An unidentified stem borer is causing concern for some growers in northern New South Wales. The moth lays its egg within 30cm of the apex of the dominant leader and just above a leaf base. On hatching the new larva enters the center of the shoot and chews its way either up or down before pupation. Before pupation the larva chews the wood in the shoot down to the cambial layer. Over time the shoot desiccates before snapping off in the wind and releasing the moth. The extent of damage only becomes obvious once the shoots desiccate and become a straw colour. Despite its apparent increase in some plantations, for reasons unknown, the damage caused by the moth is probably not significant.
With the push for producers to achieve organic status, the use of predators and parasites to control pests is an attractive management option. However, despite the existence of many beneficial insects attacking all stages of the pests in tea tree they are a relatively ineffective form of pest control and cannot be relied upon. The main problem is the lag in the development of the pests and the beneficial species. Beneficial insects usually become effective towards the end of a cycle of pest activity. They are at their greatest abundance after most of the damage has occurred. Current management practises on plantations do not allow for refugia in which beneficial insects can shelter or over-winter. The development of beneficial insect populations recommences each season within the plantation and depends on migration. The parasites and predators found are generalists and attack most pests found within tea tree.
PROBLEMS ASSOCIATED WITH CHEMICAL USAGE FOR PEST CONTROL
When P. tigrina or other pests invade a plantation, and the foliage practically disappears over night, the incentive to use an insecticide is high. This practice is safe if only registered chemicals are used in accordance to their label. Withholding periods before harvesting must be followed. Registered chemicals applied at inappropriate rates or frequencies could cause residues and their presence will influence marketing opportunities.
The unique nature of the crop, i.e. oil in oil sacks within the leaf, the solubility of most insecticides in the oil itself and the extraction process increases the chances of residues. Contract distillers must be aware of the possibility of residue carry over from one job to the next and take steps to clean the still between runs. Given the potential for tea tree oil, the industry must take responsibility for registering new chemicals after their proper screening for efficacy and residues. The industry should not rely on chemicals registered for use in other crops. Softer control options like Bacillus thuringiensis (Elliot et al. 1992) and organic methods are available, but need evaluating for use on tea tree.
On the evidence available P. tigrina, mites and psyllids cause the greatest damage within plantations and P. tigrina is the best recognised pest. Understanding conditions necessary for the development of pest outbreaks, i.e. threshold temperatures for the survival of eggs and larvae, allows the generation of a model that accounts for field behaviour. In conjunction with a reliable trapping method, e.g. colour trapping for P. tigrina, viable management strategies and action thresholds can be developed. These strategies if applied will be cost effective, minimise the use of insecticides and reduce the risk of residues in the oil.
The lack of biological data prevents the development of models for pests other than P. tigrina and the collection of such data should be an industry priority. Without such data the misuse of chemicals will continue to occur, increasing the possibility of oil contamination in the marketplace.
Finally, not all insects found in plantations are pests. The full pest complex of tea tree remains unknown, action thresholds are non-existent and the significance of any pest depends on the weather patterns.
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