Chapter 4. RECOMMENDATIONS FOR MONITORING AND MINIMIZING THE ENVIRONMENTAL IMPACT OF TSETSE CONTROL OPERATIONS

In general two types of environmental monitoring can be distinguished, namely chemical and biological monitoring (see FAO/UNEP, 1975).

Chemical monitoring refers to the measurement of residues of the pesticide in the environmental media, the abiotic components such as water, air, soil and bottom sediment and the biotic components such as plants, tissues of vertebrate and invertebrate animals and human tissues (e.g. blood, adipose tissue).

Biological monitoring concerns the measurement of certain changes in the biota. A large number of parameters at the different levels of biological integration can be used for this purpose. At the higher integration levels, the ecosystem and the population, the measurement of diversity and species abundance, the assessment of morbidity and mortality may be used. At a lower level of biological integration, e.g. social groups of organisms and at the individual level, various physiological (reproduction, enzyme activity) and behavioural parameters can be considered.

In the FAO/UNEP Report on Impact Monitoring (FAO/UNEP, 1975) it was stated that although either chemical or biological monitoring results may provide indications of environmental impacts, correlation between the two types of data may be very important for the establishment of cause and effect relationships.

The selection of the environmental components for the purpose of both chemical and biological monitoring requires a careful judgement of the ecosystems concerned, because it will never be possible to examine all abiotic and biotic elements on their residue content and to include all species of organisms in the biological judgement. Chemical compounds introduced in the environment are in general not randomly distributed but follow certain distinct pathways depending on their physico-chemical properties and biodegradability. The fate of the compound determines the likelihood that certain organisms will be exposed and possibly may experience some undesirable effect from the compound concerned. The success of the studies will thus markedly depend on a proper selection of the variables (indicator objects and indicator organisms) to be monitored.

Monitoring the environmental impact

As far as chemical monitoring in relation to tsetse control operations is concerned, relatively much attention has been paid to the measurement of the fate of the spray-deposits including studies on the degree of decomposition in the vegetation. For this purpose analyses have been made for instance of treeleaves and bark samples. Particularly as far as some of the side-effect studies are concerned residue levels have also been determined in tissues of non-target organisms found dead and in tissues of organisms like fish, fish-eating birds and insectivorous birds collected in the neighbourhood of sprayed areas. The levels found in organisms may vary markedly from one animal group to another, depending on factors as feeding pattern and trophic position in the food-web. Moreover there may be an appreciable variability within the groups due to factors like sex, age and phase of the life-cycle. For instance in fish residue levels may change considerably in the course of spawning. In general it can be stated that proper chemical monitoring should not only be looked at as a problem of chemical analysis but in particular also as a problem of proper sampling. Due to failures (insufficient samples, non-representative samples) the results of chemical monitoring studies have not always provided the data the scientists search for.

In the side-effect studies carried out sofar in connection with chemical control or eradication of tsetse the following biological parameters have been taken into consideration: assessment of acute mortality and morbidity in vertebrate species and invertebrate orders and families, short and long term population census in birds, fish, crustaceans and insects and general assessments of population changes in other vertebrates.

Studies carried out up till now clearly indicate that certain non-target animal species are much more sensitive than others. For instance as far as residual high dose rate applications of Dieldrin and Endosulfan are concerned, the flycatchers (family Muscicapidae) proved to be very vulnerable, as compared to the other insectivorous birds, while with respect to the pyrethroids shrimps are far more vulnerable than certain aquatic insects. Therefore it should be recommended to use the vulnerable species as biological indicators in the assessment of the environmental impact of future applications of the pesticides mentioned above. In table 4.1. some organisms are listed which can be used as biological indicators in relation to the use of insecticides in tsetse fly control. Adequate field observation methods should be selected in order to obtain a representative picture of the population dynamics of the species involved.

Minimizing the environmental impact

The biological significance of a decrease abundance or disappearance of certain species of insectivorous birds, non-target insects or fish for the African environment still cannot be assessed properly, although mass mortality of fish certainly can be considered as an obvious undesirable effect. However the more subtile changes in the occurrence of other groups of organisms could also give rise to certain undesirable changes in the savanna ecosystems. For instance areas could lose their natural resistance against certain pests in agriculture and forestry. Therefore preferably those methods of tsetse control should be applied which cause least damage to non-target organisms. In relation to the latter the following recommendations are made:

1. Particularly in large-scale operations pre-spray assessments should be made of the ecological value of the various habitats occuring in the areas to be sprayed in order to learn whether damage to fish and wildlife in certain areas could give rise to a serious deterioration of local amenities (availability of fish; bee-keeping; loss of species in nature reserves etc.). In this connection it should be stressed that certain components of an environment may be more vulnerable than others, for instance in savanna areas fringing forests around streams and ponds guarantee the survival of many savanna species and fish because these are the places where water and food are available throughout the dry season and where the bush fires generally will not penetrate. Therefore they are to be considered as vulnerable compared to other places where tsetse may occur, like patches of upland forest (Doka Woodland in West Africa) and fringing forests around dry streambeds.

Table 4.1. Organisms which can be used as biological indicators in relation to applications of insecticides in tsetse control

2. Considering (1) attempts should be made to apply pesticides as discriminatively as possible by making optimal use of the available techniques. This may imply that certain combinations of methods are utilized, for instance a combination of aerial- and groundspraying, a combined use of different insecticides or aerial spraying in combination with sterile male techniques.

3. The development of improved methods of tsetse control by means of selection of the most adequate pesticides, formulations and application devices should be continued in order to reduce the environmental impact as much as possible. In certain parts of Africa aerial applications of ULV formulations of Endosulfan and pyrethroids at relatively low dose rates have been applied successfully in certain habitats. Generally these methods cause at least little damage to the vertebrate groups of organisms.

4. Studies on side-effects of tsetse control operations should be continued. Sofar side-effects have been mainly studied in the Sudan savanna, the Northern Guinea savanna and comparable types of dry savanna elsewhere in Africa. These should be continued in relation to pesticide applications in the moister areas and preferably in close connection with the field trials devised for the selection of improved control methods. Information on possible long-term effects on non-target arthropods, fish and certain other vertebrates are urgently required. The duration of field observations should be chosen in correspondence with the lifecycle characteristics of the species involved and taking in consideration the year to year variations in the ecological conditions of the areas concerned, as well as seasonal variations in abundance and behaviour. Pre-spray observations should be carried out over a period of at least three successive seasons in order to compensate for interseasonal variation.

5. To date the scientific staff responsible for the implementation of tsetse control operations generally consists of veterinarians, entomologists and pesticide formulation and application specialists. It is suggested to include an environmental toxicologist in the staff who will be responsible for the ecological assessments and organisation of side-effect studies and whose advise should be seriously taken into consideration in relation to the selection of the most appropriate control techniques.

Some remarks need to be made on the possible secondary effects of tsetse control operations, viz. the changes that may occur in the environment as soon as the tsetse flies have disappeared. It is claimed that eradication of the flies may lead to overgrazing, deforestation, erosion and ultimately desertification (Ormerod 1976, 1978). As was also mentioned by Jordan (1979), the presence of tsetse flies certainly prevents overgrazing, but there is no evidence that overgrazing has occurred following the largest anti-tsetse campaign yet undertaken (Nigeria). However, considering a possible expansion of the tsetse control activities in Africa, it is recommended that larger elimination campaigns should be instituted only as part of a larger program of development including land use planning, into which it is integrated.