Rodent damage control in commercial forestry in the Natal Midlands, South Africa.

Abstract

Rodents cause damage in commercial forests by gnawing at the bark of the trees. It is currently estimated that rodent damage in commercial forestry costs the industry R50 million per annum. The species of rodents which cause the damage are not known, neither is the reason behind this behaviour. Through stomach analysis it has been established that 3 species are involved Otomys irroratus, Rhabdomys pumilio and Mastomys natalensis, however this behaviour is confined to the winter. Chemical analysis of the bark reveals that the percentage concentration of nitrogen varies seasonally. The period of high concentration correlates with periods when the natural food of the rodents is restricted and when bark gnawing is most prevalent. In the past the industry's response to the damage has been to treat the areas with rodenticides. Using standard CMR methods, the two commercially-licensed rodenticides and raptor perches were tested to examine their efficacy as rodent control strategies. It was found that at a lower application of I block of rodenticide every third tree there is little difference in the effectiveness of the rodenticide brands and there is also little reduction in the abundance of the rodents. At a higher application rate of I block per tree, the abundance of rodents is reduced but termination of the treatment results in the rodent numbers quickly recovering, indeed they surpassed their original population numbers within 4 months. Apart from the environmental dangers of applying such concentrations of poison, this is clearly an uneconomic solution. My results indicate that at first planting, the sites should be provisioned with raptor perches at a density of 16 ha⁻². Contrary to accepted policy the perches do not require cross pieces, which add to the expense and offer no advantage in raptor residency time. In areas of very high rodent abundance the provisioning of tree collars provide physical protection to the trees. An additional benefit of the collars is that the collars cause a beneficial microclimate around the tree which enhances its growth rate. When the tree is around 2 years old it should be pruned to a height of I ffi, the slash being left in the inter-row. Results show that trees treated in such a way experience no further attack and the rodents browse on the prunings. As the cost of the perches is reduced and the cost of pruning is non-recurring the recommendations provide an economical and environmentally sympathetic alternative to rodenticide application.