Host Plant Nutrition and Outbreaks of the Coconut Caterpillar, Opisina arenosella Walker in Sri Lanka

The coconut caterpillar, Opisina arenosella, causes locally serious outbreaks with severe defoliation to coconut palms and subsequent loss of yield. However, there appears to be a natural resistance in the host plant to 0. arenosella attack, for infestations are confined to mature and senescing fronds and even within a heavily attacked plantation a few palms may remain unattacked or only slightly damaged. Also previous work had indicated that potassium deficiency benefited O. arenosella. Therefore experiments were carried out to examine the possible role in host resistance of major plant nutrients and also of certain amino acids. Comparisons were made between plantations with a history of frequent outbreaks ('attacked') and those that had never been attacked ('unattacked'). The results showed that there were no significant differences between mean amounts of foliar potassium, nitrogen, phosphorus, calcium and magnesium at 'attacked' and 'unattacked' sites. However, these samples were taken at the end of unusually long dry seasons when differences in potassium would be expected to be relatively small because uptake is limited from dry soil. Potassium levels were highest in the youngest fronds which are those not normally attacked whereas peak amounts of nitrogen occurred in those fronds which are most susceptible to attack. Also amounts of 'amide' were notably higher at 'unattacked' sites. It was concluded that further work was required on the relationship between major plant nutrients and levels of attack by 0. arenosella, particularly in relation to changes in nutrient concentrations which occur at different times of the year. Thus, palms may be more susceptible to pest attack during critical periods which need to be defined in order to optimise control strategies.