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).

 

 

Resistance Management

The creation of resistant strains in the field can be countered by using insecticides according to resistant management strategies.

Three basic strategies have been developed and refined (Georghiou, 1994):

• MODERATION
• SATURATION
• MULTIPLE ATTACK

These concepts can be, and are in many cases, combined. For example, the Multiple Attack may be used in the Moderation and Saturation concepts.

A Saturation area on a single farm can be protected (with fly screens on the windows and ventilation units, doors with curtains etc.) from a wider Moderation area.

Climate, reinfestation pressure, manure management practices etc. also play a part in the choice of the correct strategy.

The aim is always:

to avoid strong selection of consecutive generations of a fly population with a single insecticide.

“Population” is a key word in this statement.

In some areas a single fly population will stretch over a whole neighborhood. A local saturation strategy may turn out to be a Moderation strategy when the true extent of the fly population is discovered.

Untreated insects within the population dilute the selection pressure. “Refugia” are deliberately left or created as a source of susceptible insects in cropping systems. This should be considered in fly control, on a case-by-case basis.

In other areas the whole fly population will be within a single building, and in some systems it is possible to eliminate the whole fly population, e.g. when all animals are removed in the winter and the building is disinfected. This “saturation” treatment eliminates selection pressure as well.

Advances in biochemistry, molecular genetics (Hemingway & Ranson, 2000), ecology, population dynamics, monitoring etc. have made the house fly resistance problem less alarming in recent years. Workable solutions have been developed in many situations.

However, strategies continue to be developed further, and local recommendations still need to be periodically updated with state-of-the-art information.

 

 

Moderation Strategy

This concept entails the acceptance of “nuisance thresholds”.

In many systems we can accept the presence of a certain number of flies. There are many beneficial organisms keeping fly populations lower than they would otherwise be.

However, at certain times of the year fly numbers are no longer tolerable and short-lived products can be used.

Granular baits, spread where flies congregate, are exposed to daily farm activities and functionally short-lived.

Paint-on baits present a depot of attractive material which is actively imbibed by the insects. A concentration can be chosen to minimise the possibility of selection at sublethal doses. Flies that find the depot are killed and the others are not selected. This again follows the concept of moderation.

Baits have little direct effect on beneficial organisms, as they are carefully placed for maximum effect on the adult flies.

It is also possible to target key breeding sites, using a narrow-spectrum larvicide on “hot spots” at the start of the season (or as needed).

Natural biological control agents have a good chance of survival in this system. In fact, larvae surviving a “hot spot” treatment (and possibly carrying genes for resistance) may either be eliminated by (hungry!) parasites and predators, or mate with incoming adults which have not been exposed to the larvicide.

 

 

Saturation Strategy

Using this management strategy, flies are eliminated completely from a certain area.

The area is cleaned out and then thoroughly treated, for example with a larvicide.

Residual sprays are not used as part of a saturation strategy, as they remain partially effective for a long period of time.

However, in some all-in, all-out husbandry systems, it is possible to treat the structure adequately for one batch of animals, then clean well and retreat for the next.

Treatment of suitable objects, sacks or boards, which are taken away (cleaned and/or retreated) while still fully effective, reduces the problem in buildings that are in constant use.

 

 

Multiple Attack Strategy

Here, a combination of unrelated products is used. Using this model, if insects survive, say, a larvicide treatment, they are killed as adults by a bait.

The larvicide and bait must be from complementary chemical groups showing no cross-resistance.

Multiple attack may combine a chemical and biological component. The chemical can be a bait or a narrow-spectrum larvicide; the biological agent can be a beetle or parasitic wasp.

Bait could also be used, combined with a fly which competes with house flies in the larval environment.