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 As
well as causing a nuisance with their stings (midges, wasps, stomoxyine
flies) or by their mere approach and skin contact (houseflies), flying
insects can directly transmit pathogenic microorganisms to humans and pets
as well as to economically useful animals and plants. The damage that can
thereby arise may also occur indirectly as a result of foodstuffs and
articles of daily use being infected with germs.
The housefly (musca
domestica), the stable fly (musca stabulans) as well as
carrion- and faeces-visiting fleshflies (sarcophaga spec.)
greenbottles (lucilia spec), phormiae and bluebottles (calliphora
spec.) are regarded as being particularly effective vectors of disease.
Transmission
of pathogens
With
flies the transmission of pathogenic microorganisms is inevitable, every
fly being able to carry up to five million germs including the pathogens
of such serious diseases as typhoid, cholera, dysentery, polio, pneumonia
and foot and mouth disease. On a suitable substratum germs multiply very
rapidly. Thus, with the aid of a culture medium (blood agar), it can very
easily be demonstrated that a housefly, for example, leaves behind its own
bacterial trail.
The manner in which a housefly goes about its food intake promotes the
transmission of microorganisms in several ways:
1. By
walking around on the surface of the food “facultatively tactile”
contamination takes place.
2. Houseflies,
in common with other species of fly having a proboscis, cannot take
nourishment in the form of solids. The proboscis secretes digestive fluids
onto the food so as provide liquefaction and accomplish partial
extracorporeal digestion. The resultant solution is then sucked in by
dabbing. Fig. 1 is a schematic representation of this process.
Virtually simultaneously a dropping of faeces is excreted, releasing germs
from the fly’s intestines. This mode of transmission is termed
“facultatively excretory.” If a foodstuff is moist and contains
protein, it is also used as an egg depositary. A housefly lays a total of
around 2’000 eggs. Given a suitable temperature the maggots hatch out
within a matter of hours.
Reducing
the numbers of injurious flying insects has therefore always been a major
objective of human endeavour.
It
is a well known fact that insects will fly towards sources of light.
Anyone can observe this phenomenon on a summer’s evening. Provided they
have a fundamentally phototropic disposition, nocturnally active insects
will fly towards any source of light, naked flame (candles, oil and gas
lamps) and electric light of any spectral composition that can be
perceived by the insect eye. Diurnal flyers do not do this to the same
extent. They are more strongly attracted if the light source emits
portions of long-wave A-region ultraviolet light in the 365 nm range.
While night flyers interpret any visible light source as signifying an
open flying space, the day flyer - whose movements in any case take place
in daylight - requires a more specific signal indicative of open flying
space. Just such a signal is provided by long-wave ultraviolet light,
which as far as the insect is concerned can only emanate from the sun or,
as global radiation, from a cloudless sky. Many winged insects have
undergone specific evolutionary adaptation to this type of signal: the
ultraviolet-sensitive receptor in the compound eye exhibits maximum
absorption at 365 nm.
Abb.1: Schematische Darstellung einer
Stubenfliege beim Auftupfen eines
verflüssigten und vorverdauten Nahrungspartikels.
UV-A
light traps
Knowledge
of this phenomenon has been exploited for years in the design of flytraps
employing A-region ultraviolet light. The source of light is provided by
tube-type fluorescent lamps with a rating of between 4 and 40 Watt. They
emit a bluish-white light which always contains a certain portion of light
in the 365 nm radiation range.
If an insect’s behavioural circumstances are conducive to flight, this
light will exert a “luring” effect as it contains for the bug the
basic information that open flying space is available. With this type of
trap, however, no differentiation can be made between so-called injurious,
indifferent and useful species.
It is hardly surprising, therefore, that the use of flytrap devices
employing this luring principle is banned in outdoor areas. And they will
remain restricted to indoor application. Their deployment is appropriate
and necessary in rooms where hygiene is of the essence, e.g. in food
production and processing operations, in all clean-room areas in the
chemical and pharmaceutical industry, in corresponding research
establishments and in hospitals; other areas of application are animal
accommodation (including for experimental animals) and of course hotels
and homes whenever flying insects become bothersome.
In extensive tests which have been carried out here since 1974, the
results of which have been reported on several times (Fuchs 1975, Mainhart
1980, Fuchs 1992), it has been established that all traps become more
effective as the amount of A-region ultraviolet light increases. An upper
limit has not been detected to date. UV-reflective surface areas behind
the tube-type fluorescent lamp increase the amount of UV-A light and thus
enhance trapping performance.
Additional orientation aids can be offered to the flying insect as it
makes its approach. For example, the trapping rate increases if the
housing of the particular device provides a stark contrast between the
light source and its background. Inter alia, the UVA-light-emitting
fluorescent tube with its 50 - 60 Hz flicker produces a “lighthouse”
effect. This is because the compound eyes of a flying insect possess a
much higher fusion frequency than, for example, the human eye. Whereas for
us a tube-type fluorescent lamp emits light uniformly, to such an insect
it appears to be constantly switching on and off.
Trapping
principles
In
conjunction with the luring effect of A-region ultraviolet light two
trapping/bug-kill principles are applied:
1) Arranged in the immediate vicinity of the ultraviolet-light-emitting
tube(s) is a high-voltage grid or a grid/plate combination designed to
electrocute the insect as it lands by means of a short-circuit spark. For
safety reasons the amperage is low (up to 15 mA), the voltage being mostly
several thousand V.
