SIMULIIDAE (Black flies)

Introduction Morphological Features

IntroductionContents

Simuliids are small stout flies with an unusually pronounced curved thorax, this has led to the colloquial name of buffalo gnats. Black flies belong to the suborder Nematocera and the family Simmuliidae. It is only the females that feed on blood with males being nectar feeders. The family contains approximately 1500 species and four genera, the Austrosimulium, Cnephia, Prosimulium and Simulium are of economic importance. Black flies have a life cycle with an aquatic larval stage which can occur in very dense numbers in fast flowing waters. Certain species of the genus Cnephia such as C.pecuarum prefer slower flowing waters and in areas such as the southern Mississippi in the USA this species has bred and developed so prolifically that vast dense swarms have occurred. The flies can form into swarms so thick that severe economic losses can result and in extreme cases death of livestock. Death can be caused by a combination of anaphylactic shock from bites, blood loss and respiratory problems due to the inhalation of flies. Black flies are important vectors of various pathogens the most important of which is the filarial worm Onchocera volvulus which causes river blindness in humans. Other pathogens carried by mechanical transmission include myxomatosis, Leucocytozoon and avian trypanosomes which infect both domestic and wild birds. Less frequently they have been incriminated in the transmission of Venezuelan equine encephalitis, an arbovirus.

Morphological features Contents

Wing length varies from 1.5-6.5mm, the anterior veins are usually prominent on the scaleless wings (fig 1) below. I= pronounced anterioral veins. The actual body length ranges from 1-5mm and colouration can vary from black through to silvery grey. Orange and yellowish setae are sometimes present on the thorax and abdomen. The flies have a very distinctive humped thorax which can be seen in the image at the top of this page. The antennae of both sexes are small and comprise of oval segments which look like slightly compressed beads. Antennal segments can range from 9-12. Several sensilla are present on the antennae, these include olfactory receptors, contact chemoreceptors and mechanoreceptors. Individual species show considerable differences in the type and density of the various sensilla as well as between mammalophilic and ornithophilic males and females. Further differences are seen between autogenous and anautogenous species.

The palps extend further than the proboscis, most of the palps length is derived from the terminal segment. The third segment on the five segmented palps contain the Lutz organ, this projects outward forming a sensory peg. The palps are held in a curved position when resting by muscles situated within the basal segment. This positioning of the palps serves to orientate the Lutz organ forward. Four types of sensilla are found on the palps, the sensilla chaetica- thought to be mechanoreceptors, capitate pegs which are found tightly grouped on the floor of the Lutz organ, thick walled pegs, believed to be chemoreceptors and campaniform organs, which are mechanoreceptors. Males of all species have been shown to have a lower mean score of sensilla than females.

The compound eyes are well developed especially in the males. The males posses compound eyes which are composed of two different types of ommatidia. The males eyes cover the majority of the head with only the areas from which the proboscis, the clypeus and the frons into which the antennae are inserted being uncovered. The males eyes are divided across the "equatorial line" into a ventral and dorsal area. The ommatidia in the ventral area shows normal type ommatidia whilst those in the dorsal region are greatly enlarged being much longer than normal and extending through the basement membrane. Pigmentation between the two regions of the eye also differs with a reddish-brown pigment in the ventral cells and a lightish-brown pigment in the dorsal region. The dorsal eye is designed to detect the small females as they fly over male swarms. The eye enables the male to locate the female against the blue backdrop of the sky.

