Forestry Invasive Alien Insect Pests in Zimbabwe.
Global trade has enabled modern societies to benefit from the unprecedented movement and establishment of species around the world. Agriculture, forestry, fisheries, the pet trade, the horticultural industry, and many industrial consumers of raw materials today depend on species that are native to distant parts of the world. The lives of people everywhere have been greatly enriched by their access to a greater share of the world’s biological diversity, and expanding global trade is providing additional opportunities for further such enrichment. But these movements of species by humans are also in some cases having negative impacts on ecosystems and the species of which they are composed, as well as on local economies.
One major challenge to the above is the introduction of Invasive Alien species (IAS). These species, include animals (especially insects), plants, and disease organisms that are transported to new environments inadvertently and have significant negative effects on human welfare. Invasive alien species are now recognized as one of the greatest biological threats to our planet’s environmental and economic well-being. A plant or animal transported beyond the ecosystem in which it occurs naturally may multiply out of control, endangering native species in the invaded ecosystem, undermining agriculture, threatening public health, or creating other unwanted – and often irreversible — disruptions. Most nations are already grappling with complex and costly invasive species problems, such as zebra mussels affecting fisheries, mollusc diversity, and electric power generation in Canada and the USA, water hyacinth choking African waterways, rats wiping out native birds on Pacific islands, and deadly new parasites attacking victims in both temperate and tropical countries. Addressing the problem of IAS is urgent because the threat is growing daily, and the economic and environmental impacts are severe. Zimbabwe has not been spared and has been invaded by two eucalyptus invasive pests namely: Thaumastocoris peregrinus (Bronze bug) and Leptocybe invasa (Blue gum Chalcid).
Numerous international instruments, binding and non-binding, have been developed to deal with certain aspects of the problem of IAS. The most comprehensive is the 1992 Convention on Biological Diversity (CBD), which calls on its parties to “prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats, or species” (Article 8h). A much older instrument is the 1952 International Plant Protection Convention (IPPC), which applies primarily to plant pests, based on a system of phytosanitary certificates; regional agreements further strengthen the IPPC.
The expanding impact of IAS on both global economies and the environment suggests that these international instruments have been insufficient to prevent and combat IAS effectively.
Furthermore, expanding international trade is moving ever more organisms more rapidly around the world, thereby increasing the threat of these species to native ecosystems and potentially overwhelming government efforts to prevent unwanted invasions.
Leptocybe invasa (Blue gum Chalcid)
The blue gum chalcid, Leptocybe invasa Fisher & LaSalle, 2004, is a newly described insect that is a major pest of young eucalypt trees and seedlings. Believed native to Australia, it is currently spreading through Africa, Asia and the Pacific, Europe and the Near East. Information on the taxonomy, distribution, biology and economic impacts of the blue gum chalcid are still being investigated. L. invasa Distribution
This insect is believed to originate from Australia (Asia and the Pacific) although its distribution in that country is still unknown.
History of its introduction in other countries
This pest has been introduced in Africa, Asia and the Pacific, Europe and the Near East.
- Africa: Algeria, Kenya (2002), Morocco, South Africa (2007), the United
Republic of Tanzania (2005), Uganda (2002) and Zimbabwe (2008).
- Asia and the Pacific: India, New Zealand, Thailand, Viet Nam.
- Europe: France, Italy, Portugal, Spain, Turkey.
- Near East: Islamic Republic of Iran, Israel, Jordan, the Syrian Arab Republic.
The female Chalcid is a small wasp, brown in colour with a slight to distinctive blue to green metallic shine. The average length is 1.2 mm. With the exception of one record describing males in Turkey, only females of this species, have been observed. Larvae are minute, white and legless.
The blue gum chalcid has a relatively narrow host range attacking eucalypt species.
Suitable host species include Eucalyptus saligna, E. grandis, E. deanei, E. globulus ssp. globulus, E. nitens, E. botryoides, E. camaldulensis, E. gunnii, E. robusta, E. bridgesiana, E. viminalis and E. tereticornis. In Zimbabwe L. invasa has been recorded on the following species: E. camaldulensis, E. grandis and E. tereticornis.
