Pesticide Residues in Coastal Tropical Ecosystems: Distribution, fate and effects - Chapter 3 pps

As in most tropical countries in Africa, pesticides are alsoextensively used in the public health arena to control diseases such as malaria a nonhemorrhagic fever caused by protozoans of

Zimbabwe is situated within the African tropics (Lat 15° to 22°S and Long 26° to

34°E), occupies an area of 390,580 km2 and has a population of about 11 M TheZambezi River forms the boundary with Zambia to the north, and the LimpopoRiver forms the boundary with South Africa to the south The eastern highlandsthat form the rim of the African Plateau (before descent to the Mozambique CoastalPlain), constitute the greater part of the border with Mozambique In the west,the boundary with Botswana follows the eastern limit of the Kalahari Desert.Zimbabwe is a landlocked country Its GDP for 1990 and 1994 was Z$14,643

M and Z$39,775 M, respectively The agriculture sector contributes about 13percent of GDP, while the export of agricultural products contributes 50 percent

of the country’s total annual export earnings (Central Statistical Office, 1990).The use of pesticides plays a major role in maintaining these high levels of agri-cultural production As in most tropical countries in Africa, pesticides are alsoextensively used in the public health arena to control diseases such as malaria (a

nonhemorrhagic fever caused by protozoans of the genus Plasmodium Marchiafava and Celli and vectored by Anopheles Meigen spp (Diptera: Culicidae) mosquitoes),

African trypanosomiasis (African sleeping sickness, a nonhemorrhagic fever caused

by protozoans of the genus Trypanosoma Gruby and vectored by tsetse flies, Glossina spp (Diptera: Glossinidae), and typhoid (a bacterial illness caused by Salmonella

typhi spread in contaminated food and water) Of late, there has been concern

about the possible effects the use of pesticides has on tropical environments,including tropical marine and fresh water ecosystems The effect of pesticides onthe environment depends on several factors such as climate, in particular tempera-ture and rainfall; soil type and the nature of the vegetative cover; biotic activity;light intensity; agricultural practices; and mode of introduction of the pesticide into

a particular environmental compartment These factors determine the persistence

of a pesticide in a specific environment, and in this respect, OC pesticides as agroup have been found to be the most persistent

OC pesticides have been used extensively since the early 1960s to control thetsetse fly and malaria vectors in southern east-central Africa, i.e Zimbabwe,

Mozambique, and South Africa (Ford, 1971) DDT has been used for this purposesince 1962, while dieldrin was used during the period 1962 to 1967 (Ford, 1971;

Mpofu, 1987; Whitwell et al., 1974) Endosulfan and BHC are currently used,

especially for aerial spraying (Chapman, 1976)

DDT was used in Zimbabwe for more than four and a half decades, from 1946

to 1983 (Chikuni, 1996) In addition to its use to control the tsetse fly and malariavectors, DDT was used extensively for the control of agricultural pests such as the

maize stalkborer Busseola fusca Fuller (Lepidoptera: Noctuidae), cotton cutworm

Agrotis spp (Lepidoptera: Noctuidae), and cotton bollworm Helicoverpa armigera

Habner (Lepidoptera: Noctuidae) DDT was used in agriculture until 1983 whenthis use was banned However, it is still registered with the Ministry of Health’sHazardous Substances Control Board as a ‘hazardous substance class 1’, i.e achemical that can endanger humans and domestic and wild animals, and itsprocurement and use are restricted to cover tsetse and mosquito control only(Chikuni, 1996) Other OC pesticides registered for use in agriculture in Zimbabweinclude dieldrin, endosulfan, BHC, aldrin, chlordane, dicofol, and chlorthal-dimethyl