Life cycle Contents

The eggs of simuliids are laid on the surface of flowing water where they gradually sink. Some species have oviposition sites that include rocks and vegetation these are however always in very close association with water and are found in the splash zone so they are wetted immediately on oviposition. The eggs are oviposited some distance upstream from the emergence site to compensate for the larval drift. The female therefore exhibits positive anemotaxis in order to orientate upstream. It is thought that the shimmering off the surface of the running water acts as a stimulus to females for ovipositing. Eggs are laid in batches ranging from 100-600. Certain species have been shown to prefer particular colour substrates when ovipositing with S. verecundum and S. vittatum showing a strong preference for yellow. The particular parameters preferred by individual species for ovipositing will reduce interspecific competition within the same river section. Females of the same species often oviposit in close proximity and it is thought that ovipositing females release an aggregation pheromone. Eggs range in size from 100-400µm and are oval to triangular in shape. The eggs of all species are suspectable to desiccation. This is seen even in those species which have eggs that have long development periods due to being dormant in wet river sediments. A. pestilens has this development cycle but even the eggs of this species succumb to desiccation at relative humidities of 96% or less. Eggs of most species hatch within a matter of days with a mean period of sixteen days for S. damnosum. The delayed hatching seen in multiple layers of eggs is due to oxygen deprivation in the deeper layers. Temperature also effects development time and hatching of eggs, in species such as Stegoptera mutata temperatures as low as -15 have been survived. In temperate climates numerous species exhibit a resting phase during the egg stage which is under the control of environmental changes such as the seasons. In tropical species some simuliids are able to survive periods within the egg and then hatch when the flow rate reaches a threshold velocity.

The larva of simuliids do not have a fixed number of instar stages, the majority show a range from 6-9. The larva of black fly are either browsers, feeding on organic material attached to the substrate, or filter feeders. Browsers are not common and found mostly in the two genera Twinia and Gymnopais. Those larva with filters can also browse although they generally have no need for this method. Larva of black flies have delicate cephalic fans comprised from a series of rays shown in the figure opposite. The filter feeding mechanism is passive with debris being filtered directly from the passing current. Another morphological feature of the larva to facilitate feeding is a series of hooks at the rear of the abdomen. These are used to anchor the larva to a silk pad which the larva produce. The combination of the silk pad and abdominal hooks maintain the larvae position on the substrate in the fast flowing water. The cephalic fans are held into the current and periodically contracted to bring food particles to the cibarium by the mandibular brushes. On the body lying ventral to the head is a proleg which serves to produce two water currents traveling to the left and right of the head and into the cephalic fans. Samples of gut content has shown that bacteria can contribute a considerable portion of the diet. This fact is important for control methods especially that of Bacillus thuringiensis var.israelensis (Bti) which acts only after being ingested. The figure below shows a bacterium caught within a cephalic fan.

Both photomicrographs curtesy of Dept of Biological Sciences. University of Alberta ©

A certain amount of dispersal is possible in response to either changes in velocity of the water current or in response to particulate matter in the water course. Movement is either by a looping motion with the rear being brought next to the anchored head, then the head being moved forward whilst the rear remains anchored and so on. Or alternatively a silken thread is released from the salivary glands and drifting to objects downstream is achieved by traveling in the current until the silken thread snags onto a protruding object. The metamorphosis to the pupal stage occurs actually within the body of the larva. The silken cocoon is spun whilst still encapsulated by the larval cuticle. When the cocoon is completed then the larval cuticle is shed. The pupa rupture the larval cuticle in order to emerge and does so when a ventral split results due to pupal movement. Pupae may be found grouped closely together or singularly. Cocoon structure differs between groups with Prosimulium and some Cnephia having the least formed cocoons. Before spinning a cocoon larva move to the downstream side of stones where drag forces are not so great. The age of pupae can be roughly assessed by its colour with dark pupa being ones from which adults are ready to emerge. A more careful examination reveals the red pigment of the imago's eye visible through the cocoon. Emergence of the imago from the cocoon takes around one minute. Gas is released by the young imago and this separates the imaginal and pupal cuticles. Peristaltic movement occurs in the abdominal region and after a short period the thorax splits leaving a T-shape slit. It is via this split that the young imago emerges, thorax first followed by the head and abdomen. When the forelegs become free the fly extends them out into the current, the current then pulls the fly out from the exuvia. The fly is surrounded by a gas bubble and so does not get wetted whilst traveling up to the surface. The fly is able to break the surface tension of the water due to its gas bubble and fly away immediately.

