Chapter: 5
V.1 URBAN FORESTRY
V.2 TERMITE PESTS AND MANAGEMENT BY
CONTINENT
SOUTHEAST ASIA
AUSTRALIA
SOUTH
EUROPE
SOUTH AMERICA
V.3 ALTERNATIVE MANAGEMENT SYSTEMS OF TERMITES IN FORESTRY
CURRENT CHEMICAL
CONTROL
SOIL TREATMENT
TREATMENT OF SEEDLINGS
BEFORE TRANSPLANTING
BAITING TECHNIQUES
ALTERNATIVE CONTROL:
TRADITIONAL METHODS
SELECTING LOW RISK
SITES
USE OF SPECIES
SUITABLE FOR A REGION
RESISTANT TREE
SPECIES
REDUCTION OF
MECHANICAL DAMAGE
MAINTENANCE OF PLANT
VIGOUR
REMOVAL OF NESTS
INCREASING
BIODIVERSITY
INTER-PLANTING
ALTERNATIVE
CONTROL: NEW AND POTENTIAL METHODS
RESISTANT
TREE VARIETIES & GENETIC ENGINEERING
BIOLOGICAL
CONTROL
BAITING
SYSTEMS
TREATMENT OF SOIL AND
SEEDLINGS WITH NEW GENERATION INSECTICIDES
Termites can have a significant impact on plantation and urban forestry, as well as on agricultural tree crops. Many other insect pest species cause damage to various parts of the tree, but often do not cause mortality. Some termite species, however, are able to kill apparently healthy trees and, therefore, have the potential to cause much greater losses. Even where termites do not cause death of the tree, they may cause damage to the bole, for example, by consuming the heartwood and, thereby, hollowing the trunk and reducing the value of the tree as a source of timber. The extent to which termites are a problem to trees and the nature of loss they cause are very much related to the geographic region concerned. There are a number of current chemical control methods employed, however, a number of alternative, traditional control methods, largely relating to silvicultural practices or plantation management, are also very important, and should be considered before chemical intervention is attempted. Some of these control strategies are also applicable to urban forestry.
V.1
Urban Forestry
In urban forestry, where trees are planted in highly populated areas along streets or in parks, special consideration needs to be given to the types of control strategies that are applicable, because of the environment the trees grow in and the proximity to human dwellings. A number of current chemical control methods are employed, but these have a relatively high environmental impact, and many governments generally refrain from these chemical control methods in urban areas. Some alternative traditional methods of control such as the use of species suitable for a region, use of resistant tree species, reduction of mechanical damage, maintenance of plant vigour and inter-planting may be applicable in some circumstances, depending on the termite pest species in the geographic region concerned. Additionally, some new and potential alternative control methods, such as baiting systems, hold a lot of promise for managing termites in urban plantings.
V.2 Termite Pests and Management by Continent
In southeast Asia, the termite species responsible for most losses in agricultural tree crops and forestry is the subterranean termite, Coptotermes curvignathus, which has the ability to kill healthy trees. Only one other species in the region can attack trees, Microcerotermes dubius. The latter species is only very occasionally a problem in forest plantations, because its aerial nests are more readily destroyed than the subterranean nests of the former species. Coptotermes curvignathus attacks a wide range of tree species, but some tree species are more resistant to attack than others.
Coptotermes curvignathus usually builds mud and carton sheaths around the bases of trees that it attacks, and undermines the bark beneath with numerous perforations into the living tissue, eventually killing the tree as a result of impaired food transport from the crown above to the root system below. Sometimes it may first penetrate into the heartwood before working outwards to kill the tree in the same manner.
Forest plantation tree species attacked
by Coptotermes curvignathus in Southeast Asia include
pines and all other
species of conifers, which are particularly susceptible, rubber trees (Hevea
brasiliensis), Acacia mangium, Paraserianthes falcataria and Gmelina arborea. Most species of conifers suffer high mortality when
attacked by this termite species. Acacia mangium is more commonly
infested within the heartwood, through pruning wounds, than killed by the
termite. Nevertheless, healthy trees can still be killed, although the
mortality is much lower than in conifers. Teak (Tectona grandis) is
relatively resistant, but has problems with the drywood termite, Neotermes
tectonae. Many native tree species of forestry importance that occur in
natural forests are susceptible to the termite, for example, Koompassia
malaccensis and Buchanania sessifolia. In addition, many species of
trees planted in urban areas are also attacked.
