Preventive Measures

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This article is divided into the following sections:

• Monoculture versus Polyculture
• Cultural Diversification
• Prevention or Cure?
• Preventive Measures in Forestry

Monoculture versus Polyculture

In the above example the termites attacking tree crops in a plantation are considered as a pest. In fact the termites are not a pest per se, but the conditions in the plantation are suitable for the termites to multiply and to cause damage to the tree crops and therefore man judges the termites to be a pest. Actually man is responsible for an insect becoming a pest, because man creates artificial conditions unsuitable for the crop and suitable for a pest.

A cultural system with only one particular crop species, called monoculture, is far more susceptible to insect attack than natural systems. The plants in a monoculture are often weakened and their natural resistance is decreased because there is high competition between the plants for nutrients, light, water, etc. Additionally, because of the low diversity in a monoculture there are hardly any suitable conditions for enemies that control a pest in a natural way. Furthermore, the source of food for a particular pest is abundant in a monoculture. This condition is most favourable for a pest to multiply rapidly, because the availability of food is the most important limiting factor for the growth of insect populations. Finally, a monoculture is not only reduced in terms of the number of species but also in terms of age classes since all plants are the same age. This is important because many pests attack their host only at a particular age. For instance only four to eleven year old Hoop pines are attacked by Hylurdrectonus araucariae. If all trees belong to the same age class, all of them will quite likely become the target of a pest attack.

In contrast, a polycultural system has a higher diversity of planted crops. As a result there is less competition between the different species due to the different requirements of each species. Furthermore, the food plant of a particular pest is less abundant and might be dispersed, occurring only here and there so that it is more difficult for the pest to locate its host. Due to the species diversity in a polyculture there is a variety of predators and parasites that help to control the pest naturally. Therefore, a polyculture is usually less susceptible to insect pests.

Cultural Diversification

Agriculturalists often benefit from the advantage of a more diverse system like a polyculture in terms of pest susceptibility. In large-scale forestry however, it is a bit more complex since the planted species ideally require the same time from planting to harvesting so that the whole plantation can be clear-felled. Furthermore, particular crop species might suppress the growth of an adjacent species. A diversification of a monoculture can be gained, for instance by planting small but long strips of different crops next to each other as shown below. The diversity is usually higher in the transition zone (ecotone) between two different habitats or compartments with different crops. Higher species diversity in general contributes towards the increased natural resistance of the crops as well as higher numbers of natural enemies. The diversification by means of age classes, can further help to increase plant resistance and decrease the susceptibility to pest infestation. These considerations are a basic concept of pest control and should be part of the planning and establishment of plantations.

Diversification of Cultural Systems: small strips of Kamarere in the foreground followed by Erima, Talis and secondary forest in the background at the Stettin Bay Lumber Company reforestation area near Kimbe, West New Britain; this setup has more diverse ecotones indicated by thick black lines (upper drawing) than the theoretical setup shown in the bottom drawing

Prevention or Cure?

In the ancient Greek society physicians were paid for maintaining their patient’s health, the doctors’ job was to prevent illnesses. Once a patient became sick the doctor was sacked because he apparently failed to do his job properly. The contemporary ‘Western’ health concept is different or even contrary. We visit the Haus Sik when we feel ill and most physicians do little to prevent their patients from becoming sick. Since plants are living creatures, agriculturalists and foresters have the obligation to attempt to maintain our crops in a healthy condition and to prevent diseases. The nursery, garden and plantation should be a Health Centre rather than a Haus Sik. The prevention of diseases is the key to the successful cultivation of crops. Prevention is often more effective, cheaper, environmentally more sound and more appropriate than curative measures. In forestry there are hardly any effective curative measures available and therefore in most cases there is no other choice apart form preventive measures. Regular monitoring is also of great importance so that disorders can be recognised at an early stage and curative measures can be taken if necessary.

Preventive Measures in Forestry

This section outlines possible preventive measures to minimise the risk of pest infestation. The measures are recommended by the Forest Research Institute (FRI) as well as by experienced senior foresters and apply to any tree species.