This chapter discusses the use of pesticides in Zimbabwe and how this hasimpacted on Lake Kariba, a tropical freshwater ecosystem Lake Kariba is one ofthe world’s largest man-made lakes It was constructed in the mid-1950s, started

to fill in 1958, and reached full capacity in 1963 Situated in south-central Africa(between Lat 16°30' to 18°S, and Long 27° to 39°E), the lake is politically shared

by Zambia and Zimbabwe, with the international border bisecting the lakelongitudinally (Figure 3.1) Its physical dimensions are given in Table 3.1.Geographically Lake Kariba is part of the middle Zambezi region and lies in arift valley (the Gwembe Valley), overlooked on both sides by steep escarpments.The mean maximum temperature is 30.4°C, while the minimum annual meantemperature is 18.2°C Rainfall around the lake region is generally low; the annual

mean for the period 1951 to 1986 was 734 mm (Leggett et al., 1991) Generally the

wet season occurs in the months of December to March, with occasional storms

Table 3.1 The physical dimensions of Lake Kariba, Zimbabwe at the

normal operating water level (see Figure 3.1) a

Water level (above sea level) 485 m

a Source Balon and Coche (1974).

b Occasional deeper ‘holes’ not considered.

in October, April, and May There is no rainfall from June to September Evidence

of a pesticide residue build-up in Lake Kariba has been reported by several workers

(Billings and Phelps, 1972; Whitwell et al., 1974; Greichus et al., 1978; Wessels et

al., 1980) Thus, in 1980, Wessels et al were prompted to warn that ‘in view of the

extensive fishery development taking place on the lake, the problem of residues inhuman food may become a serious matter’ The background and extent of theproblem are the subject of this chapter

The use of pesticides in Zimbabwe is regulated in terms of the Pesticide Regulations

of 1977, under the Fertilizer, Farm Feeds and Remedies Act (Chapter 111) of

1952 (Government of Zimbabwe, 1952) and the Hazardous Substances and ArticlesAct (Chapter 322) of 1972 (Government of Zimbabwe, 1972) The Fertilizer, FarmFeeds and Remedies Act (Chapter 111) is administered by the Ministry of Lands,Agriculture and Rural Resettlement This act prohibits the sale or distribution ofFigure 3.1 Lake Kariba with inflows from Zimbabwean rivers

Source: Wessels et al (1972).

pesticides unless they have been registered with the Plant Protection ResearchInstitute Registration is carried out under the Hazardous Substances and ArticlesAct (Chapter 322) which is administered by the Ministry of Health and ChildWelfare.

Before registration, pesticides are classified on the basis of their acute oral lethaldose (LD50) and persistence after application The classification (poison group) isindicated by a green, amber, red, or purple triangle on the label for LD50 values ofgreater than 2,001, 501 to 2,000, 101 to 500, or 0.1 to 100 mg kg–1, respectively.Pesticides can only be imported into the country after they have been registered It

is also a requirement that all imported pesticides be registered in the country oforigin

The complete list of registered pesticide products in Zimbabwe is very long(approximately 600), but compared to the total number of pesticides availableworldwide (>40,000), this number is small Several companies are involved in theformulation and marketing of pesticides in Zimbabwe The major formulatorsare (with the number of formulations registered by each company shown inbrackets): Agricura (99), Zimbabwe Fertilizer Co (ZFC) (71), Windmill (66), Bayer(62), Shell (48), Ciba-Geigy (34), Spray-quip (26), Hoechst (16), Omnichem (15),and Wellcome Environmental (17) (Mathuthu, 1993) There are several minorpesticide formulating companies including Rhone Poulenc (11), Technical Services(6), Fercochem (4), Oxyco (4) and T.S.A (2) Some formulations of Agricura andZFC are made totally from local raw materials The other companies, to a largeextent, merely import the active ingredients from which they make theirformulations

A list of pesticides commonly used for crop pests in Zimbabwe is given in Tables3.2 and 3.3 The formulations commonly marketed in Zimbabwe were selectedfollowing a market survey, which involved visits to major outlets that sell pesticides,e.g the Farmers Cooperation, Agricultural Buying and Veterinary Services, whole-sale centers, and supermarkets, in addition to interviews with farmers (Mathuthu,1993) The pesticides listed in Tables 3.2 and 3.3 are those formulations that aremost commonly used around the country The tables show the brand name anda.i of the pesticide, its poison group, and the chemical class of the compound.The poison group indicates the degree of toxicity of the pesticide and is indicated

by the symbols P (Purple), R (Red), A (Amber), and G (Green), with P indicatingthe most toxic pesticides and G the least toxic