Termites are relatively scarce in Australian rain forests. They are of negligible importance as pests of timber trees in this type of forest. In the hardwood (Eucalyptus) forests of coastal and near-coastal eastern Australia, species of Coptotermes, including C. acinaciformis and C. frenchi, very commonly infest living trees. Termite damage to forest trees mainly takes the form of a cylindrical pipe running up inside the centre of the trunk. Coptotermes spp. reach the inner heartwood through the root system. A varying and often limited number of galleries reach into surrounding wood. It is common practice to ‘box out’ the inner heartwood because it is often degraded not just by termites, but also by decay fungi. Thus, even a high incidence of termite infestation in trees does not necessarily translate into serious loss of marketable timber. Porotermes adamsoni and Neotermes insularis can also affect commercial timber trees. No specific measures are taken to manage any the termite problems in eucalypt forests.
Plantations of native and exotic pine trees do not experience termite damage, except in tropical areas where Mastotermes darwiniensis occurs. This species has prevented the successful establishment of some pine plantations in the Northern Territory. Planned large-scale plantations of species of Acacia in the same region will have to be protected from Mastotermes attack in order to succeed. Unique among termites, Mastotermes foragers often kill trees first by ring-barking them, before hollowing them out from the top down. In Australia, management measures for Mastotermes darwiniensis include baiting techniques with POP’s, and biological control.
Islands of the South Pacific with major areas of rain forest usually have a species of Neotermes, to which a number of native tree species are susceptible, while others are resistant. Damage in native trees is, as a rule, restricted to the central heartwood (‘piping’). Where rainforest is cleared to grow exotic hardwoods, Neotermes may attack any susceptible species. The most notable example is the plantings of American mahogany (Swietenia macrophylla) on Fiji. Up to 10% of trees, and possibly even more, have been lost due to termite damage. Incidences of attack vary between sites. Trees are vulnerable to attack from about two years of age onwards. Termites enter the trees via the root system and spread through both inner and outer heartwood over the full length of the bole and into the main branches. Control of Neotermes in the South Pacific islands can be achieved using biological control.
In New Guinea plantations of native and exotic pines (Araucaria, Pinus) are prone to attack by Coptotermes elisae. This species attacks trees in a manner very similar to Coptotermes curvignathus, and is managed in the same way.
In Europe,
subterranean termites attack living trees in urban areas, besides infesting wood
in buildings. The termite species generally responsible for this is
Reticulitermes santonensis, which causes an estimated several million euros
worth of damage to street trees in the city of Paris alone, where there are
about 90,000 trees worth around 2500 euros each. This species attacks deciduous
trees indiscriminately, regardless of age and species. Among tree species
attacked are the plane tree, chestnut tree, ash tree, wingnut (Pterocarya
spp.), poplar, etc. Infested trees are generally found near buildings that are
infested. Heat and moisture generated in urban areas promote the spread of the
termite. Baiting systems are currently being explored as a method of managing
this termite problem.
V.3
Alternative Management Systems
of Termites in Forestry
A number of chemical control methods are currently used to protect trees against attack by termites. These methods have remained largely the same over the years, although the chemicals used have changed to reduce the environmental impact of these treatments. Many, or all, of the organochlorines, which are persistent organic pollutants, are no longer registered for use in most countries. Current chemical control methods employed are soil treatment, treatment of seedlings before transplanting and baiting techniques.
Soil treatment
Soil
treatment to protect trees from attack by termites involves the application of
chemicals to the soil surrounding the base of the tree. This may be done at the
time of transplanting seedlings from the nurseries into the field, or it can be
done to trees already in-ground. In the former, granular formulations of
insecticides (e.g. fipronil) are usually used, since they are easy to carry into
the field, and they are applied into the planting hole. In the latter, the soil
at the base of the tree is removed to form a cavity or trench around the tree,
and liquid-based chemicals are poured in and allowed to seep into the ground by
gravity and capillary action. The cavity or trench is then filled over with the
soil originally excavated from around the tree, and treated as well. Chemicals
currently used include chlorpyrifos, imidacloprid and fipronil.