Attacks take place within 1 to 2 weeks of bud break. Eggs are laid in the epidermis of the upper sides of newly developed leaves, on both sides of the midrib, in the petioles and in the parenchyma of twigs. White minute, legless larvae develop within the host plant. Five stages of gall development have been recorded on E. camaldulensis in Israel.
- The first symptoms of cork tissue appearing at the egg insertion spot begin one
to two weeks after oviposition. A small change in the morphology of the attacked
tissue is evident, the cork scar becomes bigger and the section of the midrib that
carries the eggs often changes colour from green to pink.
- The typical bump shape of the galls develops and they reach their maximum size
of about 2.7 mm wide.
- The green surface colour fades and tends to become pink while retaining its
- Glossiness of the gall surface is lost and its colour changes to light or dark red
depending on whether the galls are present on leaves or stems.
- The galls change colour to light brown on leaves and red on stems. Emergence
holes of the adult wasps are evident
Two to three overlapping generations per year have been observed in the Islamic
Republic of Iran, Israel and Turkey.
Symptoms and Damage
The developing larvae form bump-shaped galls on the leaf midribs, petioles and stems of new growth of young eucalypt trees, coppice and nursery seedlings. Severely attacked trees show leaf fall, gnarled appearance, loss of growth and vigour, stunted growth, lodging, dieback and eventually tree death. During outbreaks wasp pressure is quite intensive and all new growth may be damaged. While the impact of the wasp on mature tree development is not yet clear, galls can be found on most leaves if the wasp occurs in large numbers.
Damage caused by Leptocybe invasa on eucalypt branches and leaf petioles, Kibaha, the
United Republic of Tanzania. Left: young galls; right: older galls with exit holes.
Photos by G. Allard of FAO.
Damage caused by L. invasa on eucalyptus coppices at the Forest Research Centre, Harare, Zimbabwe. Photos by M. Mushongahande (Forestry Commission, Zimbabwe).
Dispersal and Introduction Pathways
Possible pathways of introduction include movement of nursery stock. The adult wasps are very small and are thus incapable of long distance flight.
There are currently no control measures for L. invasa although research on possible biological control agents is on-going in Australia and Israel. The Forest Invasive Species Network for Africa (FISNA) a network to which Zimbabwe is a member sent one of its members to Israel for training and currently natural enemy rearing is on-going in South Africa at the University of Pretoria.
Ovipositing female Leptocybe invasa. Photo by Z. Mendel
**Please check for L. invasa symptoms in your nurseries and coppicing compartments and report any similar symptoms to the address below.
3.0 Thaumastocoris peregrinus (Bronze bug)
Thaumastocoris peregrinus is a sucking insect that feeds on eucalyptus leaves and is native to Australia. This bug was virtually unknown in its native range until recently, when it became a pest in the Sydney area. This build-up of populations in large centres in Australia most likely contributed to its spread to other countries like Zimbabwe and other Southern African countries.
3.1 T. peregrinus Distribution
Thaumastocoris occurs in Australia, where there are four known species, and T. peregrinus is the only described species outside Australia.
3.1.2 History of its introduction in other countries
The bronze bug is spreading fast and it may be influenced by man. It is possible that the insect is being spread by trucks carrying Eucalyptus logs, which still have leaves and branches and eucalyptus firewood. In Zimbabwe we believe it found its way into the country through the importation of infested clonal material from South Africa. Since its first sighting at a farm in Chegutu it has since spread across the whole country and apart from men transporting this pest we also believe its spread by birds that roost in tall eucalyptus trees in cities and fly away each morning to far places as they forage for food.
T. peregrinus has so far been introduced in Africa and South America as follows:
- Africa: South Africa (2003), Zimbabwe (2007), Tanzania (2008) and Kenya (2009).
- South America: Brazil (2008), Argentina (2007), Uruguay (2008).
