- River midge plague
|Last update: April 3, 2012 07:28:54 AM|
THE RIVER MIDGE PLAGUE
Dr G.F. Bath. Veterinarian
Regional Veterinary Laboratory
Grootfontein College of Agriculture
River midges, which are also known as black flies and buffalo gnats in other countries, are small two-winged insects which superficially could be said to resemble flies, except that they are far smaller. Their horn-like antennae distinguish them from other similar insects. The adults are bloodsuckers, and when they occur in large numbers are important pests of livestock. They breed only in the water of flowing streams and rivers, and so are not really important at great distances from large perennial rivers. However, they can fly several kilometres from their breeding sites and have been recorded nearly 50 km from large rivers. The female lays eggs under the water on stones or plants in the river or stream. The larvae which hatch attach themselves very firmly to the rocks or weeds, and feed on organic matter in the passing water. The larvae then pupate still attached to the stone or rock, after which an adult midge emerges, rises to the surface, and flies away.
Because of its characteristic breeding requirements, the river midge is confined to the larger rivers of South Africa. Locally the only river of real importance is the Orange River. Simulium chutteri is probably the river midge species which is most important here. Several other small insects look superficially similar; some are bloodsuckers while others are completely harmless. One of the bloodsuckers is the saltpan midge Leptoconops karteszi which breeds in pans with a salt concentration higher than 500/ug/g (parts per million). The apparent anomaly of finding a river midge at very great distances from rivers is usually because it is this or another insect, and not the true river midge. The saltpan midge is present over much of the Karoo, and often quite close to the Orange River. It is therefore important that a positive identification of such insects be made.
Over the past number of years the river midge problem has increased alarmingly. While this may be partly due to the very good rains which have fallen, the increase in midge numbers is more strongly related to the building of large storage dams and control sluices along the course of our larger rivers. It seems certain that the presence of these storage dams, while a great blessing in so many other ways, has increased the river midge problem enormously. While in previous years even the largest rivers had long periods of sluggish flow or near-emptiness, today the regular, controlled release of water from these storage dams for various purposes such as irrigation and power generation has brought about a situation in which they can be considered perennial. This, coupled with our generally warm climate, has ushered in ideal breeding conditions for the midges. There is enough water to keep the breeding sites under water, the flow never becomes too weak, and food-rich water is continually being released from the dams. The result has been that whereas formerly it was a fairly small, localised and seasonal problem, the river midge plague has spread to large districts, is not strictly confined to the wetter months and has become much more severe in its effect on farm animals.
The midges are a danger in several ways. In overwhelming numbers they may even kill small animals like poultry, especially when they are close to the river and unable to take any evasive action. The midges are voracious bloodsuckers and in such numbers they may cause severe anaemia in other stock.
Much more commonly, however, they are responsible for production losses due to the irritant effect they have on animals. The bite is painful and as the nuisance increases, animals become more concerned with evading the midges than with any other activity, such as grazing or ruminating. Sometimes the situation may reach the point where animals can be seen dashing about in a frenzied way, looking for respite from the persistent midges. Under these conditions it is quite obvious that losses in production must occur. Whether it be weight gain, milk or wool production, the result is that the farmer can incur severe economic losses though no animals die. It is impossible to quantify the extent of the loss on each farm, but the figures for badly affected farms is probably considerable.
Finally the midges are known to be the carrier of a number of diseases, many of which fortunately do not occur in South Africa as yet. While no certainty exists on all the diseases which these midges can transmit locally, it is probable that they played a role in the mechanical spread of Rift Valley Fever and Wessels Bron Disease in the past few seasons. It is also more than likely that they can spread Bluetongue of sheep during large outbreaks, while it is quite possible that diseases like Scabby Mouth (Vuilbek) of sheep, and Three Day Stiffsickness and Gallsickness of cattle can be transmitted by the river midge. Any other disease which can be spread by infected blood could also become epidemic where midge populations are very high. Diseases in which the cause is as yet uncertain, like the so-called "cancer ear" of sheep, may be transmitted or directly caused by the midges.
There are therefore many good reasons to consider the river midge plague along the larger rivers of South Africa as a real threat to the economic viability of nearby farms, and a problem which is worth a great deal of effort to control, although total eradication is probably impossible.
Control of the plague is a very difficult matter, given the breeding habits of the insect. For a long time considerable effort has been expended on finding ways of protecting stock from midge attack. While various types of insect repellent may be useful for limited periods, practical and economic considerations have eventually ruled out these repellents as a long-term solution to the problem. Every animal on each farm would have to be thoroughly treated, virtually all over the body, and at short intervals. The cost in terms of labour, equipment and remedies would be very high. However, individual animals may be so treated, and exceptionally valuable stock could even be kept indoors behind fine mesh screens. Especially in the evenings and at night when the midges are most active. Several proprietary brands of insect repellent are available.
Treatment of the sores and ulcers caused by these bites is similar to that for any open wound. Until the animal has completely recovered, it should also be protected from further midge attack as just described.
Because the plague is so difficult to control on the animal. A lot of work has also been done on finding methods of controlling the breeding of the river midge. In the fifties an effective method of controlling breeding was discovered in the introduction of very small quantities of DDT to the water stream in which the midges breed. The larvae were killed off on a grand scale, while the DDT level in the water was so calculated to be harmless to fish. However, later concern on the effect of the slow build-up of DDT residues in the environment, and the eventual effect on man, led to this method of control falling into disfavour.
A new biological approach was then sought to overcome this objection. A team of researchers, led by Dr C.J. Howell of the Onderstepoort Research Institute, and including officials of the C.S.I.R. and the Department of Water Affairs, was constituted to investigate practical methods of control by manipulation of the water flow of affected rivers. They have carried out their initial investigations and experiments in the Vaal River between the Warrenton weir and Windsorton, this being a stretch of river with very high concentrations of river midge larvae and with a control point at the weir so that it was ideally suited to such experiments. The purpose of the water flow manipulation is to prevent a regular flow of water, thereby making the river unsuited to breeding for the river midge for various reasons. Firstly, the midge larvae are attached to rocks or vegetation and therefore need a continuous flow of water to get their food requirements. By breaking the water flow, conditions become unfavourable for feeding. Secondly, a break of water supply of several days will lower the water level to the extent that much of the riverbed becomes exposed, so that most larvae and pupae are killed.
In the Vaal River experiments, the water supply was entirely closed for two days every two weeks. Subsequent experiments increased the length of closure to 2½ and 3 days, and in some cases this was done every week. Although these experiments are still in progress, it can already be said that this method of control is very promising. Marked reductions in the number of midges in these regions have been recorded, and it seems that this will become the control method of choice.
The success obtained in the Vaal River has encouraged the research team to extend the same principle to the Orange River. Here, however, electric power turbines at the dams, which are used mainly as peak period boosters, have brought a further consideration into the planning. The matter is presently being worked out in cooperation with Escom, and the first trials should begin this year.
When these investigations are completed, it is hoped that the river midge plague along the Orange River will also be brought under control.
Karoo Agric 1 (1), 33-44