The house fly is one of the most highly-developed insects, with rapid and efficient reproduction. The adult is omnivorous, highly adaptable, and appears to have "the greatest ability to develop resistance to insecticides over the widest geographical area” (Agarwal, 1979: UNEP report).
Resistance to insecticides is an evolutionary process. With hindsight, it is therefore clear why this species has rapidly developed resistance to various insecticides.
The build up of resistance to insecticides, from DDT onwards, was very carefully monitored in Denmark between 1948 and 1992 (Keiding, 1999). Studying resistance in houseflies also enabled decisions to be made regarding resistance prevention in less adaptable (and less easily cultured) insects.
Regular and widespread use of residual insecticides in animal houses in Denmark led to rapid development of resistance, although these treatments remained partially effective for a long period of time.
Key factors were the exposure of sequential generations of semi-isolated populations, with great seasonal fluctuations, to sub-lethal doses of insecticide.
Each such selected (pressurised) population leaves survivors. Some of these flies survived because they carried genetic information which enabled them to deal with the insecticides.
Exposure was repeated with the offspring and the genetic traits strengthened. With time, selection for resistance, and other fitness factors (especially winter hardiness), can overlap so that the final populations are both resistant and fit.
Once this stage is reached, resistance will not disappear when selection pressure stops.
The use of a sugar bait in Denmark, painted on fly resting places, from 1958 onwards, did not induce important resistance. On the other hand, intensive, regular use of aerosols did create resistant populations.
Indiscriminate use of an insecticide with a broad spectrum of activity against adults and larvae is likely to produce resistance against the individual chemical. Killing beneficial mites, wasps, beetles and spiders further aggravates the situation.
Cross-resistance has also become a major consideration, as resistance to older products has been found to confer resistance to newer products.
This can be because the chemicals share a mutual target site within the insect (e.g. DDT and synthetic pyrethroids), or because of broadly effective mechanisms (e.g. penetration resistance), or because specific biochemical mechanisms affect both molecules (e.g. elevated esterase levels).
As resistance combined with fitness tends to be stable in house fly populations, sequential build up of multiresistant strains was seen.
Many factors: genetic, biological and operational play a role in the evolution of resistance (Georghiou & Taylor, 1986). They can be evaluated in a 'Resistance Risk Assessment' (Keiding, 1986).
Novel compounds should be chosen on their ability to kill multiresistant strains and on their lack of resistance development under selection pressure in the laboratory (Keiding et al, 1991, 1992).
