George O'Meara's hand
with a "Mosquito Hawk"
Electronic Mosquito Repellers:
Published in Wing Beats, Summer 2000
F. CORO (1)* AND S. SUÁREZ (2)
A bibliographic review about the use of electroacoustic devices with alleged repellent action on the females of different species of hematophagous mosquitoes is presented. 16 references conclude that these devices do not protect human beings from mosquito bites. The names of 10 of the devices tested as well as those of 19 of the main species of mosquitoes present in the field tests are mentioned. The mosquito species used in laboratory trials are also listed. These tests have been carried out in very different ecological conditions, from Alaska to Equatorial Africa. It is also stressed out that the high intensity ultrasonic frequencies emitted by some of these devices produces a potentially harmful effect on man.
The development of resistance mechanisms by many mosquito species to most, if not all, of the chemicals used up to now to control them, together with the pollution that these substances elicit in the environment, have made researchers to search for physical methods to which no resistance will appear and that would be environmentally "clean". Among these are acoustical devices, some of which have been designed as attractants and others as repellers for mosquitoes. Although acoustic attractants have evident theoretical advantages, there is not still a unanimous criterion about their effectiveness in the field. On the other hand, the alleged repellent effects of different commercial acoustic devices have not been demonstrated in research work done in the field and in laboratory conditions.
The aim of this article is to present a bibliographic review about the acoustic control of hematophagous mosquitoes. We consider that this method is still valid as an alternative and in other cases as a complement of other methods used to control these disease-carrying vectors.
All human beings with normal audition detect, the annoying for us, buzz produced by mosquitoes while flying. This sound is the by-product of their wing beat, and its fundamental frequency is in the range between 180 and 800 cycles per second (Hz), varying with species, sex, age and temperature (Kahm et al. 1945, Belton and Costello 1979, Tamarina et al. 1980, Belton 1986, Ogawa and Kanda 1986, Clements 1999). It has been demonstrated that males are attracted by the fundamental frequency of the female wing beat (Ikeshoji 1981, Ikeshoji et al. 1990, Clements 1999).
The first field tests done in the World using sound recordings of mosquitoes to attract males were done in at that time swampy area of Havana (Kahn et al. 1949). These American researchers used the sounds produced by Anopheles albimanus Wiedemann recorded in a disk plate and reproduced in the field in the area of the Husillo Swamp. However, the effective range of this acoustic trap was small, since the increase of sound intensity elicited a repelling effect. On the other hand, the attractant effect was exerted only to the flying males, without affecting those perching in the vegetation.
After this initial trial, various researchers have done experiments on the acoustic behavior of different mosquito species, using the frequencies present in the spectrum of female wing beat. Both in the studies done before 1970 (Wishart and Riordan 1959, Belton 1967), and during the 1980's mainly in Asia (Ikeshoji et al. 1985, Kanda et al. 1987, Ogawa 1988, Leemingsawat 1989) the aim was to attract the males. As a conclusion about his analysis on acoustic traps, Service (1993) states that it does not seem probable that they will be widely used, because they have a small range of attractancy, they mainly attract males and usually they have to be complemented with attractant odors, as those produced by living animals and carbon dioxide. However, we think that these outdraws do not rule out the possibility of doing further laboratory and field tests to develop acoustic traps for sampling mosquito populations of high epidemiological interest, and consequently applying the needed control measures only at the convenient moments, eliciting the least pollution and mosquito resistance possible.
Another form of using acoustic waves for controlling hematophagous mosquitoes, and that is the main aim of this review, started from an article published in Popular Electronics (Greenlee 1970), in which the construction of a simple electronic circuit that generates a frequency between 2000 and 2500 Hz is described. This signal emitted by a loudspeaker (in the original electronic device an earphone was used) is supposed to elicit repulsion in mosquito females. Greenlee (1970) states that this is an experimental device, and that the author does not offer absolute guarantee that it will avoid mosquito bites. Soon after this, at the beginning of the 1970's, "Electronic Mosquito Repellers" appeared commercially in the market. Greenlee (1970) does not refer to any scientific result that would support the repulsive effect of the frequency used by his device, which is much higher than that of the wing beat of most mosquito species (Clements 1999).
