The baobab tree, native to Africa, Australia, and Madagascar, is an iconic part of the landscape in those regions. These majestic trees can live for thousands of years and develop massive trunks that store water during dry seasons. However, borers pose a threat to the health and longevity of baobab trees. Read on to learn how to identify and get rid of borers on your baobab tree.
What Are Borers?
Borers are insects that bore into tree trunks and branches, excavating tunnels in the wood as they feed The larvae of certain moths and beetles are the most common tree borers. They can cause significant damage over time by destroying vascular tissues that transport water and nutrients through the tree
Some of the most prevalent baobab tree borers include
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Longhorn beetle borers – Large black or gray beetles with long antennae. Larvae bore deep into the trunk.
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Bark beetle borers – Small black beetles that excavate tunnels under the bark
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Clearwing moth borers – day-flying moths resembling wasps. Larvae bore into trunk and roots.
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Carpenterworms – Caterpillars of a large noctuid moth. Tunnel extensively in the heartwood.
If left uncontrolled, borers can eventually kill a baobab by girdling the trunk, weakening branches, or creating entry points for disease. Catching infestations early is key.
Signs of Borer Infestation
Watch for these common signs that your baobab tree may have borers:
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Small holes on the bark, oozing sap – Entry points for borers
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Sawdust-like frass around holes or cracks – Excavated wood chewed up by larvae
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Wilting, dying leaves and branches – Damage to water and nutrient transport
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Hollowed out trunk/branches – Extensive tunneling weakens wood
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Loose, cracking bark – Larvae feeding under the surface
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Evidence of beetles or moths – Chewed bark, sightings of adults
Thoroughly inspect the entire tree, especially the trunk base and any damaged areas, to check for borer activity. The sooner you detect an infestation, the better your chances of saving the tree.
Getting Rid of Baobab Tree Borers
If your baobab shows signs of borers, take action right away using these methods:
Physical Removal
Manually extract borers from their tunnels using a knife or stiff wire. Scrape out larvae, cocoons, frass, and any wood showing tunneling. Remove loose bark that may harbor borers. Seal larger holes with grafting compound once clear.
Systemic Insecticide
Apply a systemic insecticide containing emamectin benzoate in early spring so it distributes through the tree and kills larvae within tunnels. Avoid treating during hot, dry conditions that inhibit absorption.
Targeted Insecticide
Use a spray insecticide directly on borer entry holes and areas with active infestation to kill larvae on contact. Spinosad or permethrin work well for baobab borers. Time applications before adult emergence in spring.
Nematode Application
Introduce beneficial nematodes into the soil around infested trees. The nematodes locate borers underground and parasitize the larvae. Effective for root and trunk borers.
Trunk Injection
Inject small amounts of insecticide directly into the active tunnels using specialized equipment. Allows pesticide to penetrate the infested tissue. Best for moderate to severe infestations.
Horticultural Oil
Applying horticultural oil helps smother borer eggs and makes it harder for larvae to penetrate the bark. Use as a preventive treatment to deter infestation.
Combining multiple methods often yields the best results when dealing with baobab borer infestations. It’s imperative to eliminate all signs of borers and avoid reinfestation.
Preventing Baobab Tree Borers
Once your baobab tree is pest-free, implement these measures to help prevent future borer problems:
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Maintain tree health with adequate water, nutrients, and pruning out deadwood. Healthy trees resist borers better.
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Shield bark and roots from sunscald and damage. Inspect regularly for cracks or wounds where borers enter.
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Clean up fallen branches, stumps, and debris which serve as borer breeding sites.
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Encourage natural predators like woodpeckers which feed on borer larvae. Avoid pesticide overuse.
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Divert borers with trap trees. Debark logs placed near the baobab draws borers away.
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Apply preventive trunk sprays. Coat the trunk in early spring with horticultural oil or insecticidal soap before borers become active.
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Install physical barriers. Wrap susceptible tree trunks with fine mesh screen to obstruct borer movement.
With diligent monitoring and a proactive pest management approach, you can protect your beloved baobab tree from destruction by borers. Consistent inspection paired with targeted treatments when signs of infestation arise offers the best defense against these harmful pests. Do not delay in responding to borer damage to give your baobab tree the greatest chance of survival and recovery. Your efforts will allow you to continue enjoying the splendor, shade, and bounty this iconic tree provides for years to come.