Having flown from the aquatic larval site the flies form into swarms. The stimulus for these swarms is visual markers which contrast against the sky, such as branches of trees. Some male swarms have been found to swarm around the hosts of the females flies. Males of Boothora erythrocephala form small swarms of 5-15 which fly directly towards the flapping ears of cows. This is in response to the biting of female flies which cause the cow to flap its ears in irritation. The females are seized in the air but mating takes place on the ground with insemination occurring in a few seconds. Different species obviously have variations on this theme but mating in association with the host is common. It is this behaviour that can lead to respiratory problems or even death in livestock due to the large number of flies involved. The feeding of black flies especially that of mammalophilic species takes place predominantly during the day. Some species such as Eusimulium latipes, S. tuberosum and S. reptans have been caught in light traps at night. The peak biting time for diurnal species is soon after sunrise and again, although to a lesser extent in the late afternoon. In Africa forests however, probably due to reduced fluctuations in light intensity and temperature the biting is reasonably constant throughout the day. The site of the bites tends to concentrate on protruding body parts and flat underparts. When the female alights onto the host if the temperature is within 28-32⁰C then she will begin to probe. Feeding takes around 4-5 minutes and involves the labrum anchoring onto the skin with the use of paired hooks. The two maxillae are alternatively pushed downwards by flexing the occipital sections. The mandibles then exhibit a rapid slicing or scissor like motion. The mandibles and maxillae which actually penetrate the skin are compressed between the forwardly curving labrum and the backward pointing hypopharynx. Cutting into the skin occurs until a capillary is reached. The fly then feeds on the resulting blood pool. Cibarial and pharyngeal pumps bring the blood up and into the fly's gut where it is taken straight to the midgut, the crop is only used for sugary solutions.

Photomicrograph of female black fly. Curtesy of Dept of Biological Sciences. University of Alberta ©

Trapping MethodsContents

Larval Monitoring

Adult Monitoring

Traps for adult black flies fall into two catergories, either attractant or mechanical. Mechanical traps are reasonably limited in design being either suction traps or intake traps. The attractant traps show a wider range of design mainly in the type of attractant used. The easiest bait or attractant to use is yourself. This usually involves a person sitting or standing with one or two catchers armed with nets. When human bait is used it has been discovered that clothing colour influences the amount of flies caught and higher catches are obtained when dark-skinned people are used over lighter skinned people. Most trapping using this method usually occurs over a 2-3 hour time period. To compensate for peak biting times of different species the time periods need to be altered so as to include all possible biting times. As previously mentioned black flies can sometimes occur in such huge swarms that it becomes impossible to catch them all and so reduced sample sizes result with this method under these circumstances. The main draw back of this method is that human bait will only attract the anthrophilic species. To attract the mammalophilic and ornithophilic species other bait animals are used. When using bait animals they can be either tethered or if a bird is being used caged. These traps are then either monitored constantly by a collector or inspected periodically. An obvious disadvantage of this is the effect of the human collector. This human presence will serve to either attract or repel the number of black flies coming to the trap. Once the black flies have alighted onto the bait animal they are removed for identification and counting. Another trap method is to place a box with a hole cut in one corner over the box containing the bait animal. Black flies enter through the hole and are trapped in a small cage covered in a fine mesh. The outer box trap has a sliding base which can be closed and allows another box trap to be placed over the bait when it is removed. Other trap designs have included using a mesh around a bait animal which just allows small blackfly species such as S.adersi and S. griseicolle through but they are unable to exit through the mesh after taking a blood meal. The use of CO₂ has also been incorporated into traps as a chemical attractant. Sometimes this is combined with animals but more often on its own. CO₂ is released either from cylinders containing the gas or by sublimation of dry ice. Light traps can be modified to become CO₂ traps by the removal of the bulb and insertion of a nozzle to disperse the gas. Trials with this method used in New Jersey caught Cnephia pecuarum, S. vittatum, and Prosimulium magnum along with seven other species. The flies once within the trap become suck to an adhesive coated band which can be periodically removed and the flies counted. Varying rates of CO₂ release have been experimented with and 50ml/min is the usual release rate for optimum catches. Capture rates will also vary depending on trap height and particular species will form the majority of the catch at particular heights. Visual traps can be either those resembling the silhouette of a host or the three dimensional shape of a host. Other traps have little or no relationship to host shape, such as squares, cicles and triangles. The traps are covered with adhesive and a varying array of shapes and sizes gives a better range of species diversity in a defined area (defined by active trap area) than using just one design. Silhouettes alone have shown little use in trapping S. damnosum but have become more effective when combined with CO₂ release. Silhouette traps in the shape of birds have also been shown to be more effective when moved slowly along the ground, attracting ornithophilic species such as S. latipes. Three dimensional traps in the shape of sheep and cows have been used to some effect and can be designed to catch live flies. Catch rates have varied greatly however with these traps depending on geographical location.