The Agricultural Chemicals Industry Association (ACIA) represents allmanufacturers and distributors of agrochemical and animal health products inZimbabwe The ACIA is a member of the International Group of National Associa-tions of Agrochemical Manufacturers (GIFAP) Through GIFAP, the ACIA hasendorsed the FAO’s code of conduct on the distribution and use of agrochemicals(Mbanga, 1996)

Table 3.2 Pesticides used in Zimbabwe: grouped according to chemical classes of the compoundsa

BHC 75%

continued…

Table 3.2 continued

Baygon residual spray propoxur 2.0% +

Garden insecticide

Killem insect aerosol tetramethrin 0.2% +

Thiram 80% W.P thiram 80% (disulphide) R carbamate (fungicide)

Copper oxychloride

copper oxychloride 6.5% inorganic salt

Citrocyclin 90 tetracycline +

Agrifume EDB 4.5 ethylene dibromide 42.2% P organohalide

Snail and slug killer metaldehyde 2% G organic acid derivative

derivative

continued…

Table 3.2 continued

Cotogard 500 F.W fluometuron 25% +

Gesagra 500 F.W metolachlor 25% +

TCA 90 grass killer sodium

Tordon 101 mixture 2,4-D amine salt 39.6% P phenoxy acid

derivative

Gardomil 500 F.W terbuthylazine 36.7% +

Notes:

a Reproduced with permission, Table 23 in SADC ELMS Report Series 35 (1993).

b Poison group (see text for LC50s corresponding to each group): A (amber) = toxic; G (green)

= non-toxic; R (red) = highly toxic; P (purple) = extremely toxic.

Table 3.3 List of commonly used dipping chemicals in Zimbabwea

Brand name Active ingredient Poison group % a.i Class of compound

Note:

a Adapted from SADC ELMS Report Series 35 (1993).

Pe s t i c i d e p o l l u t i o n f r o m a g r i c u l t u r e

Zimbabwe is a landlocked country where elevation and rainfall are highlycorrelated Rainfall varies from below 300 mm annually in the low-lying areas inthe south and southeast, to more than 1,500 mm in the mountains borderingMozambique Rain falls from November to March, and only about one-third ofthe country is suitable for intensive agriculture Figure 3.2 shows the agro-ecologicalregions of Zimbabwe (ENDA, 1991) Traditionally tobacco has been the primaryagricultural commodity, although cotton, tea, citrus, livestock, wheat, sugar, andmaize are also important Figure 3.3 shows that all rivers within Zimbabwe originatefrom the high veld – veld or veldt is the extensive grassland region of eastern andsouthern Africa – where most of the intensive agriculture is practiced These riversdrain to the Zambezi River and Lake Kariba in the north, and to the LimpopoRiver in the south The Zambezi and Limpopo rivers are the two major riversflowing to the Indian Ocean

A major climatic factor in the dispersal of pesticides from agricultural use inZimbabwe is the fact that rain is usually in the form of short, heavy tropical stormswhich result in high erosive runoff during the periods that most pesticides areapplied in agriculture, i.e between November and January This high erosive runoffleads to silting behind dams, so that much of the applied pesticides find their waydirectly into river and lake sediments (Zaranyika and Makhubalo, 1996) Evidence

of a build-up of OC pesticide residues in Lake Kariba sediments has been reported

(Zaranyika et al., 1994).