PSHB host list1
REPRODUCTIVE HOST TREES2
Exotic species:
Acacia longifolia (long-leaved wattle); Acacia mearnsii (black wattle); Acacia melanoxylon (blackwood); Acer buergerianum (trident (Chinese) maple); Acer negundo (boxelder); Acer palmatum (Japanese maple); Acer saccharinum (silver maple); Brachychiton discolour (pink flame tree); Casuarina cunninghamiana (beefwood); Gleditsia triacanthos (honey locust); Liquidambar styraciflua (American sweetgum); Magnolia grandiflora (southern magnolia); Persea americana (avocado); Photinia x fraseri (Christmas berry); Platanus x acerifolia (London plane); Populus alba (white poplar); Populus x canescens (grey poplar); Populus nigra (Lombardy poplar); Populus simonii (Chinese cottonwood); Quercus palustris (pin oak); Quercus robur (English oak); Quercus suber (cork oak); Ricinus communis (castor bean); Salix alba (white willow); Ulmus parvifolia (Chinese elm); Viburnum odoratissimum (sweet viburnum), Wisteria sinensis (Chinese wisteria).
Native SA Species:
Afzelia quanzensis (pod mahogany); Anisodontea scabrosa (rough-leaf African mallow); Bauhinia galpinii (pride of de Kaap); Brachylaena discolor (coast silver oak); Calpurnia aurea (wild laburnum); Combretum erythrophyllum (river bushwillow); Combretum krausii (forest bushwillow); Diospyros glabra (Cape star-apple); Erythrina caffra (coastal coral tree); Kiggelaria africana (wild peach); Podalyria calyptrata (water blossom pea); Psoralea aphylla (leafless fountain bush); Psoralea pinata (fountain bush); Salix mucronata (Cape willow); Sparrmannia africana (African hemp); Trema orientalis (pigeon wood); Trichilia emetica (Natal mahogany); Vepris lanceolata (white ironwood); Virgilia oroboides subsp. ferruginea (keurboom).
NON-REPRODUCTIVE HOST TREES3
Exotic Species
Bauhinia purpurea (butterfly orchid tree); Betula pendula (silver birch); Bougainvillea sp. (bougainvillea); Camellia japonica (common camelia); Carya illinoinensis (pecan nut); Ceiba pentandra (kapok); Cinnamomum camphora (camphor); Citrus limon (lemon); Citrus sinensis (orange); Diospyros kaki (persimmon); Eriobotrya japonica (loquat); Erythrina livingstoniana (aloe coral tree); Eucalyptus camaldulensis (river red gum); Ficus carica (common fig); Fraxinus americana (American ash); Fraxinus excelsior (European ash); Hakea salicifolia (willow-leaved hakea); Hibiscus rosa-sinensis (hibiscus); Jacaranda mimosifolia (jacaranda); Macadamia sp. (macadamia nut); Malus domestica (apple); Melia azedarach (syringa); Metasequoia glyptostroboides (dawn redwood); Morus nigra (black mulberry); Olea europaea subsp. europaea (cultivated olive); Platanus occidentalis (American plane); Platanus racemosa (Californian plane); Plumeria rubra (frangipani); Prunus avium (sweet cherry); Prunus cerasifera (cherry plum); Prunus nigra (black plum); Prunus persica (peach); Psidium guajava (guava); Quercus rugosa (net leaf oak); Robinia pseudoacacia (black locust); Salix babylonica (weeping willow); Schinus molle (pepper tree); Solanum mauritianum (bugweed); Taxodium distichum (swamp cypress); Ulmus minor = Ulmus procera (English elm); Vitis vinifera (grape vine).