Treatment of
seedlings before transplanting
Treating seedlings before transplanting reduces the labour involved in
field-based treatments, but provides less protection to the seedlings. The
seedlings are usually prepared with granular insecticide mixed into the soil at
the time of planting in the nursery.
Aggregation and baiting techniques, using the POP chemical mirex as the active
ingredient, are used primarily in Australia
as a management measure for Mastotermes darwiniensis (see Australia, Termites
in Agroecosystems).
Alternative control: traditional methods
Many traditional methods of control of termites in forest plantations have a sound basis in the principles of ecology. They are often simple and cost effective means of minimising termite problems through silvicultural practices that involve thoughtful planning before establishing a plantation and suitable practices following establishment. These, some of which are inter-related, are listed below:
-
Removal of Nests
Termite pest species have their own native habitats in which they are most abundant. For example, in Southeast Asia, the tree-killing termite species, Coptotermes curvignathus, is abundant in peat swamps. The abundance of termite species in particular plantation sites is often, therefore, reflective of the history of the site. Plantations established in areas once occupied by peat swamps in Southeast Asia often have a relatively high incidence of attack by Coptotermes curvignathus. One way of avoiding the problem of termites in forest plantations is not to plant susceptible species of trees on sites known to be high-risk.
Use of species suitable for a region
Tree species-site matching is an important aspect of termite management. Trees grown in regions to which they are not suited may be more stressed and, hence, more prone to attack by termites. Acacia mangium for example, which is widely planted in parts of Southeast Asia, does better in a climate with a marked season, such as a wet and dry season. In other areas, it tends to develop a bushy growth form, which requires more pruning and predisposes the trees to attack as a result of mechanical injury to the trees. Terrain, climate and soil properties (physical and chemical) should be suitable for the tree species chosen, bearing in mind its native habitat and its degree of tolerance outside the conditions in which it naturally grows.
Some
tree species are naturally resistant to certain termites. However, a tree
species that is resistant to one termite species may not be resistant to
another. Thus, it is important to identify the termite pest species that is a
problem in the geographic
area concerned. In southeast Asia, Coptotermes curvignathus
is the primary species that kills trees in plantations, and it has a wide host
range.
Reduction of mechanical damage
Reducing mechanical damage to trees can minimise damage by termites. Such mechanical damage can occur from pruning wounds, or during weed control or thinning operations, especially where heavy machinery is used. Wounds and scars on tree trunks or branches serve as entry points for termites into the heartwood of the tree, sometimes after fungal infections of the wounds. Termite species that kill trees, such as Coptotermes curvignathus in Southeast Asia, may also preferentially attack trees with wounds, and may locate trees by the exudates that flow from such wounds or dissolve in rainwater. All wounds should be treated with a wound dressing, but minimising any form of mechanical damage to trees is advisable, as wound dressings do not adequately prevent fungal and termite infestations. Besides mechanical injury, it is thought that stress also predisposes trees to attack, therefore, maintaining plant vigour is an important means of minimising damage by termites.
Healthy trees are likely to be less readily attacked by termites. Trees under water or nutrient stress, or deprived of sufficient light, due to overly high planting densities or lack of thinning, may be more prone to attack. At the extremes of such conditions, termites that are not normally pests of plantation trees may cause damage or even accelerate death of the trees. But it is also likely that termites that have the ability to kill healthy living trees may preferentially attack stressed trees growing under poor conditions. For similar reasons, reduction of mechanical damage to trees due to silvicultural practices is also important. Selecting trees that are suited to the region in which they are planted is also an integral part of ensuring that plant vigour is maintained.
Nests of pest termite species that are easily visible can be removed manually, thus reducing their population. This has been used in South America to control arboreal nesting species, particularly Nasutitermes, which may occur in high densities on fruit trees. It is also applicable to Microcerotermes dubius in Southeast Asia, which may occur very occasionally in forest plantations. Likewise, drywood termites, such as Neotermes tectonae on teak, can also be controlled by removal of affected branches.
