Flies & Filth

There are many thousands of different species of flies, however, relatively few interact with humans. Still, those that do are among the most destructive of pest species, spreading many diseases to man, domesticated animals and agricultural and horticultural crops.

The housefly and its close relatives are not true vectors of any disease in the way that the mosquito is inextricably linked with the spread of malaria and yellow fever. But they are capable of harboring disease-causing organisms in and on their bodies, and therefore the control of these flies by pest management professionals should be considered vitally important.

GENERAL FLY BIOLOGY. The great success of flies as worldwide pests can be attributed to their highly adaptive life cycle. As is true of many insects, flies have a life cycle in which the juvenile stage, the larva, exploits a different food resource from the adult. This characteristic is advantageous for flies, as it allows them to make use of diverse habitats. In addition, the adult female fly, which has an acutely developed sensory system, is able to locate suitable areas in which to lay her eggs, such that the larvae which emerge from the eggs are able to have adequate food, and be in an area of adequate humidity and the correct temperature.

Furthermore, this pest’s ability to fly with speed and agility has given it an edge over many other insect species, as is evidenced by their worldwide distribution.

This mobility is also of great use at the local level where adult flies move in many directions and frequently enter areas of human habitation and work. This is when their status as pests becomes evident.

Increasingly in areas where humans live, eat and work there is an extremely low tolerance to flies. Houseflies, lesser houseflies, fruit flies and fungus flies can all frequently be encountered in premises where humans are to be found, particularly in areas where plants are grown, as many of the larvae of these flies live on the organic debris that is found in the soil, potted plants, etc.

A single fly, no matter how small, in the wrong place can constitute a pest problem, and as outlined above the great mobility of flies means that no location is totally immune from invasion. The nuisance value of a fly becomes paramount in sensitive areas such as computer company clean rooms, food processing plants, hospital operating rooms, etc., and in these areas the need for control is vital.

As mentioned previously, adult female flies use their complex sensory systems to choose suitable areas of rotting vegetation and decaying animal matter in which to lay their eggs and for the larval stages to develop into pupae. The adults emerge from the pupae in these unsavory sites and during the emergence process can become contaminated. Often they then move into sensitive food preparation, food processing, or food consumption areas seeking feeding sites for themselves as adults.

The possibility of contamination of human food with pathogens has been proven over the years by a number of experiments in which certain disease-causing agents have been found to be able to survive on the outside surface of flies, among the numerous hairs, within their digestive system, and also in their blood system.

Various experiments have focused on a number of disease-causing organisms. Klebsiella bacteria, which are often the causative agents of many lower respiratory tract infections and urinary tract infections, have been found on the outside cuticle and within the gut of a large proportion of flies where an outbreak of Klebsiella has been diagnosed. Cam-pylobacter, a bacterial species which has increasingly been found to be important in public health and is the causative agent of acute gastro-enteritis type complaints, has been isolated from many fly populations. Streptococci, which are often found in skin infections and gut infections in humans, have also been isolated from flies in many situations. And there is increasing evidence that Chlamydia, a parasite that causes a variety of complaints in humans, can be spread by flies. Furthermore, the fungal pathogens Candida sp., Mucor sp., and Aspergillus sp., which cause varying complaints, have all been isolated from flies.

The spread of these disease-causing agents from the fly is most often associated with the feeding process. Flies are fluid feeders, and although they feed on solids, they need to make the food liquid before they can take it into their fore gut for storage and then to the mid gut for digestion. Flies do this by producing large quantities of saliva (their salivary glands stretch throughout their whole body), which is poured onto the food down the salivary canal of the mouthparts. Flies also frequently regurgitate some fore-gut contents onto their food while feeding. It is at this stage that any pathogenic organisms, which could be located in the salivary glands or the fore gut, are spread.

Since the food from which the flies are feeding may have originally been prepared for human consumption, human disease and suffering can result. In addition, during the feeding process flies frequently defecate, which in turn can spread any pathogens from the hind-gut of the fly onto food and food preparation areas. It is clear from the dangers posed by these situations, that there is a very serious need to control flies.