Native SA Species:
Adansonia digitata (baobab); Afrocarpus falcatus (Outeniqua yellowwood); Albizia adianthifolia (flat crown); Buddleja saligna (false olive); Calodendrum capense (Cape chestnut); Celtis africana (white stinkwood); Commiphora harveyii (copper stem corkwood); Cordia caffra (septee tree); Cussonia spicata (cabbage tree); Diospyros dichrophylla (star apple); Diospyros lycoides (monkey plum); Diospyros whyteana (bladdernut); Dombeya rotundifolia (wild pear); Dovyalis caffra (kei apple); Ekebergia capensis (Cape ash); Erythrina lysistemon (common coral tree); Ficus natalensis (Natal fig); Ficus sur (Cape fig); Grewia occidentalis (cross berry); Gymnosporia buxifolia (spike thorn); Halleria lucida (tree fuschia); Harpephyllum caffrum (wild plum); Ilex mitis (Cape holly); Leonotis leonurus (wild tobacco); Melianthus major (honey flower/Kruidjie-roer-my-nie); Nuxia floribunda (forest elder); Olea europea subsp. africana (wild olive); Olinia ventosa (hard pear); Osteospermum moniliferum (bietou); Podocarpus henkelii (Henkel’s yellowwood); Protea mundii (forest sugarbush); Prunus africana (red stinkwood); Rapanea melanophloeos (Cape beech); Schotia brachypetala (weeping boerbean/huilboerboon); Searsia chirindensis (red currant); Searsia lansea (karree); Senegalia burkei (black monkey-thorn); Senegalia (Acacia) galpinii (monkey-thorn); Syzygium cordatum (waterberry); Vachellia (Acacia) karroo (sweet thorn); Vachellia (Acacia) sieberiana var. woodii (paper bark thorn); Virgilia divaricata (keurboom).
1 Only tree species on which the presence of the beetle and/or Fusarium fungus have been confirmed with DNA sequences are listed here.
2 Host trees in which both the beetles and the fungus establish, and where the beetle successfully reproduce. In most cases the reproductive hosts will eventually be killed by the fungus.
3 Host trees that are attacked but the beetles do not establish breeding galleries. The fungus may, or may not cause disease. Trees are generally not expected to die.
The Polyphagous Shot Hole Borer (PSHB) and its fungus in South Africa
The beetle-fungus symbiosis
The discovery of this beetle and fungus in South Africa is of major concern to farmers, foresters, landscapers, home owners and ecologists, as together, these organisms can be aggressive tree killers. The PSHB is a 2mm long ambrosia beetle native to Southeast Asia. The PSHB has a symbiotic relationship with three species of fungi. These include the tree pathogen, Fusarium euwallaceae. This fungus provides a food source for the beetle and its larvae, but in susceptible trees, it kills the vascular tissue, causing branch dieback and tree death.
In its native environment in Southeast Asia, it seems as if the beetle and fungus do not cause serious damage because tree species have evolved with the beetle-fungus complex and have resistance towards them, and because there are most likely a suite of natural enemies of the beetle. However, the beetle and fungus were somehow introduced into Israel and California in the early 2000s where they caused serious damage on several ornamental trees as well as avocado trees.
Identification of the beetle and fungus
Until December 2018 the PSHB was known as Euwallacea nr. fornicatus. However, Gomez et al. (2019) showed with DNA sequences that Euwallacea fornicatus is actually a species complex including four closely related, but distinct species. These four species of Shot Hole Borer are very similar in shape, and can only be distinguished by specialists under a microscope or with DNA sequences. The four species carry different fungal species, have different host ranges, and different geographical distributions (Gomez et al. 2018). Although they suggested the name Euwallacea whitfordiodendrus should be used for the PSHB, this was an error that was corrected in a subsequent paper by Smith et al. (2019). The correct names of four Shot Hole Borer species in the E. fornicatus complex are:
1. Tea Shot Hole Borer A [TSHB-a = Euwallacea perbrevis (Schedl 1951)]
{Distribution: Asia (American Samoa, China, Indonesia, Papua New Guinea, Samoa, Sri Lanka, Taiwan, Thailand), Australia, and introduced in the USA (Florida and Hawaii)}
2. Tea Shot Hole Borer B [TSHB-b = Euwallacea fornicatior (Eggers 1923)]
{Distribution: Asia (Malaysia, Papua New Guinea, Singapore, Sri Lanka)}
3. Polyphagous Shot Hole Borer [PSHB = Euwallacea fornicatus(Eichhoff 1868), previously referred to as Euwallacea nr. fornicatus and Euwallacea whitfordiodendrus (Schedl 1942)]
{Distribution: Asia (China, Hong Kong, Japan, Taiwan, Thailand, and Vietnam) and introduced in USA (California), Israel, and South Africa}
4. Kuroshio Shot Hole Borer [KSHB = Euwallacea kuroshio Gomez and Hulcr 2018]
{Distribution: Asia (Indonesia, Japan, and Taiwan) and introduced in Mexico and USA (California)}
To date, only the PSHB has been found in South Africa. However, both the PSHB and KSHB have invaded California, while TSHB-a has invaded Florida. These species carry similar fungi, have similar life styles and similar effects on trees. With our borders being open for trade to Southeast Asia, the risk that one of the other species can be introduced is very high. Co-occurring species increase the chances for interbreeding which will enhance the adaptability of the beetles to new hosts and new environments, posing a greater threat. From a management perspective, they are dealt with in the same ways. In California they have thus started referring to the PSHB and KSHB jointly with the single term Invasive shot hole borers (ISHB). It is recommended that for management and legislative purposes we also adopt the term ISHB in South Africa.