Modern large-size devices of this type turn in an extraordinarily high
trapping performance but do not always meet hygiene requirements. In the
most favourable case the insect is killed immediately, falls vertically
into a catching bowl attached to the bottom of the unit and is thus
initially hygienically removed. However, a draught may blow dead insects
or insect particles out of the catching bowl. After all, a housefly weighs
only around a milligram.
Very often, however, the insect is torn to pieces by the short-circuit
spark. Particles are hurled out of the device and contaminate surfaces and
various objects - depending on the use to which the room is put - and in
the worst case foodstuffs are affected. The bottom line is non-compliance
with statutory hygiene requirements.
Even though it appears otherwise, the least objectionable scenario from
the hygienic point of view is when an insect, mostly a large one such as a
wasp or a bluebottle, remains attached to the electrical grid, drying out
and burning in the electric arc. This is because all germs are thereby
destroyed. Other disadvantages arise, however. During the burning process
the high voltage system is down and further insects coming into land are
not destroyed. There is an unpleasant smell of burning. The vapours
blacken the high-voltage grid, reducing the level of ultraviolet
reflection and hence the luring effect. - In rooms whose atmosphere
entails the risk of explosion, the use of electrical traps is out of the
question.
2) A further fly-kill principle which has only been used in conjunction
with ultraviolet-light traps over the past few years involves the use of
adhesive-coated surfaces positioned in a semi-circular arrangement behind
or next to the UV-A lamps. The transparent adhesive substance, applied to
thin cardboard, is exposed by peeling off a protective foil. It reflects
A-region ultraviolet light, remains sticky for a very long time and traps
insects up to the size of a hornet securely and hygienically. (Normal
adhesives quickly lose their stickiness when subjected to irradiation by
A-region ultraviolet light). From the aspect of hygiene, therefore, this
trapping principle is definitely the preferred solution.
The smallest device currently available on the world market employing this
combination without an electric grid is called the FANGREFLEKTOR FR 3003.
When placed in a 40m3 room containing 200 houseflies, this unit achieves a
trapping rate of 100% in just 4 hours, at an LT50 (= time after which 50%
of the flies are caught) of 43 minutes. In the case of
high-voltage-grid-type devices, this level of performance is only achieved
by larger-size and appreciably more expensive industrial units. The
flytrap reflector’s easy-to-change foil has a surface area of 890 cm2;
the U-shaped tubular lamp emitting A-region ultraviolet light is rated at
10 Watt.
The biggest units of this type currently available are the FR 8008-series
flytrap reflectors. The FR 8008 features two foil holders with a total
surface area of 4,800 cm2 and operates with two 60-cm long 20-Watt tubes.
Two further versions are offered, one featuring two shielded 40-Watt UV-A
fluorescent tubes and the other equipped with two explosion-proof lamps
each rated at 20 Watt.
Size comparison of the two
traps.
The average
trapping performances.
FR8008 o---o 200 houseflies in the same
room
FR8008 o---o 20 houseflies in the same room
FR3003 x---x 200 houseflies in the same room
FR3003 x---x 20 houseflies in the same room
The
FR 8008 catches 200 houseflies in the same room in one hour and thirty
minutes, with the LT50 mark being reached in just a quarter of an hour.
It is interesting to note that, related to the size of the traps, the
trapping times evidently do not depend on the
number of flies used in the experiment.
If there are 20 flies in the room, these are not eliminated in a shorter
time than 200 flies (Fig. 3). In a room of the same size the insects’
inclination to fly increases with the amount of ultraviolet light emitted.
Clearly, a higher amount of ultraviolet light also increases the luring
distance. Even in homogenous fly populations of musca domestica, however,
the intention to seek open flying space is evidently statistically
congruent. From this it must be concluded that in rooms where hygiene is
of the essence only large traps should be used, even if the occurrence of
flies is low.
The widely held view that where the incidence of pests is low a small-size
trap will suffice is erroneous. Rather, before recommending a trap size (=
specification of trap capacity) the question that has to be asked is
“how many flies or other winged insects is the user prepared to tolerate
in a room and for how long?”
References
-Fuchs, M.E.A. Der
Einsatz des elektrischen Insektentöters “Voltinex” in der
Versuchstierhaltung (The use of the “Voltinex” electic insect killer
in accommodation for experimental animals)
D.prakt. Schädlingsbek. Vol. 27 (1975) pp. 153-154
- Meinhart, A.: 1980. Fluginsektenfallen nach dem Prinzip der UV-Anlockung
- Möglichkeiten zur Steigerung der Fängigkeit gegenüber Musca domestica.
(Flying-insect traps employing the principle of luring with ultraviolet
light - scope for increasing trapping performance in the battle against
musca domestica). Dissertation at EWH Rheinland-Pfalz, Koblenz section.
- Fuchs, M.E.A.: Wespen alternativ bekämpfen (An alternative approach to
combating wasps). D.prakt. Schädlingsbek. Vol. 44 (1992) pp. 196-198.
Author’s address:
RD Dr. M.E.A. Fuchs, Zentrales Institut der Bundeswehr Koblenz
- Ernst-Rodenwald-Institut - Medizinische Zoologie, pob. 73 40, D-56065
Koblenz
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