Insects with a good memory for places
An alternative approach to wasp control
Dr. Manfred Fuchs, Koblenz, Germany
*(All seasonal indications are related to Europe)*

After an early spring with no major cold spells, followed by a long hot and dry summer, numerous large colonies of wasps have to be expected. The result is a corresponding increase in the nuisance they cause, and in the health risk they pose: not only with their stings but also - though less visibly - through their contamination of foodstuffs and beverages. This is because they are carriers of pathogenic micro-organisms.

This would not be such a big problem if these pests only went for foodstuffs that contain sugar - a substance that inhibits the breeding of germs. Wasps, however are not mainly attracted to sweet things. The attentive observer will notice that protein-containing foods such as meat and sausage are also prime target for the wasp's air-raids. This is because wasp larvae have a very high protein requirement which is mainly met by wasps attacking other insects, especially flies.

As stables, manure heaps, faeces, and decomposing animal cadavers are highly suitable depositories for flies' eggs, they are major attraction for wasps, which duly pick up pathogenic germs. Back in 1952/53 and in 1960 Döhring (1) found wasps on rubbish dumps and in pigsties almost 100 % contaminated with faecal germs.

Anyone who has seen a dense swarm of wasps going to work on a not yet too severely decomposed animal cadaver is aware of the very real threat this can subsequently pose to human hygiene. The insects often infect their stings with puss pathogens which can later be injected into human skin together with the wasp's own poison.

In such cases, the body's reaction to the sting is particularly severe. Swelling occurs around the area stung, followed by painful inflammation and festering that may even lead to blood poisoning.

Of the central European wasp varieties, three are often to be found in the vicinity of people: the common wasp (Paravespula vulgaris) the German wasp (Paravespula germanica), and the hornet (Vespa crabro). The hornet is the biggest wasp and the one which is most likely to sting. With all varieties of social wasps, it is always the female workers and queen that sting. The males, which are to be found with the young queens or with the autumn queens in the months from August to October do not have a sting.

As is well known, the nests consists of a paperlike mass. They are built and constantly repaired and upgraded by the female insects using wood meal and saliva - in the spring the queen does the work alone after having survived the winter. If weathered wood is used, nests have a largely greyish colour. A nest covering of stripy appearance indicates the use of various types of wood. No matter how large a nest is , it was always built during a single breeding season and will never be used a second time. Only the fertilized young queens survive the winter and each build a new nest in the spring - usually from late April onwards. Initially the queen also searches for food and handles the business of brooding on her own, until the first female workers are hatched by mid June. In the weeks and months that follow, the wasp colony undergoes massive expansion. The highest population count is reached in the second half of August, and numbers gradually begin to dwindle only from late September onwards. The population then declines sharply, and by mid-November all the female workers, the nest's founder (the old queen) and the males have died out. The wasp year begins anew the following spring when the young queens restart the cycle.

The airborne wasp presence is at it height in the months from August to October. A female worker flying out of the nest has one of three reasons for doing so: she is looking for nest-building material, water or food. Once she has found a bountiful source, she will return there time and again. Her task is made easier by an excellent memory for places and by an ability to gauge where the sun is in the sky even in cloudy weather, taking into account positional changes over the course of the day in relation to the food source. This is a capability which wasps have in common with bees, which have a higher level of organisation. Unlike bees, however, they never create stocks of food.

The main source of nourishment for all wasps and hornets, particular the brood, is provided by other insects and spiders. As high-protein food quickly spoils, it is not possible to keep stocks. That is the main reason why colonies of wasps die out in the autumn.

In warmer climes, wasp species which are also indigenous in central Europe, do build nests for use over several years. These may reach astounding sizes. There are basically three ways of combating wasps:

1. Eliminating the queens in the spring or autumn - this prevents wasps' nests from being built in the first place.
2. Eliminating the female workers in the summer - this has the effect of weakening the population: in the best possible scenario, the brood dies out. Eliminating the female workers in late summer and autumn - essentially this merely reduces the level of nuisance.
3. Mechanical destruction of the nest or rapid destruction of the population using insecticides.

The first procedure is the most elegant solution. The problem is that the queens do not make themselves noticeable in the spring, and catching them in traps is the exception rather than the rule. The autumnal success rate is a lot higher. As the female workers die out, the young queens have to look for food themselves. They can then often be caught in jars containing bait liquids (sour fruit juices, malt beer, slightly sugared vinegar - in summer months, do not use sugar-, honey- or syrup-water, as this will attract bees), if these are set as traps in late summer. The second approach is steeped in tradition, and will serve at best to reduce the wasp nuisance if conventional traps are hung up in the vicinity of the nest. What is more, will give the person concerned the satisfaction of his or her having "paid back" the wasps for their unwanted attentions.

A large nest accommodating thousands of female workers will not be appreciable weakened by a few hundred specimens being caught.