A large diversity of host trees
Polyphagous refers to the ability of the PSHB to infest many different tree species. In California, surveys in botanical gardens revealed that more than 200 tree species have been infested, damaged and often killed, by these organisms (Eskalen et al. 2013). An important distinction is being made between different types of infestations. Reproductive host trees are trees that the beetle infests and where it successfully establishes a breeding gallery in which the fungus grows, where eggs are laid, and larvae develop into mature adults, thus completing its life cycle. The majority of reproductive hosts eventually succumb to the disease symptoms caused by the fungus. Non-reproductive host trees are attacked by the beetle, but the beetles do not establish breeding galleries. The fungus may, or may not cause disease. Trees are generally not expected to die.
A problem when compiling these lists is that sometimes PSHB can infest a stressed tree (e.g. as result of drought, too much water, root damage, etc.). Such an individual, stressed tree might then become a reproductive host, whereas healthy growing individuals of the same species are barely affected. When trees are assessed for Fusarium Disease or whether it is a reproductive host, other stress factors on the tree should always be considered.
The invasion in South Africa
Since its discovery in KwaZulu-Natal in 2017, the FABI team has confirmed the presence of the PSHB in eight of the nine provinces in South Africa. The only exception as of March 2021 is Limpopo, but that might merely be because appropriate samples have not been received from that province. Below is a list of all the host trees in South Africa on which the presence of the beetle and/or fungus have been confirmed with DNA sequences in FABI.
Based on the experiences in California and Israel, avocado trees are among the most susceptible agricultural crop trees to PHSB infestation and FD. To date, PSHB has been found on some backyard avocado trees in Sandton and Knysna, but its presence has not yet been confirmed in any commercial orchards in South Africa (van den Berg et al. 2019). PSHB has been detected in pecan and macadamia orchards, but effects seem to be limited on these crops at the moment, most likely as these do not seem to be reproductive hosts Other fruit trees in private gardens on which PSHB has been detected include lemon, orange, guava, peach, and grapevine. However, at this point, there is no evidence suggesting that PSHB pose a threat to these crops, but producers should carefully monitor and report any infestations.
In addition to agriculture, commercial forestry is another sector that is concerned. PSHB infestations have been observed on a small number of roadside wattle trees – but to date, no trees in commercial plantations have been infested. Based on observations on Acacia spp. from commercial forestry in SE Asia, however, there is concern around the threat posed to species of importance to the South African forestry industry.
The most visible impact of the PSHB invasion in South Africa is in urban forests on street, park and garden trees, and this became the focus of many articles in the media. Many trees have been killed by PSHB in Sandton, George, and Knysna, while reports from Sedgefield, Bloemfontein, the Ekhurhuleni metro, Jankempdorp, Hartbeesfontein, Pietermaritzburg and Durban suggest that the impact is becoming worse in those areas. The most common trees to be killed are English oak, Chinese maple, Japanese maple, boxelder and sweetgum.
Of great concern is the recent discovery of PSHB on London plane and sweetgum in Somerset West in the Cape Peninsula. With the major impact of the disease on oak trees in especially the George and Knysna areas, it is inevitable that the famous oaks of Stellenbosch and the surrounding wine farms will be dramatically impacted.
