Suivi des populations larvaires de diptères simuliidae dans la vallée de la Semois

Context

Black flies are Diptera Nematocera belonging to the Simuliidae family. The worldwide dispersion comprises over 1,500 species, some twenty of which have been found in Belgium (notably Simulium equinum, S. erythrocephalum, S. lineatum, S. morsitans, S. ornatum and S. reptans). The larval form of the insect develops mainly in fast-flowing rivers and streams, where it attaches to stones or aquatic vegetation. It occurs in large numbers for instance in the river water crowfoot communities (Ranunculus fluitans Lam.) of the Semois Valley in the Ardennes. Most are blood-sucking species, the females requiring blood which they obtain from mammals (cattle, game and humans) in order to reproduce. Swarms numbering tens of thousands of flies form as soon as the weather warms up in spring and are likely to swoop down on cattle near infested streams or rivers. The attacks are most virulent when the cattle first go out to pasture and sometimes cause the death, due to anaphylactic shock, of badly bitten animals. Similar lethal attacks occur from time to time. The first major attacks were noted in 1986 along the Lhomme and Serpont rivers in the Libin region, and in 1992 in the Semois Valley.

Objectives

In view of the risks faced by farmers in the affected valleys, the Biological Control and Genetic Resources Department has been working on a preventive control strategy aimed at limiting the risk of recurrence as far as possible in those regions where cattle losses have occurred.

Description of tasks

Regular monitoring of black fly larval and pupal populations is carried out between April and October annually on sections of rivers at risk (the Semois and also the Ourthe, the Vierre, etc.). To this end a technique for harvesting and counting larvae and pupae present on the crowfoot has been developed. Population dynamics can thus be compared from one year to another. Biological control trials using a selective microbial larvicide containing Bacillus thuringiensis (var. israelensis) were also conducted until 2003 to assess the effectiveness and practical conditions of use of such a product. The impact of the treatment on larval populations in the treated areas is still the subject of investigation. The effects of the physico-chemical and bacteriological characteristics of the river water on the larval and pupal populations were also studied in cooperation with the Luxembourg University Foundation (University of Liege, Arlon campus). Initial results have not shown any clear correlations between population levels occurring and the various parameters studied, however.

Expected results

This on going project ought to deepen our understanding of the population dynamics of the different black fly species occurring along the Semois and lead to a more precise assessment of the risks these blood-sucking Diptera may pose to cattle in the valley.