CONTROLLING FLIES. When controlling any insect the proven technique is to concentrate on a vulnerable stage in the life cycle and direct the control measures at that stage. Flies have four developmental stages in their life cycles: the egg, larva, pupa and adult. The egg stage is very short, and the pupae are well protected against the entry of any insecticidal solutions. This leaves only the larval and adult stages as the most effective and realistic targets of control measures.

Larval control is generally carried out by using conventional insecticides to treat the areas where the larvae are feeding or likely to feed. These measures can be highly successful, but they also can be very wasteful, if a large amount of insecticide is used for situations in which it may not be necessary. Recent advances in the biological control of fly larvae may provide an answer. Bacterial control using various varieties of Bacillus thuringiensis against certain fly larvae, particularly those found in aquatic habitats, has been successful, and further developments are taking place in this area to extend the range of species which can be controlled by these techniques.

The Bacillus is a naturally occurring organism which produces a toxic element within the digestive tract of the larval stage of the insect and eventually kills the insect. The organism has also been used to control many caterpillar pests, and it is used in products for the control of sewage flies and mosquitoes.

Adult fly control can be divided into two categories: chemical measures and physical measures. The chemical treatment of adult fly populations with insecticides has limited use. There may be some occasions when an infestation involving large numbers of flies calls for the use of a space treatment, but in general there is little point in filling vast areas of space with insecticides to kill a few flies. It is wasteful and also not desirable, particularly in food preparation or retail outlets.

Physical measures to control adult flies can also be divided into two categories: prevention of entry and trapping.

Prevention of entry, which involves keeping adults out of premises, is of course one way to control flies, but this is extremely difficult to do effectively. The use of door screens, air curtains and window screens has often been successful, but these methods frequently suffer in their effectiveness due to human intervention. When fly screens are attached to doors or windows it is frequently perceived, rightly or wrongly, that they affect the air circulation, and there is always the tendency for people to prop open screened doors and netted windows to encourage free flow of air, thus completely defeating the purpose for which the screens were put in place.

If adult flies do enter the premises then it is generally possible to trap them in a number of ways. When using traps to control adult flies you are exploiting the senses of the fly. Adult flies are always navigating throughout their habitat using, in particular, their sense of sight and their sense of smell. If you design a trap to attract the flies visually and then add an odor to it, you are then making use of the fly’s senses.

Fly traps which use ultraviolet (UV) light as an attractant are the most successful at attracting many flying insects and throughout the world are becoming the industry standard for fly control. UV fly traps exploit a factor that has been in action for a long time! Insects for many thousands of years have used the UV rays from the sun to navigate by and almost all insects have a strong visual response to UV light. In other words, their eyes respond to light at the UV end of the spectrum. We have studied this response in many flies using sophisticated electrophysiological methods and we have found that the UV rays around 365 nanometers in the spectrum are the most readily detected by most flies which are considered as pests in the public health arena; the houseflies, fruit flies, cluster flies, fungus flies, etc.

It should be emphasized that UV rays around 365 nanometers are totally safe, as these are not the low frequency wavelengths which cause sun burn or have germicidal properties. The tubes employed in the high quality electrical fly traps on the market produce UV rays, which are invisible to humans, peaking around the 365 nm wavelength, thus exploiting the visual responses of the flies.

Much research goes in to the production of these UV fly attracting units. Work is carried out to investigate the most attractive color of tubes, the most effective orientation of tubes within the units, and the transformer design and glue board technology. All this research leads to constant changes and improvements.

One area of current research on improving the catch of such traps involves, among other things, incorporating an odor attractant. Flies are attracted to many odors, most of which are pretty foul smelling to us, but research into distilling a particularly attractive combination which will attract the flies and not repulse humans is underway.

Many thousands of different combinations are being tried and research like this is extremely time consuming, but I am sure some useful results will appear in the near future.

Dr. Moray Anderson is technical director of The Killgerm Group, Birmingham, England.