Most unpredictable is the impact that the PSHB invasion will have on our native forests. Several native tree species were found to be infested in the gardens of Sandton, George and Knysna, with species like the coral tree, keurboom and Cape willow being particularly vulnerable and often killed. It is now known that the beetle is spreading from the urban areas into native forests close to the towns of George, Knysna and Durban. However, which species will be affected and to what extent, is unpredictable.
The PSHB Research Network
PSHB as an invasive is relatively new to science, and has only been studied since the outbreaks in California and Israel in the early 2000s. There are thus many unanswered questions about this pest that is essential to understand to be able to better reduce its impact. The research team at FABI is engaging with various government agencies, municipalities and industries to advise on policy and strategy, and to secure funding for research projects. In the process they have established a research network including academics from seven other universities who will collaborate on various aspects of the PSHB invasion in the different regions.
The team appreciate the funding from the Department of Environment, Forestry and Fisheries (DEFF), SAPPA, SAMAC, Hans Merensky Foundation, DSI NRF Centre of Excellence in Plant Health Biotechnology, SANBI.
What can be done?
There is no way in which the PSHB invasion in South Africa can be stopped. However, some treatments and management strategies can reduce its impact.
Municipalities should:
- Train staff to recognize and cut down heavily-infested reproductive host trees from streets and public areas.
- Infested branches can be cut if the main stem is not infested (unlikely, as PSHB usually infests the stem first).
- Designate dedicated dumping sites where infested wood can be dumped as it poses a risk of spreading the beetle.
- Chip wood to pieces finer than 5cm at the dumping sites.
- Provide a help desk (preferably online) where the public can report infested trees and get information.
Tree growers/home owners should:
- Try to determine whether the symptoms are really caused by PSHB (see FABI brochures).
- If unsure, ask help from municipal or other help desks, or your local arborist.
- If the tree is a heavily-infested reproductive host, cut it down.
- Infested branches can be cut if the main stem is not infested (unlikely, as PSHB usually infests the stem first).
- Dump the wood at a dedicated (by your municipality) dumping site.
- Chip the wood to finer than 5cm, allow chips to compost by keeping it wet.
- Or burn the wood on site (some beetles will fly away when the wood becomes hot or when smoke appears, so do not burn in uninfested areas).
- DO NOT MOVE FIREWOOD AROUND, ESPECIALLY NOT OUT OF THE INFESTED AREA WHERE YOU LIVE.
- Or solarize (leave in full sun) chopped wood under thoroughly-sealed clear plastic sheets for at least one month in summer or several months during winter.
- At present no chemical product is registered (legal) to use on PSHB in South Africa.
Western Cape: fill in the online report form at www.capetowninvasives.co.za
Johannesburg: email [email protected] or Whatsapp: 0828030748
Other parts of the country, especially from towns where it has not been reported, and host tree species not on the list below: email [email protected] (English please, some of our staff are foreigners who do not speak Afrikaans)
PLEASE DO NOT SEND SAMPLES TO FABI WITHOUT FIRST CONTACTING US AT [email protected]
How To Treat Tree Borers.
FAQ
Can a tree with borers be saved?
What is the best remedy against borers?
How to get rid of peach tree borers?
How do I get rid of tree borers?
Tree Borers are a destructive group of pests known to destroy wood as adults or lay eggs inside of trees where destructive larvae hatch and burrow and cause damage. Using a combination of contact insecticide treatment with Sylo Insecticide and a soil drench treatment of Dominion 2L can help to eliminate tree borers from your tree.
How do I prevent borer infestation?
Addressing tree injuries promptly can reduce the risk of borer infestation. If you spot a wounded tree, take immediate steps to care for it and minimize the likelihood of infestation. When dealing with borer infestations, pruning off affected branches is a practical approach. Inspect your cut for any signs of borer holes or tunnels.
Are there borers in a garden?
There are many insects that can be described as borers. The insects that bore into the seasoned timbers of furniture and houses are different species to those that bore into trees. The most common borer insects found in gardens are the larvae of Longicorn Beetles. Larvae of Jewel Beetles are also quite common in gardens.
What happens if a tree eats a borer?
Once inside the tree, detecting borer larvae becomes challenging until substantial damage occurs. This damage often results in structural weakness, branch dieback, girdling, and overall decline in tree health, potentially leading to the tree’s demise. Infested areas also become vulnerable to pathogen entry.