Assessment and reduction of insect infestation of cured fish in South East Asia, with laboratory studies on Chrysomya megacephala (Fab.), a principal causative agent
A survey of cured fish establishments in 5 countries of South East Asia, revealed that cured fish is a nutritionally and economically important commodity in the region and that insect infestation, in particular blowfly infestation during processing and dermestid beetle infestation during storage, are major causes of losses in cured fish. Many processors have responded by illegally applying household and agricultural insecticides to their fish. Field investigations in Indonesia and Thailand, identified Chrysomya megacephala (Fab.) as the most widespread cause of infestation during processing. Lucilia caprina (Wied.) was also a common cause of infestation. Dermestes maculatus (Degeer), D. carnivorus (Fab.), D. ater (Degeer) and Piophila casei (L.), were the most common causes of infestation during storage. These species were able to tolerate the relatively high salt concentrations of the processed fish. Field infestation reduction trials, demonstrated that salting the fish for an extended period failed to provide protection against insect infestation. Guarding the salting tank with a closely fitting lid, prevented blowfly infestation during salting. Flyscreens were found to reduce blowfly infestation during drying, but the design used, presented practical difficulties and was not acceptable to the processor. The pyrethroid insecticide Fastac (alphacypermethrin), prevented blowfly infestation during processing at concentrations as low as 0.00.3% and had a marked repellent effect against blowflies at a concentration of 0.001%. Fastac, applied at a concentration of 0.006%, protected fish against dermestid beetle infestation and damage. Fastac residues in fish treated with a 0.006% dip decreased to less than 2 rag/kg after drying and 1 week's storage. The pyrethroid insecticide deltamethrin, prevented insect infestation during processing and storage, when applied as a 0.003% dip before drying. The FAO/WHO approved insecticide piriraiphos-methyl, reduced blowfly infestation and prevented damage during processing and reduced dermestid beetle infestation during storage, when applied as a 0.03% dip before drying. This treatment resulted in residues, after processing, that were within the FAO/WHO maximum residue limit of 10 mg/kg.Spray applications of pirimiphos-methyl, at dosages of 5-20 mg/kg and deltaraethrin, at dosages of 1-3 mg/kg, were effective in reducing dermestid beetle infestation of smoked fish during storage. Laboratory investigations demonstrated that C. megacephala produced similar numbers of male and female offspring and that there was no difference between the mortalities of the 2 sexes. Female flies greatly outnumbered male flies at the processing site. Mean lifespans of C. megacephala cage populations ranged from 47-54 days and the maximum survival time ranged from 80-98 days. C. megacephala eggs matured within 3 weeks of adult emergence and the mean egg count for the adult female flies was 221.The presence of C. megacephala eggs on fish, stimulated oviposition by C. megacephala and freshly laid eggs were found to have a higher stimulatory effect than eggs which had been previously boiled. Fish being salted exerted a marked, differential attractive effect on gravid, female flies. When presented with fish of a range of salt concentrations, C. megacephala preferentially oviposited on the fish with the lowest salt concentration. In the absence of choice, C. megacephala readily oviposited on fish with relatively high salt contents of 30-40% (dwb). A feeding medium salt content of 33.8% was necessary to significantly reduce larval growth rate and salt contents in excess of 39.5% were necessary to obtain high larval mortalities. Salt contents of up to 39.5% had no effect on pupal mortality.