with several other new pests, can stress and kill eucalyptus.

Freshly cut wood, dying limbs, and trees suffering from stress, especially drought stress, attract both of these beetles. Many eucalyptus trees in California grow in unmanaged or minimally managed environments with no irrigation. Eucalyptus species that grow in wetter areas of Australia have been planted in California, so when they experience prolonged dry periods, these species are especially susceptible to being attacked and killed by borers.

IDENTIFICATION AND LIFE CYCLE

Adults of the two Phoracantha species resemble each other. Their antennae are as long as or longer than their body, and the antennae of the males have prominent spines. They differ, however, in the number of hairs on their antennae and their wing color. For example, dark brown predominates on the wing covers of P. semipunctata, while yellow to cream color predominates on wing covers of P. recurva (Table 1).

Several nights after emerging from infested wood and mating, the female beetles of P. semipunctata begin laying eggs in groups of 3 to 30 beneath the loose bark or in crevices on bark surfaces of eucalyptus trees. Females can live 1 or more months and lay up to 300 eggs, which hatch in about 1 to 2 weeks depending on temperature.

After hatching, larvae bore directly into the inner bark, or they feed temporarily on the bark surface before boring in. If feeding on the surface, outward from the egg mass they leave a distinct, dark trail 1/4 inch to several inches long that scores the bark surface. Most larval feeding occurs in the cambium, the thin layer of tissue on the bark’s inner surface. There larvae bore galleries that can extend several feet. In trees with heavy infestations, the scraping sounds the developing larvae make while chewing in the cambium are audible from a distance of several feet.

At the end of the feeding period, larvae excavate pupal chambers in the wood. They enter the chambers, packing the holes behind them with frass (excrement) and wood shavings. Larvae require about 70 days to develop in fresh wood or logs during hot summer months and up to 180 days in drier logs. Only relatively fresh logs sustain beetle larvae; old, dry logs are too hard for larvae to feed on and then successfully develop. Mature larvae can be more than 1 inch long and are cream colored and legless.

Following pupation, adult beetles emerge from the same holes by chewing through the frass plugs. During spring and summer, the beetle requires 3 to 4 months to complete its life cycle, but starting in fall and winter it can require up to 9 months. There are two and possibly three overlapping generations a year. Larvae can be present anytime during the year, and adults can emerge from pupae anytime from April through October.

The life cycle of P. recurva differs somewhat from that of P. semipunctata. P. recurva develops more quickly and completes a generation in a shorter period of time. P. recurva adults can be present for a longer period during the year and emerge as early as February.

The specific amount and frequency of water needed vary greatly depending on the site and tree species. A general recommendation is to irrigate eucalyptus trees infrequently (possibly once a month during drought periods) but with sufficient amounts so the water penetrates deeply into the soil (1 foot or more below the surface). You can achieve this by applying water slowly through drip emitters that run continuously for several days. In areas without an established irrigation system, you can use a water tank truck to temporarily flood soil. However, avoid prolonged waterlogging, especially around the root crown, because eucalyptus trees are susceptible to pathogens that cause Armillaria and Phytophthora root rots.

Protect trees from injuries. Remove infested trees and branches immediately, but do all other pruning during December and January when adult beetles aren’t present. When planning extensive limb removal for reasons other than removing dead wood or eliminating hazardous trees, space the trimming over several years. Otherwise, the tree might not have adequate foliage to produce the food it needs to maintain good health. Also, extensive pruning suddenly exposes previously shaded bark to direct sunlight, which can result in sunburn (sunscald) cankers that are susceptible to borer attack.

DAMAGE

Holes in the bark and stains or oozing liquid on limbs or trunks are common symptoms of longhorned borer damage. Foliage can discolor and wilt, and limbs can die back. Longhorned borers usually attack stressed or damaged plants, leaving vigorous, appropriately watered trees alone. In California, however, many eucalyptus trees are seasonally water stressed during hot summer months, rendering a significant proportion of them susceptible to beetle attack. Tree species with some resistance to these wood borers can produce copious amounts of resin in response to an attack.

Extensive larval feeding beneath the bark can spread around the entire circumference of a tree, girdling, or completely removing a strip of bark from, the trunk. Trees at this stage of infestation have a thin canopy with wilted or dry leaves, and the bark is cracked and packed with larval excrement. Infested trees usually die within of a few weeks of girdling, although resprouting can occur from the tree base.

MANAGEMENT

You can use the same methods to manage both Phoracantha species—reduce tree stress, properly handle eucalyptus wood, plant resistant species, and avoid activities that disrupt biological control. Pesticide applications generally aren’t effective in managing these pests.