Light traps have also shown widely varying success rates, with catches being large in Scotland but low in areas such as the Upper Volta. Better results have been given using a Monks Wood Trap (shown opposite) with daylight fluorescent tubes. When used along the Volta River the catch comprised of 95% S. damnosum. Suction traps such as the Johnson-Taylor trap have been used but these need to be operated and monitored over a period of years as they are non-attractant traps. An example of the differences in capture rates between a non-attractant suction trap and an attractant light trap can be seen in Scotland. Here a suction trap caught 693 blackflies mostly S. latipes over a period of three years whilst a light trap caught 17,500 blackflies over a four year period. One of the main problems with monitoring using a suction trap is that a supply of electricity is needed.

Vehicle mounted traps are fixed to the roof of a car or van, they have a wide aperture at the front, which is usually round, facing the same way as the vehicle. The trap then tappers down from the large front aperture eventually leading into a collection box. The overall appearance is like a large funnel. The trap is constructed from polyester netting with 13.3mesh/cm which gives reasonably smooth air flow through the trap when driven at 48km/h. A much cheaper version of this can be achieved by holding a net out of a car window or sun roof whilst moving and periodically removing caught flies. The limitations of traps for estimating population dispersal and size needs to be considered within any sampling investigation. Certain traps will always tend to be bias for a certain species and so multiple traps of varying design and position will always give a wider range of species diversity within the catches than when only one trap design is used.

Control of blackflies is carried out primarily due to their importance as vectors of Onchocerca volvulus causing the disease onchoceriasis in humans. Blackflies are also the transmitters of various other pathogens resulting in diseases in livestock which are mentioned in the introduction of this page. The implementation of control measures will therefore have some degree of overlap concerning humans and animals, thus resulting in mutual benefit. As onchoceriasis is an endemic disease in Africa there have been several control campaigns in this country. A large control initiative was undertaken in the Volta River Basin and the methods used provide a sound foundation from which other control measures can be implemented. In 1968 a meeting of the OCCGE, USAID and WHO concluded that control should be implemented through the use of larvicides. The Volta River Basin had been proposed as a control site by Waddy in 1963 and this area was selected for the campaign which began in 1970 with a team which collected information to plan and cost the control measures. In 1973 two objectives were stated:
1) to combat a disease that is widespread and severe in the area
2) remove a major obstacle to economic development
In November of 1973 a meeting of all interested parties endorsed the findings of the report started in 1970 and in January 1974 the Onchoceriasis Control Program (OCP) was initiated. It was due to run for a duration of twenty years which corresponds with the lifespan of the parasite in its host.

The areas included in the control site were Guinea and Sudan Savanna in seven countries of West Africa, Mali, Upper Volta, Ghana, Toga, Benin, Niger and the Ivory Coast. The site as a whole constituted some 654,000km² and included the whole of the Volta River Basin. The area to the north and northeast were natural limits to the vector population and the south was a forest zone. It was considered very unlikely that cytospecies from the forest region would pemeate out into the open savanna environment in any substantial numbers.

The area of the control program was evaluated in respect to suitable breeding sites for S. damnosum s.l. The number of eggs, larvae, and pupae, plus adult flies were monitored. This was achieved by making 24 sub-divisions of the control area and each sector was allocated three to five teams for vector collecting. A typical team consisted of two collectors and a driver. The OCP employed 563 full-time staff in order to carry out this entomological evaluation.