Taking measures to increase biodiversity may increase competition from non-pest termite species and, thereby, reduce populations of pest termite species. For example, in southeast Asia established colonies of the pest termite species, Coptotermes curvignathus, have a competitive edge over other termite species in plantations of susceptible tree species because of its ability to kill trees. However, encouraging a diversity of termite species in forest floor wood litter may reduce nesting opportunities for new founding colonies and, in the long term, reduce the population of the pest termite species. In addition to providing increased competition against termite pest species, increasing biodiversity can also increase natural enemies of termites, such as ants. Measures that can be taken to encourage biodiversity in forest plantations include inter-planting, retaining a litter layer, retaining ground cover, encouraging rapid canopy closure and reducing pesticide usage.
When tree species susceptible to termite attack are to be used, inter-planting them with resistant species may help reduce the overall incidence of attack in the plantation and, consequently, limit the build-up of the termite pest population. Inter-planting may also help reduce attack on susceptible tree by making it more difficult for the termites to locate them. It also helps increase biodiversity in the plantation, which may help reduce populations of termite pest species.
Alternative control: new and potential methods
A number of novel methods of termite control have been recently introduced, or may be introduced in the future. These are:
- Resistant Tree Varieties & Genetic Engineering
- Biological Control
- Baiting Systems
- Treatment of Soil and Seedling with New Generation Insecticides
Resistant Tree Varieties & Genetic Engineering
The
development of resistant tree varieties through breeding programmes and genetic
engineering is still at an early, exploratory stage, and is hampered by the long
rotation periods of forest plantation trees in comparison to annual agricultural
crops. However, it may become an important means of protection against termites
in the future. Genetic engineering for resistance usually involves the
introduction of genes from insect pathogens, such as bacteria, into the tree,
which then expresses pathogenic characteristics towards insects. The
environmental implications of such genetically modified organisms in agriculture
and forestry are controversial and still largely unstudied. Some studies
indicate that they may carry a set of environmental problems of their own.
Biological control of termites has largely focussed on the use of fungi (e.g., Metarhizium) and nematodes. It is not easily achieved in the field
because of the tendency of termite colonies to cut off and avoid infected areas
as soon as disease sets in. Nevertheless, such biological control agents can be
a substitute for chemicals when they are used to control local infestations.
The use of these agents against termites is an area of active research and, in
the future, methods may be developed for colony elimination using biological
control agents. Trials using the entomopathogenic nematode, Heterorhabditis sp.,
have given encouraging results for the control of Mastotermes
darwinensis in Australia.
Also, initial studies indicate that Neotermes colonies attacking Mahogany
in the South
Pacific Islands
can be eliminated by
both the fungus Metarhizium anisopliae and the nematode
Heterorhabditis sp. However, because Mahogany in Fiji is planted in single
rows, separated by strips of undisturbed forest, recolonisation of trees over a
plantation lifetime of about 30 years is possible.
Baiting systems using active ingredients such as moult (chitin synthesis) inhibitors, have become widely used in the control of termites affecting housing. They are effective in eliminating colonies when used consistently over an extended period of time. However, their application against termite pests in agriculture and plantation or urban forestry has not yet been widely developed or specifically tailored for the purpose. They would almost certainly prove to be effective against subterranean pest species from among the lower termites, such as Reticulitermes and Coptotermes. Systems specifically designed for the treatment of termites on trees will probably become available in the future, and will be a low-environmental-risk chemical method of control, because the chemicals can be targeted specifically at pest species of termites in baiting receptacles. Bait systems have already been tested and have shown promising results against Reticulitermes santonensis in Paris, where city officials are also co-operating with the Centre National de la Recherche Scientifique (CNRS) in a pilot study to investigate colony and population structure using molecular and chemical markers. Hopefully this knowledge will lead to a better understanding of the pest's invasion strategies and allow development of effective targeted control measures for Paris and other big cities.
Treatment
of Soil and Seedlings with New Generation Insecticides
Soil treatment and the treatment of seedlings before transplanting have been used as classical methods of control and prevention of termite attack in forest plantations for many years. These methods of control have a high environmental impact because of the large amounts of insecticides that have to be applied in open areas that are exposed to leaching. In recent years, new generation insecticides that are active at very low doses have become available. These chemicals generally also have a low toxicity to other life forms. When chemical treatment is deemed necessary, such chemicals provide an environmentally preferable alternative to traditional chemicals. Examples of these new generation insecticides are imidacloprid and fipronil. However, before adopting a chemical means of control, alternative, traditional methods of control should be considered.