Before a nest can be destroyed, however, it first has to be found. Careful observation of the wasps' movements over an approximately 50 metre radius usually leads to the nest being successfully located. The range over which wasps will usually search for food is 50 to 250 metres from the nest, although wasps are capable of flying far greater distances.

The actual task of destroying the nest, sometimes using anaesthetic substances such as laughing gas or carbon dioxide, is often performed by fire brigades. The people doing the business are subject to the most virulent attacks by the enraged wasps, and protective clothing does not always prevent stings.

Moreover, the sound of an attacking wasp differs markedly from the usual flight sound and causes other wasps to likewise attack. Also, the poison-secreting gland also produces an alarm-triggering substance which the wasp hurls in droplets at the enemy, thus leaving a marker that will tell other wasps to attack. This substance is not identical to the wasp's poison.

When going about the job of removing a wasp's nest, the professional pest controller will avoid endangering him- or herself or other people. He or she will therefore do the job late in the evening or early in the morning, when the outside temperature is as low as possible. Most female workers are then in the nest. It should not be forgotten, however, that a considerable number of wasps will spend the night outside the nest, and that flight movements will not be discontinued until the light is very weak and will resume as soon as the morning light permits (1.5 lux). Hornets quite often hunt at night when light conditions range between 0.2 and 0,03 lux. (2)

If insecticides are used, evenings and mornings are the best times. An aerosol can (with locking valve) attached, if necessary, to the end of a sufficiently long rod and held in the immediate vicinity of the nest will do the trick. However, if the use of insecticide is liable to cause problems (e.g. if the nest is to a kindergarten, school, hospital or food-processing factory), a highly effective alternative is to use UV-A-light flytrap reflectors featuring adhesive trapping surfaces. Longwave light in the 365 nanometre range attracts numerous flying insects because the sun is the only natural source of this light, and when the insect picks this light up either directly from the sun or as global radiation from a cloudless sky, the message it receives is that there is an unobstructed flight path ahead.

As is the case with many insects, colour vision for wasps begins in the longwave UV range, and the ultraviolet receptors in the wasp's eye reach maximum absorption at precisely 365 nanometres. The luring effect on a phototropically disposed wasp is therefore particularly great. A wasp flying out of the nest is constantly seeking an unimpeded flight path. It therefore takes the radiation of a UV-A lamp to be bright daylight and flies directly towards it, only to be trapped on the adhesive surface. Conversely, a Wasp that is looking for food or wood and flies to a window or to a dark piece of wood is photophobic. If it is forced to fly away to escape danger or if it has found what is was looking for, however, the mode switches over and it will once again seek an unimpeded flight path. If the light trap is now the brightest source of ultraviolet A-region light in the vicinity, the wasp will head straight for it. The picture illustrates the effectiveness of such trap (iGu® FANGREFLEKTOR^® FR 3003). Compared with units using a UV-A lamp surrounded by a high -voltage grid, the adhesive foil offers a number of advantages: it keeps a firm hold on the insect, whereas high-voltage bug killers are liable to hurl the electrocuted insects out of the device and possibly into foodstuffs (unhygienic); and the insect may still be able to sting (dangerous) If they remain on the grid, they will burn there, giving of a very unpleasant smell. The trapping principle embodied by the adhesive surface is therefore preferable.

Bug control tests carried out in the Koblenz area (Germany) over recent years have shown that the wasp nuisance in bakeries, on fruitstalls, in cafes and in beer gardens can be appreciable reduced. The direct elimination of female workers flying out of the nest efficiently decimates a wasp colony. Within a few days, the population collapses, provided the trap is well placed and the adhesive foils are exchanged when full.

References:
(1) Kemper, H.; Döhring, E. !1967: Die sozialen Faltenwespen Mitteleuropas: published by Verlag Paul Parey, Berlin/Hamburg 1967
(2) Edwards, R. (1980): Social wasps. Their biology and control. The Rentokil Library, Rentokil Ltd., Felcourt, East Grinstead, 1980 ISBN 0 0906 564 018.

Author: RD Dr. M. E. A. Fuchs, Zentrales Institut des Sanitätsdienstes der Bundeswehr Koblenz (Central Institute of the German Armed Forces' Medical Corps, Koblenz), Ernst Rodenwaldt Institut - Medizinische Zoologie.

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Insect Control using light traps
How effective are modern devices?

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 micro-organisms 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 micro-organisms 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 micro-organisms 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.

Schematic representation of a housefly sucking in a liquefied
and pre-digested food particle by dabbing

Virtually simultaneously a dropping of faeces is excreted, releasing germs from the fly's intestines. This mode of transmission is termed "facultative 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.

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.

One of the smallest devices currently available on the world market employing this combination without an electric grid is called the FANGREFLEKTOR iGu FR 3003 from iGu Transtrade Ltd., Christchurch, New Zealand. 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 iGu flytrap reflector's easy-to-change foil has a surface area of 630 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 iGu's 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.

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. 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?"

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