Preventive Measures

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This article is divided into the following sections:

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.

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Michael F. Schneider, 1999