In order to achieve control a suitable insecticide was needed and several underwent trials. The eventual choice was an emulsifiable concentrate of temphos called Abate®. This offered a good combination of high larval kill rates, safety in handling, relatively minor effects on non-target organisms, low toxicity to fish and fairly rapid biodegradability. The method of deployment for the insecticide was that of aerial application. The main factors influencing this decision were the rapid development of the aquatic stages and the large area of the control program. In the West African conditions were temperatures varied between 22⁰C and 32⁰C and sometimes reaching 35⁰C the development from egg to pupa could occur in as little as 8 days. The Abate® insecticide was only effective on the larval stages thus both eggs and pupa would remain viable after spraying. A weekly treatment cycle from the air was thus selected. Application methods were varied according to the season. At the beginning of the spray program the "vide-vite" system was used. This is the rapid discharge of a measured amount of insecticide. In the wet season this method worked well. This was because the rivers were in spate and so considerable mixing and travelling of the insecticide occur giving good kill rates on larvae for distances up to 40km downstream from the drop zone. In the dry season however the water flow was insufficient to mix or carry the insecticide effectively and so in some river stretches it was necessary to treat every 1km or so. The concentration of Abate® was usually between 0.03ppm and 0.10ppm, the lower concentration was used in the wet season and the higher concentration in the dry season.

The spraying regimen was a huge logistical exercise and over 100 fuel and insecticide depots were set up throughout the control area. In March of 1976 the control program was using five helicopters and one fixed-wing plane. The entomological survey collected data which was plotted on a 1:1 000 000 scale map. Colour coding was used to indicate the breeding status of various regions. Red indicated where S. damnosum s.l. were successfully breeding and green for areas were no evidence of breeding was found. Where and when necessary ground applications of insecticides were carried out where treatment failure had been reported. The information from the pilots was important in respect to changing hydrological conditions which had a direct effect on insecticide kill rates. In order to maximize this source of information pilots were kept on the same flight circuit so they had time to familiarize themselves with river and weather conditions in their area.

Most larval breeding sites were successfully controlled after two weeks of spraying. It became apparent however that certain small localized areas were very difficult to treat due mainly to their construction, usually areas that were man made. The main problems were found in areas of dams, broken bridges and causeways. Monitoring of the spray sites revealed good non-target populations even 24hrs after spraying, species being identified included non-vector blackfly species such as S. adersi, S. griseicolle and S. schoutedeni. The OCP also monitored the biting rate within the control area, termed the Annual Biting Rate (ABR) and the Annual Transmission Potential. These two criteria were estimates of the number of bites that would be incurred by a person sitting at the trap points all day for a year. And the number of microfilariae this person would receive from these bites.

Work by WHO scientists at Geneva agreed that figures of ABR of less than 1000 and ATP of less than 100 would be satisfactory in allowing resettlement into the area without a likelyhood of ocular lesions. The use of these figures soon became realized when the area of the confluence of the White and Red Voltas was studied. In this heavily populated region onchoceriasis was hyperendemic and even though the ABR was at 587 well below the threshold figure of 1000 the ATP was 102. The monitoring of adult flies by trapping methods also elucidated the problem of invasion of adult flies from outside the control area. Some of these flies penetrated some 250-300km into the control zone. These flies seemed to have been brought in on prevailing winds from the southwest and northeast. The overall outcome was an extension of the control area onto the valleys of Sassandra, Marahoué and southern Bandama rivers in Ivory Coast.

The control programme as a whole has been extremely successful. The influx of blackflies from outside the original control area was originally estimated wrong, but much effort in this area rectified this. The use of "dribble bar" release systems is now being used in some areas, this method involves the use of a long pipe along which are a series of holes. The pipe is positioned so it spans the water course and the holes allow the release of Abate®, and Bti. The programme has saved countless people in the region from disabilitating ocular lesions and has helped uncover many biological facts of the blackflies life history.