Recently, smallholder vegetable production has rapidly expanded in Zimbabwe

Sibanda et al (2000) found these small farmers use some cultural control methods

and occasionally botanical pesticides but for the most part they rely on conventionalsynthetic pesticides for controlling the range of serious pests and diseases thataffects nonindigenous vegetables Synthetic pesticides are usually applied usinglever-operated knapsack sprayers, although occasionally less orthodox applicationmethods are employed The primary concerns based on these practices are due toshortcomings in protective clothing for applicators, large deviations from recom-mended doses (based on the adage that if a little is good, then more is better), andexcessive runoff to the soil Both of the latter concerns can lead to a build-up ofpesticide residues in streams and lakes

P E S T I C I D E U S E I N T H E C O N T R O L O F D I S E A S E

V E C T O R S

Ts e t s e f l y i n f e s t a t i o n i n e a s t e r n c e n t r a l A f r i c a

Tsetse fly infestation in Africa was reviewed by Ford (1971) In southern east-central

Africa, infestation is mainly by Glossina morsitans Westwood (Diptera: Glossinidae) and G pallidipes Austen Figure 3.4 shows the areas infested G morsitans infests the

Brachystegia-Fulbernardia woodlands of Mozambique and Zimbabwe below

1,200 m above sea level, and the Colophospernum Mopane woodlands in the

Zambezi valley G pallidipes is also found throughout these areas inhabiting thicket

or forest-edge areas Until eliminated by insecticides from Zululand (du Toit, 1959),

it extended further south in Africa than any other species Figure 3.5 shows the

distribution of G morsitans and G pallidipes in Zimbabwe, southern Mozambique

and South Africa in 1959

Ts e t s e f l y i n Z i m b a b w e

Zimbabwe forms a single natural geographical system, centered upon the watershedthat separates the Zambezi from the Limpopo and Sabi-Lundi river systems (seeFigure 3.2 Agro-ecological regions of Zimbabwe (from ENDA-Zimbabwe, 1991)

Figure 3.3 Zimbabwe with its major drainage systems

Source: Billings and Phelps (1972).

Figure 3.4 Distribution of G morsitans and G pallidipes in S.E central Africa (adapted

from Ford, 1971) With permission of Oxford University Press.

Figure 3.3) before they descend from the African Plateau into the MozambiqueCoastal Plain The eastern highlands form the rim of the plateau in the east Inthe southeast, between the Sabi and the Limpopo, descent to the coastal plain ismore gradual This region lies in an arid zone of low rainfall (200 to 400 mm peryear), centered over the Limpopo, but extending chiefly northwards Here, tempera-tures are high In the northeast, the Nyanga Mountains slope down to the ZambeziValley.

Glossina were found below about 900 m above sea level in the Limpopo Valley,

and below about 1,200 m in the Zambezi Valley The economically importantportion of Zimbabwe lies above these altitudes, so that in the north and south arelowlands, both of which were once tsetse infested The Limpopo Basin lost itsinfestation during the Great Rinderpest Panzootic (1889 to 1896), except for theMozambique Plain (Ford, 1971) The Zambezi Valley, at least on the Zimbabweside, also lost most of its tsetse infestation, except for small pockets

Figure 3.5 Distribution of G morsitans in Zimbabwe, southern Mozambique and South

Africa in 1959 (adapted from Ford, 1971) With permission of Oxford

University Press.

Pollution of the Zambezi River (including Lake Kariba) and the MozambiqueCoastal Plain by OC pesticide residues is related, in the main, to efforts by thethree countries – Zimbabwe, South Africa, and Mozambique to prevent therecovery and spread of the tsetse fly in the region Prior to 1961, control of thespread of the tsetse fly had been carried out by means of brush clearing and gamedestruction (Robertson and Kluge, 1968) Ever since 1962, the three countrieshave combined their tsetse control campaigns by carrying out annual applications

of persistent pesticides to dry-season resting and refuge sites within the infested

areas (Robertson et al., 1972).

