Biting Flies Because of the large number of species of fly that bite livestock, it is only possible to discuss a few of the most important ones in this article, Horse flies (Tabanidae sp.) and Stable flies (Stomoxys sp). Horse Flies, despite their name, do not just bite horses; they are quite happy to feast upon cattle, sheep, deer, camels, dogs, humans – anything that contains blood really. Unlike many other insects that live upon the blood of other creatures, horse flies do not have a long, thin proboscis that they pierce the skin with. Instead, they have sharp scissor-like mouthparts with which they cut into the skin or hide causing it to bleed. When a small pool of blood has formed, the fly sucks it up with its tongue. Needless to say, the wound created is extremely painful to the animal, causing it much distress. Studies have shown that animals bitten by as few as 60 flies in a 24 hour period can have their weight gain affected by up to 100 grams per day1 because they are not able to graze in peace. Milk yields can also be significantly affected. The life cycle of the Horse Fly, or Cleg as they are sometimes known, involves water – this, accompanied by their increased activity during wet weather, gives the Common Horse Fly its Latin name of Haematopota pluvialis = ‘blood drinker of the rains’. The eggs are not laid directly into water, but usually on vegetation that is overhanging water. Between 200 and 1,000 eggs are laid which hatch after around four days. The larvae can take several months to develop depending on the ambient temperature. Consequently, larvae often over-winter in colder climates. The larvae start off as they mean to continue and are carnivorous. They usually feed on other insect larvae, crustaceans, snails and earthworms and have even known to be cannibalistic. The pupal stage lasts for two or three weeks after which the adult fly emerges. The female fly needs blood to enable her to develop the eggs, so the first port of a call is a suitable animal to bite. After feeding, mating is the next priority. The male flies congregate in large swarms above the trees to which the female flies are attracted. Mating takes place and the cycle begins again. Stable Flies are approximately 8mm long and look very similar to the common House Fly. But, unlike the House Fly that is happy to lick at crumbs on your kitchen work surface and sugar bowl, the Stable Fly has a taste for blood and is armed with a fearsome piercing proboscis that boldly protrudes from the front of its head. They insert this large proboscis into the hide or skin in order to suck the blood and a large deep hole is the result. The flies will feed several times a day causing much pain to the animal and damage to the hide. They tend to bite mostly around the lower parts of the animal, so hide damage is more likely to be found in the shanks and belly areas. Their eggs are usually laid in moist rotting vegetation with manure laden bedding being a popular choice. When hatched, the larvae take around two weeks to develop and pupate. The pupae develop into adult flies within one to two weeks. The adults can live for up to one month and each female fly will lay many hundreds of eggs in her lifetime. So, several generations can be produced within one summer, or they may even breed all year round in warmer climates. Prevention As with many biting insects, biting flies are known to transmit serious blood-borne parasites and diseases such as bovine leukaemia. Therefore, their control is of
paramount importance as they have economic significance for the farmer as well as the tanner who has to contend with the damaged hides and skins. Such is the distress caused to the animal, some have developed their own strategies to cope with the nuisance of biting flies. When fly numbers are at their greatest, cattle will group together, usually with a fight to be in the middle of the group rather than at the outside. They swish their tails and stamp their feet to keep the flies from
settling on them. They will even stand in deep water if it is available. In the case of Stable Flies, good housekeeping is one recommended means of control – regular removal of dirty bedding in which the flies breed is highly effective. However, for animals that are not housed, pesticides are recommended. Cattle can be manually sprayed with pesticides that will remain active in the coat of the animal for up to a week. However, regular applications are necessary. Spraying the cattle in combination with regular spraying of the area where they graze can be effective, especially if the area is sprayed at the times when the flies congregate, eg the evening. Where large areas of land are involved, aircraft are sometimes employed to do the spraying. Insecticidal ear tags have been found ineffective against Stable Flies because they tend to attack the lower parts of the animal. In the USA research has been conducted into using biological control of Stable flies². There is a species of wasp that lays its eggs in the pupae of Stable Flies resulting in the death of the fly. Although the research is still ongoing, results to date have unfortunately not shown significant reductions in fly numbers. Scientists have also observed that within herds of cattle, some
animals can be noticeably less affected by biting flies than others. Research into this phenomenon has found that some animals produce and release volatile chemicals that act as an insect repellent³. By synthesising these chemicals and attaching them in a slow-release form to the animal they have proved successful in keeping the flies away. It is envisaged that by selectively breeding animals that display this natural repellency, the use of chemical insecticides can be reduced. REFERENCES
1. M Hall & J Chainey, Natural History Museum, London, UK,
Paolo Bettella and Jose Luis Aramayo, Museo de Historia Natural Universidad, Santa Cruz, Bolivia
2. Stable Fly Control on Cattle, John B Campbell, University of Nebraska
3. www.rothamsted.ac.uk/forthepublic/SnapshotMosquitoes.html