Cultural Management

Certain species of eucalyptus are more resistant to longhorned borers, especially those adapted to drier conditions (Table 2). Be aware that a few species such as blue gum (Eucalyptus globulus) have become invasive, and other species can be better choices for planting depending on the area you live in. Also, longhorned borers can attack any species of eucalyptus if trees are heavily stressed, such as when planted in poor soil or deprived of sufficient water. A well-maintained tree of a susceptible species can be at less risk of infestation than a neglected tree of a resistant species.

Consider providing supplemental water on a regular schedule during prolonged dry periods, particularly if seasonal rainfall has been below normal. If a tree has received regular irrigation, avoid prolonged interruptions to watering, particularly during the summer when insect pests are most active. Suddenly cutting off irrigation to trees that have been receiving water regularly will cause trees to become water stressed and susceptible to heavy attack. If irrigating, apply water to the ground below the edge of the outer canopy and not in close near the trunk. Avoid frequent, shallow watering, which often occurs for lawns. Such a watering schedule is likely to promote tree root disease.

Eucalyptus logs used for firewood and dead or dying branches and trees are the beetles’ primary breeding sites. Sanitation is critical for reducing beetle numbers. Until the wood dries below a critical level, it remains highly attractive to egg-laying beetles. Anything you can do to speed the drying process, such as cutting and splitting wood and, even more importantly, removing bark from felled logs, helps reduce the length of time it can support beetle development. If infestation already has occurred, you should treat or destroy the wood. Bury, burn (where permitted), or chip infested eucalyptus. Chipped eucalyptus makes an excellent mulch.

Alternatively, solarize wood by placing it in a sunny location and covering it with ultraviolet-resistant plastic. This prevents new beetles from attacking and resident beetles from emerging and flying to nearby living eucalyptus. You can store properly solarized wood for firewood.

To solarize, tightly seal wood beneath thick (10 mil), clear plastic sheets in a sunny location for several months. To be effective, this treatment requires keeping wood piles small; using high-quality, clear plastic resistant that is to ultraviolet light degradation; thoroughly sealing the edges; and promptly patching holes to prevent beetles from escaping. For more information on solarization, see the publications by Donaldson and Seybold (1998) and Sanborn (1996) in References.

OTHER EUCALYPTUS PESTS

Several other introduced insects also attack eucalyptus. Pest managers should learn how management methods such as conserving natural enemies and planting resistant species can help control these pests, which include leaf-feeding beetles (tortoise beetles), eucalyptus gall wasps, and at least six species of psyllids.

Some of these insects now are now under effective biological control, including the bluegum psyllid (Ctenarytaina eucalypti), the eucalyptus snout beetle or gumtree weevil (Gonipterus scutellatus), and especially in Southern California the eucalyptus redgum lerp psyllid (Glycaspis brimblecombei). Inappropriate actions such as spraying persistent, broad-spectrum insecticides—those that remain in the environment at levels toxic to natural enemies for more than a few days—can harm natural enemies and can cause outbreaks of these other pests.

For more information, see: Pest Notes: Psyllids, Eucalyptus Redgum Lerp Psyllid, and Eucalyptus Tortoise Beetles.

Biological Control

Biological control with natural enemies has provided a partial solution to longhorned borer problems. At least one native California wasp, Helcostizus rufiscutum (Ichneumonidae), parasitizes P. semipunctata but only rarely; consequently, an egg parasite has been introduced from Australia for biological control.

The wasps are highly specialized parasites, attacking only certain beetles. They present no threat to humans, pets, or livestock. By reducing beetle populations to a lower level, natural enemies have reduced borer damage, because vigorous trees can survive a few attacks.

The egg parasite is Avetianella longoi (family Encyrtidae), a 1/16-inch-long, host-specific, introduced wasp that lays its eggs within the eggs of the beetles. Parasitized eggs develop brown shells through which you can observe the body and dark eye spots of developing wasps. A. longoi disperses rapidly and efficiently finds borer eggs. This wasp has become widely established in California, typically attacking and killing more than 90% of P. semipunctata eggs in the field. However, it is less effective against the more recently introduced P. recurva. Conserve parasites by avoiding the spraying of eucalyptus bark or foliage with broad-spectrum, persistent insecticides.

Chemical Control

Early work from both California and other regions where beetles have become established indicated insecticides might not be suitable for managing eucalyptus longhorned borer populations, but more recent studies indicate newer, systemic insecticides (compounds that move within trees after being applied to the soil) might protect individual trees if applied before beetle colonization. Exercise care when using any insecticide, because of the potential for drift, environmental impact, effects on beneficial or nontarget insects, and risk of exposure to humans in urban environments.

REFERENCES

Paine, T. D., J. G. Millar, and L. M. Hanks. 1995. Integrated program protects trees from eucalyptus longhorned borer. Calif. Agric. 49(1): 34–37.

PUBLICATION INFORMATION

Authors: T. D. Paine, Entomology, UC Riverside; S. H. Dreistadt, UC Statewide IPM Program, Davis; and J. G. Millar, Entomology, UC Riverside.

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