Regarding behaviour, it has been shown to be gregarious, both in field and laboratory conditions. Eggs, nymphs and adults were presented often on the same leaf. Eggs are black with a light concavity central area and are arranged in clusters, mainly in protected places such as deformities on the leaf blade and twigs. In the laboratory females have been observed laying at least one egg per day and eggs might be found singly or in clusters. New emerged nymphs move quickly over leaves, until they begin feeding. The insect is hyperactive and in the field our researchers have found that T. peregrinus is attracted to white colour which gives an opportunity to manage this pest using white coloured sticky traps.
T. peregrinus attacks both pure eucalyptus species including hybrids.
- South Africa: The main hosts are E. botryoides, E. camaldulensis, E. dorrigoensis, E. grandis, E. paniculata, E. saligna, E. scoparia, E. smithii, E. syderoxylon, E. tereticornis, E. viminalis, E. grandis x camaldulensis andE. grandis x urophylla.
- Brazil: Recorded T. peregrinus hosts include: E. camaldulensis, E. tereticornis, E. viminalis, E. nicholli, E. scoparia and E. grandis x E. camaldulensis hybrid were the most susceptible.
- Argentina: The main hosts include Eucalyptus camaldulensis, E. tereticornis and E. viminalis.
- Australia: T. peregrinus is established on the following species: Eucalyptus nicholli, E. scoparia, E. benthamii, E. globulus globulus, E. globulus bicostata and E. maidenii.
- Zimbabwe: T. peregrinus has been recorded on the following eucalyptus species: E. tereticornis, E. camaldulensis, E. grandis
The insect is a small phytophage bug, with a flattened body, specific to Eucalyptus trees. It is approximately 3 mm long and very agile (active). The head has developed mandible plates, antenna with four segments, with dark apical segments. A short rostrum and pulvilli are absent in the tarsus. Adults are light brown in color with darker areas. The male genital capsule is asymmetrical, with the opening at the right side. It has five instars, with a developmental period of about 20 days at temperatures between 17 to 20°C. Fecundity is close to 60 eggs. Eggs are black and laid in clusters on leaves and twigs.
Symptoms and Damage
Thaumastocoris peregrinus is a sucking insect that feeds on eucalyptus leaves. Feeding causes leaf whitening and bronzing, followed by drying and falling. The attacked trees show bronze leaves and there can be defoliation of severely infested branches.
T. peregrinus eggs laid in a cluster laid near the midrib (left) and nymphs feeding below a leaf (right). Photos by M. Mushongahande (Forestry Commission of Zimbabwe).
T. peregrinus adults and a nymph feeding below a leaf (the brown marks are caused by feeding and they coalasce to cause bronzing) (left). A solitary adult on firewood that was to be transported to an area outside the source area (right). Photos by M. Mushongahande (Forestry Commission of Zimbabwe).
Dispersal and Introduction Pathways
Possible pathways of introduction include movement of nursery stock. In Zimbabwe we suspect this pest could have been introduced in the country through contaminated eucalyptus clonal materials which originated from South Africa. This emphasises the importance of pyhtosanitary issues in any importation and exportation of forestry products. In country T. peregrinus’ dispersal has been aided by birds and the movement of eucalyptus timber, firewood from infested areas to non-infested areas.
When T. peregrinus was introduced in Zimbabwe it became apparent that virtually nothing was known about it across the globe. This was even in its Australian native land, where it was virtually unknown until recently when it became a pest on street trees in Sydney. There is currently no easily available control measure for Thaumastocoris, neither in South African or elsewhere in the world. One potential control option at this stage is to apply chemical pesticides.
The only reliable solution to the T. peregrinus infestation in Zimbabwe clearly lies in biological control. This is the chosen route to deal with most forest pests because it offers a relatively safe and effective means to reduce insect populations although it takes long to implement. Zimbabwe will benefit from work currently being done in South Africa and Israel through its membership to the Forest Invasive Species Network for Africa (FISNA).
If you have seen these pests or suspect symptoms similar to those caused by the two pests please report immediately to the undersigned:
I. Makowe (Entomologist)
Forest Research Centre
Box HG 595, Highlands
Cell: 0774 639 714 or 0716 184 557