In general two types of environmental monitoring can be distinguished, viz. chemical and biological monitoring (e.g. see FAO/UNEP, 1975).
Chemical monitoring refers to the measurement of residues of the pesticide in the environmental media, viz. 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 can be used. At a lower level of biological integration, e.g. social groups of organisms and the individual level, various physiological (reproduction, enzyme activity) and behavioural parameters can be considered.
As was mentioned also in the FAO/UNEP Report on Impact Monitoring (FAO/UNEP, 1975), although either chemical or biological monitoring results may provide indications of environmental impacts, correllation 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 likelyhood 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 for instance been made 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, insectivorous birds etc. 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 etc.) 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 and general assessments of population changes in other vertebrates. Relatively much attention has been paid to birds, as it is felt that they reflect a variety of conditions in the ecosystem because (1) they represent various trophic groups and (2) they occupy a large range of ecological niches in the environment
Studies carried out up till now clearly indicate that certain non-target species of birds and other animals are much more sensitive than others. For instance as far as dieldrin and endosulfan are concerned, the flycatchers (family Muscicapidae) which occur in many species all over Africa appeared to be very vulnerable. It could be recommended therefore to use these birds 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 applications of endosulfan and dieldrin.
Minimizing the environmental impact
The biological significance of a decreased abundance or disappearance of certain species like insectivorous birds, non-target insects, fish etc. for the African environment still cannot be assessed properly, although mass mortality in fish certainly can be considered as an obvious undesirable effect. However the more subtle 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:
Particularly in large-scale operations pre-spray assessments should be made of the ecological value of the various habitats occurring 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 parts of an environment may be more vulnerable than others, e.g. 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 in comparison to other places where tsetse may occur, such as patches of upland forest (e.g. 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 endosulfan and dieldrin
|Tantalus Monkey||Cercopithecus aethiops (Linnaeus)||+|
|Gambian Sun Squirrel||Heliosciuris gambianus (Ogilby)||+|
|Fruit Bats||Pteropinae spp. Trouessart||+|
|Blue Flycatcher||Elminia longicauda (Swainson)||+||+|
|Puffback Flycatcher||Batis spp.||+||+|
|Paradise Flycatcher||Tchitrea viridis (Müller)||+||+|
|Wattle-eye||Platysteira cyanea (Müller)||+||+|
|Robin Chats||Cossypha spp.||+||+|
|Olive-thrush||Turdus olivaceus (Linnaeus)||+|
|Blue-breasted King-fisher||Halcyon malimbicus (Shaw)||+|
|Pigmy Kingfisher||Ispidina picta (Boddaert)||+||+|
|(reptiles, amphibians, fish)||+||++|
|Dragon Flies||Odonata spp.||+|
Considering (1) attempts should be made to apply pesticides as discriminatively as possible by making an optimal use of the available techniques. This may imply that use is made of certain combinations of methods (e.g. aerial spraying in combination with groundspraying; aerial spraying in combination with sterile male techniques; combined use of different pesticides etc.). For instance, according to the present knowledge, helicopter applications of either dieldrin or endosulfan could be limited to the habitats whose ecological value is ‘relatively low’, while those which are relatively valuable could be sprayed by groundspray techniques using the most appropriate pesticide available.
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 (fixed-wing) applications of ULV formulations of endosulfan at relatively low dose-rates have been applied successfully in certain habitats (e.g. Okavango Delta) and it was reported that no serious side-effects occurred. However it is not known yet whether these applications are applicable to the various other habitats where tsetse occurs.
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 insects, fish and certain other vertebrates are urgently required.
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 a 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.
Secondary impacts on the environment which may result from the invasion of man and cattle in areas reclaimed from tsetse have not been considered in the present report.