It is understandable then that these electronic acoustic devices were almost simultaneously tested in different parts of the World: Delaware (Kutz 1974), Alaska (Gorham 1974) and the former USSR (Rasnitsyn et al. 1974). Both in these first field trials, as well as in those done some time later in California (García et al. 1976) and Florida (Schreck et al. 1977) in the USA, in the Canadian provinces of Ontario (Helson and Wright 1977), British Columbia (Belton 1981) and Quebec (Lewis et al. 1982), in the villages of Keneba and Bansang in Gambia, West Africa (Snow 1977) and in the former German Democratic Republic (Iglisch 1983) the results have all been negative, i.e. none of the electronic acoustic devices tested (Table 1) showed a significant decrease of mosquito bites in the people using them. If one adds to these the also negative results obtained in Zaire by L. Cook (mentioned by Curtis and White 1982), then it may be said that the field trials of these devices have been done in very different latitudes and ecological conditions, from Alaska to Equatorial Africa. In these tests hematophagous mosquitoes of different habits and significance as vectors of diseases or that only represent annoying pests for outdoor activities of man, including tourism, have been present (Table 2).
In the laboratory tests, inseminated females have been used, since they are the ones that bite for blood sucking, thus assuring the development of their eggs. Using the devices mentioned in Table 1 and the mosquito species listed in Table 3, researchers also reported negative results regarding the repellent effect of sound frequencies ranging from 2000 to 6500 Hz on mosquito females.
Besides the already mentioned electronic acoustic devices, in the 1980's appeared others that emitted frequencies above 20,000 Hz, i.e. above the spectrum audible to man, thus named ultrasonic frequencies. The echolocation calls of many species of insectivorous bats are ultrasonic (Neuweiler 1984), and maybe this fact together with what it is said in the documents of the Patent granted to White (1975) for the construction of an electronic device for insect repelling regarding that mosquito females have antipathy for frequencies between 36,000 and 38,000 Hz, made that the next generation of these devices generated ultrasonic frequencies. Some of these devices have been tested with different species of mosquitoes in laboratory conditions (Table 3), always with results contrary to those claimed in their advertisement (Schreck et al. 1984, Foster and Lutes 1985). Schreck et al (1984) also report that these ultrasonic frequencies do not affect either the behavior of a cockroach species, Blatella germanica.
With the devices that generate ultrasonic frequencies one has to know also the intensity at which they are emitted, since high intensities, above 90 dB SPL (0 dB SPL= 20 µPa), may be harmful to man, whom while not hearing these frequencies, does not avoid them. One of the devices tested by Schreck et al. (1984) emitted intensities above 90 dB SPL.
Another type of electronic mosquito repeller is claimed to "mimic perfectly the sound of dragon-flies, natural enemies of mosquitoes". In this case Schreiber et al (1991) measured that the frequency emitted was 30 Hz, much lower than the minimum frequency reported for the mosquito wing beat (Clements 1999). Schreiber et al (1991) with field and laboratory tests in Florida and Curtis (1994) with laboratory trials in London, England also report negative results of this device in eliciting a repellent effect on mosquito females.
Curtis et al. (1990) and Clements (1999) point out that undoubtedly the wing beat sound made by mosquito females have an attractant effect on males, but that females do not seem to respond to acoustic stimuli. In any case, the great majority of the electronic acoustic devices tested emit frequencies much higher than those of mosquito wing beat, and at least 16 different studies coincide in stating that they were not effective in decreasing mosquito bites in the people using them.
In England two commercial companies were fined for making unfundamented advertisement in announcing their electronic mosquito repellers (Curtis et al. 1994). These authors continue saying that it would be desirable that similar legal procedures were done in those countries in which they are possible, since these companies are not only defrauding considerable amounts of money from the consumers, but they are giving them a false feeling of security that these electronic devices will protect them from diseases transmitted by mosquitoes, such as malaria. Curtis (1994) details the judicial trials that conducted to the mentioned fines, and concludes that electronic acoustic devices are still advertised and even worst, sold claiming that they repel mosquitoes.
In this review article we give 16 direct and one indirect references about the alleged "effectiveness" of electronic mosquito repellers. The data from this scientific literature clearly demonstrate that these devices do not protect the people that use them against the always annoying, and some times dangerous, mosquito bites.
Address of corresponding author: Dr. Frank Coro