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Field trials to control flies on animal farms in Malta were held during 1973 with tetrachlorvinphos, iodofenphos, fenitrothion, bioresmethrin and dimethoate applied as cover sprays to walls and ceilings. Results were assessed by fly population counts (using sticky fly-paper traps exposed for 24 h at weekly intervals), bioassay tests and visual assessments of flies on individual farms. Tetrachlorvinphos gave the best results and was applied as a cover spray on 12 large animal farms in the northwestern part of Gozo during 1974. Results, using the same criteria as in the 1973 trials, indicated that, although high fly mortality was observed and deposits on walls remained toxic throughout the season, effective control of houseflies and stableflies was not achieved.It is suggested that residual spraying alone on the larger animal farms was not sufficient to overcome the breeding potential of the flies and that a residual spraying campaign could only be successful if more extensively applied and supplemented by a comprehensive programme of farm hygiene improvement.

Bioassays were carried out under laboratory conditions to determine the residual toxicities to the housefly, Musca domestica L., and the stablefly, Sromoxys calcitrans L., of a range of insecticide formulations sprayed on wood, Maltese limestone, cement-washed and lime-washed stone. Stomoxys were more susceptible to all deposits than were Musca. Wettable powders were more effective than emulsions; some being still highly toxic to flies a year after spraying. Most formulations had a shorter life on stone than on wood but a covering of cement could extend the residual life markedly. With tetrachlorvinphos w.p. on stone, respraying nine weeks after the initial spraying resulted in higher kills and longer residual life following the second spraying than after the first.

The results of bioassays using the WHO topical application method are given for 25 insecticides against susceptible laboratory strains of Musca domestica L. and Stomoxys calcitrans L. Many of these were also assayed against M. domestica and S. calcitrans (13 and 9 insecticides respectively) collected from farms in Malta. Comparisons of median lethal doses show resistance of Maltese M. domestica to DDT, dieldrin and malathion as well as increased tolerance to trichlorphon and fenchlorphos. Monitoring tests in subsequent years showed an increase in the tolerance to trichlorphon and fenchlorphos of Maltese M. domestica.