Studies on the extent of damages caused by pests of stored copra and controle of the important pests

Abstract

The magnitude and intensity of insect pest attack in stored copra, in the Southern Districts of Kerala, were assessed in an elaborate survey, adopting a random sampling technique for two years. The survey revealed that insects cause significant damage even in well preserved good quality copra stocked for more than five months. N. rufipes, O. surinamensis, A. advena and L. serricorne were the major pests recorded. The immature stages of N. rufipes, adults and immature stages of the other pests were seen inside the tunnels made between the endosperm and testa. N. rufipes and O. surinamensis were distributed widely, while the predominant insects observed at Nedumangad were A. advena and L. serricorne. All the insects were found breeding on copra from June to October. A positive correlation between moisture content of copra and the insect incidence was observed in the studies. The influence of A. flavus on the extent of damage caused by these pests showed that the presence of the mould was not a predisposing factor for insect attack. The extent of damage in copra due to the infestation by N. rufipes was 12.2 per cent at the end of sixth month whereas O. surinamensis caused only three per cent damage. When combind with fungus, the damage caused by N. rufipes was reduced by 50 per cent. This effect was not observed on O. surinamensis. The infection by the fungus adversely affected the oil content of copra and this effect was not altered by the preceeding or succeeding infestation of the insect. The insect attack alone caused significant reduction in oil yield of copra during the sixth month after exposure only. The quality of oil was adversely affected when infested by insects and fungus independently. The infestation of insects preceeding or succeeding fungal infection did not alter the adverse effect caused by them independently. The quantitative loss caused by the attack of N. rufipes and O. surinamensis to copra obtained from different varieties of coconut did not show significant variations. Among the different varieties tested LM and LO were more favourable to N. rufipes and DxT and TxD to O. surinamensis. Two to six fold increase in the longevity of the adults of N. rufipes was observed when they were provided with immature stages in addition to copra for feeding. N. rufipes and O. surinamensis caused significantly greater damage to copra under eight per cent and six per cent moisture levels respectively. The development of N. rufipes was totally arrested in copra with four per cent moisture content. The development of immature stages and adult population of N. rufipes were higher in copra containing eight per cent moisture level and those of O. surinamensis in copra containing six per cent moisture. The response to moisture fluctuations was more conspicuously seen in O. surinamensis than in N. rufipes. Though the population of insects were found to be high in reused gunny bag followed by heap storage, the extent of damage was higher in heap storage than in reused gunny bags. High populations of N. rufipes and O. surinamensis were recorded from heap and reused gunny bags respectively. Low populations of insects were recorded in copra stocked in polythene/alkathene lined gunny bags and netted polythene bags. But the oil extracted from copra stored in these types of bags gave significantly higher acid values. In the trials done to find out a safe prophylactic method of control against pests of copra, malathion and phoxim proved more toxic to N. rufipes and malathion and fenitrothion to O. surinamensis. When these insecticides were evaluated for their persistence on gunny bags, malathion 0.4 per cent gave protection up to five months and fenitrothion 0.8 per cent up to six months. The residues of malathion and fenitrothion came below tolerance limits, 15 and 60 days after treatment respectively . For complete control of the major pests of copra 4.5 g/m (2.5 g ai/m ) of aluminium phosphate with one day exposure period was found adequate. The residue of phosphine in the copra fumigated as above was below tolerance limit.