• Site preparation: Termite-infested tree stumps should be removed immediately after clear felling if replanting is intended. Therefore the logged site should be searched for subterranean nests before the area disappears in tall kunai grass. Infested stumps can be easily found since they often have a hollow ‘pipe’. Ideally the infested stumps are uprooted, an exercise which is very difficult. Even a bulldozer has difficulty pulling out the stump of a newly felled tree. Previously explosives, injected below the stump and ignited were used for that purpose. The explosives were prepared from a mixture of diesel and ammonium nitrate, contained in fertiliser. The problem with this method was the availability of experts for the safe handling of the explosives. In addition waste wood that would offer additional food for the termites should be removed. Finally it is recommended that compacted soil be broken up prior to replanting.
• Nursery: The right species and provenance suiting the soil type and climatic conditions in the proposed area, and only high quality seed material should be used. The seedlings should be kept in the nursery until they are large and vigorous enough to be planted to the field. How long they are kept in the nursery involves a compromise of several factors. If the seedlings are kept in the nursery for a longer time, the bark will be thick enough and will be no longer a suitable source of food for grasshoppers. Furthermore the mortality rate of the seedlings will be lower. On the other hand, the tap root will quite likely start to coil and this will encourage termite attack. Additionally, each day in the nursery costs money to carry out watering and weeding of the seedlings.
• Planting: Only healthy and vigorous seedlings should be used. Wider spacing should be taken into consideration since less refilling, tending and thinning has to be done. Apart from reducing the costs of these operations, a wider spacing results in faster growing trees because of less competition and a reduction in the potential damage done to the trees during each of these operations. The tubed seedlings are not supposed to be exposed to the heat of the sun but should be placed somewhere in the shade. The temperatures under the black polythene can be as hot as 80° C, enough to cook the roots of a seedling. The tube has to be removed prior to planting. A suitable plant hole has to be dug, big enough to allow all roots to fit in. The coiled taproot of the seedling should be uncoiled, otherwise it will strangle itself as it increases in diameter. Many uprooted termite-infested seedlings show a coiled and already deteriorated tap root, through which termites started the infestation. Therefore, coiling and upwards bending of the roots should be avoided during planting, so that the root system can develop properly. Planting should be carried out in such a way that the newly planted seedlings suffer as little stress as possible. Some water and fertiliser if necessary will give the seedling the right start.
• Tending and Pruning: Any damage of the bark during pruning should be avoided. The branches are supposed to be pruned as close to the stem as possible, so that the bark grows over the sore immediately (rapid occlusion). Diversity should be allowed in a plantation rather than creating a monoculture. A diverse understorey offers attractive conditions for beneficial insects. Hygiene in the plantation is of great importance. Pest-related problems are often enhanced because waste such as pruned branches offers additional food for pests. One should consider removal or burning of any waste. Furthermore, silvicultural treatments such as tending and pruning should not be delayed but carried out as early as possible.
• Thinning: Damage of the bark of residual trees caused by falling trees should be avoided. Only experienced chainsaw operators familiar with directional felling should carry out the operation. Vehicles like skidders should only be operated, if really necessary so that soil compaction and damage of rootlets of trees along the snig tracks are minimised. The use of bandages, eg. made from the rubber of a worn-out conveyor belt should be considered in order to protect the base of the stem from being damaged by wheels. It is doubtful whether waste wood placed at the base of the trunk can protect the roots from being damaged by skidders. In other countries skidders are more and more being replaced by horses for hauling. Horses are much cheaper than a skidder and do less harm and damage to trees. Straight snig tracks can also contribute towards less damage of residual trees because damage often occurs in bends. It is advisable to leave a buffer of trees along the snig tracks which are felled at the end of the operation. Any gravel piled up for the construction of temporary roads has to be removed after the operations. If gravel is covering the bark of trees along the road, it will cause rot and damage of the trees in the long term. Waste materials such as timber or gravel should not be pushed downhill but disposed of thoroughly. Bigger pieces of timber or boulders can easily damage the bark of trees when rolling downhill. The right season for thinning, and also for pruning should be considered. Most wood-boring insects attack only green timber and as the timber dries, it becomes less suitable for an attack. Therefore fewer problems are experienced when thinning is carried out during the dry season, when the waste dries faster.
• Post Logging: The harvested timber should be removed from the forest as soon as possible to avoid infestation with wood borers resulting in timber degrade. Ideally the timber should be immediately processed after harvesting. If this is not possible, the entire log surface has to be treated with a suitable insecticide or the stems have to be continuously sprinkled with water. Another preventive measure that should be considered in some cases is to debark the logs in order to create conditions that are no longer suitable for insects associated with the bark and the sapwood.
• Regular Visits: During regular visits of the plantation, any signs of an insect attack should be thoroughly monitored. The earlier an outbreak is detected, the more successfully remedial measures can be taken and the smaller the economic loss will be.
• Other Silvicultural Practices include the diversification of single-species forests and the diversification of age classes as discussed in the previous section. The elimination of alternative or wild hosts is a further possibility to prevent insect-related problems. An example is the elimination of Lau-lau (Syzygium spp.), the wild host of Agrilus spp., adjacent to areas where Eucalyptus deglupta or Terminalia spp. plantations are established

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© Michael F. Schneider, 1999