Ts e t s e c o n t r o l u s i n g i n s e c t i c i d e s i n t h e s o u t h

-e a s t -e r n Z i m b a b w -e – M o z a m b i q u -e b o r d -e r r -e g i o n

The use of persistent insecticides in the reclamation of the Zimbabwe–Mozambique border region between the Rio Save and the Limpopo River from

G morsitans was reviewed by Robertson et al (1972) Between 1953 and 1962, an

extensive and rapid westerly and southwesterly advance of G morsitans occurred in

the lower Lundi drainage basin in the Zimbabwe–Mozambique border region,west of the Save River By mid-1962, the tsetse had advanced to within 80 km ofthe Krugger National Park in South Africa The advance seriously threatened toextend cattle trypanosomiasis over vast areas on the Nuanetsi and Limpopo basins,and, thus, became a matter of vital concern to the three countries, Zimbabwe(then southern Rhodesia), Mozambique, and South Africa Therefore, joint sprayingoperations to control the advance were started The operations involved groundapplication of persistent insecticide to tsetse resting and refuge sites To ensurethat the pesticide deposits remained lethal for as long as possible, the spraying was(and is) done in the dry season, during July to the end of September In the firsttwo years, the insecticide was applied by means of motorized machines, but, from

1964 onwards, hand-operated spraying machines were used These sprayers were(and are) fitted with special nozzles capable of throwing a variable jet of spray up

to a distance of 7.6 m and with special regulating valves capable of giving a constantoutput pressure of 2.07 bars A team of eight spray operators, four of whom are

in action at a given time, carry out the spraying operation In the field, the team ofspray operators is guided by maps made from aerial photographs of the applicationarea Each spray operator normally covers a swathe about 14 m in width.The type, formulation, and quantity of pesticide used on the Zimbabwe side ofthe Zimbabwe–Mozambique border and the year and area sprayed are shown

in Tables 3.4 and 3.5 The increased application rate of the pesticide, from 50

L km–2 in 1962 to 144 L km–2 in 1966 as shown in Table 3.4, primarily reflects theincreasing density of suitable tsetse habitats as the work progressed from theperiphery of the tsetse advance to areas of firmly established tsetse flies Duringthe same period, 1962 to 1966, spray operations were also conducted on theMozambique side of the border The use of DDT was begun in 1968 The 1970and 1971 campaigns involved some respraying of areas that had been sprayedpreviously

Table 3.4 Spraying operations using 3.1 percent dieldrin emulsion in the south-east Zimbabwe–Mozambique border region 1962–67

Year No of teams Quantity used (L) Approx area sprayed (km 2 )

a 830 km 2 in Zimbabwe and 1,300 km 2 in Mozambique.

Table 3.5 Spraying operations in the south-east Zimbabwe–Mozambique border region 1968–71

Year Territory Insecticide Area treated (km 2 ) Quantity (L)

‘War of Liberation’, but were resumed after independence in 1980 Figure 3.6

shows the areas sprayed in the periods 1982 to 1984 and 1988 In addition to theareas shown in Figure 3.6, a study of the operational maps of the Tsetse andTrypanosomiasis Control Branch of the Department of Veterinary Services showsthat DDT sprays were concentrated in the Binga area of the Zambezi Valley prior

to 1985 Between 1985 and 1990, spraying was conducted mainly east ofRuzirukuru River in the areas drained by the Sengwa, Ume, Sanyati, andGachegache rivers

The quantity of DDT used for tsetse control has been declining since 1981 (see

Figure 3.7) In 1981, about 300 T of DDT were used and that figure had dropped

to less than 10 T by 1990 (a personal communication from W Shereni, Head ofthe Tsetse and Trypanosomiasis Control Unit, Department of Veterinary Services,Ministry of Lands, Agriculture, and Rural Resettlement for Zimbabwe; unrefer-enced, see Acknowledgments) Two factors have contributed to the drop in thequantity of DDT used The first is eradication of the tsetse flies from some locations

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Figure 3.7 Annual use of DDT for tsetse control in Zimbabwe 1981–90

Figure 3.6 Areas sprayed with DDT in the Zambezi valley between 1982 and 1985, and

in 1988

Source: Department of Veterinary Sciences, Zimbabwe.