[Reprinted from the 'Annals of Tropical Medicine & Parasitology,' Vol. 56. No. 1. April, 1962]

THE EFFECT OF DDT ON THE FAUNA OF A CENTRAL AFRICAN STREAM BY H. B. N. HYNES AND T. R. WILLIAMS (From the Department of Zoology, University of Liverpool, and the Department of Parasitology and Entomology, Liverpool School of Tropical Medicine) (Received for publication December 22nd, 1961)

In recent years it has become common practice in many parts of the world to use DDT for the control of the larvae of stream-dwelling , particularly Simu/ium. The effects of insecticides on stream communities have been reviewed by Hoffmann (1959), and several authors have expressed disquiet at the possible effects of their widespread use in streams and the dangers of upsetting the ecological balance of the community (Hynes, 1960 and references therein). This effect of insecticides is well known in agriculture, where, for instance, Pimentel (1961) has recently shown that they may actually increase the numbers of a pest by destroying its natural enemies. Similarly, Hinton (1955), in a review of the effect of insecticides on the balance of natural populations, goes so far as to write ' We create our own pests but not from choice : they are one consequence of our interference with nature, with our attempts to control it.' There are, indeed, already indications that attempts to control Simu/ium with insecticides have had the reverse effect : Davies (1950) reports a very large increase in the numbers of these insects in the years following application of DDT to a stream in Ontario. It was against this background of possible danger that one of us appealed for caution in the use of DDT against Simu/ium in Africa (Hynes, 1960). Caution seems particularly called for in a region where so very little is known about the biology of the rapid streams in which Simu/ium breeds. Ecological studies of South African streams have recently been started (Harrison, 1958; Harrison and Elsworth, 1958; Oliff, 196oa, 196ob) ; but in Central Africa, apart from notes in several scattered faunistic and taxonomic works, little ecological investigation of small streams and rivers has been carried out. The only exceptions are the work of van Someren (1952) on a trout stream in Kenya, which is, however, primarily concerned with the trout themselves, and that of Marlier (1954a), who gives a general description of the fauna to be found in different types of streams in the eastern Congo. Other ecological observations have been limited to certain groups or species (Marlier, 1954b; Hynes, 1953), or to fishes (e.g., van Someren, 1952; Marlier, 1953). Against the background of our ignorance of African stream communities, we should like to quote from the agricultural world : Because communities have a structure and a physiology they should be studied as a whole. This approach has been neglected in most control studies' (Pimentel, 1961). Similarly, Stern et al. (1959), discussing problems arising from the widespread and often indiscriminate use of insecticides, state : Few studies have included basic investigations on the effects the chemicals might have on other components of the

78 79 ecosystems to which the pests belong', and later, 'because fundamental knowledge is lacking, the investigator may be unaware of the intricate nature of the biotic complex with which he is dealing, and of the destructive potential that many chemicals in use today have on the environment of the pests.' During a visit to Uganda in December, 1960, and January, 1961, we were therefore very pleased to have the opportunity of studying a stream which had been dosed with DDT. We are well aware of the limitations of our work, confined as it was to a short period, but, in that we attempted to study the entire community, we feel that our data may be of value in assessing the dangers of the use of insecticides in African streams. Corbet's (1958) description of the effect of DDT upon the fauna of the Victoria Nile is the only other published study of this kind from Central Africa, and is much more limited in its scope.

The River Manafwa The river Manafwa rises at a height of 13,900 feet on Mount Elgon in eastern Uganda, and flows in a south-westerly direction, eventually to join Lake Kioga. For the first nine miles it flows through dense and almost impenetrable forest, from which it emerges at about 6,000 feet. It then flows for five miles through cultivated land, on which plantains and coffee are the main crops, to the township of Bulucheke at 4,400 feet, and our study followed it for a further 71 miles, through similar country, to the mouth of its tributary, the river Sala, at 3,950 feet. From the forest edge, where our study began, to the mouth of the Sala, the main stream receives several tributaries of various sizes, and these steadily increase its size but do not much alter its general appearance. At the forest edge the river is about four yards wide, flowing swiftly over stones among large rounded boulders, which presumably it can move during spates. At 5,200 feet, the highest point on the motorable track up the valley, it is slightly wider, and the largest stones are about head-sized. It continues in this condition, although widening steadily to about seven yards at Bulucheke and to 10-12 yards at the mouth of the Sala. In a few places virgin rock projects up from the stream bed, which is composed of stones and gravel, but everywhere the current is swift, even at a time of low water. There are no deposits of very fine gravel, sand or silt, but we found leaves, sticks and other vegetable debris lodged amongst the stones. At the time of our visit the water was shallow (although as deep as two feet in places), but it was clear that during rainy periods, which are seasonally rather ill-defined on Mount Elgon, the water rises at least 2-3 feet. Everywhere the stones were smooth and clean and were very little coated with algae. The only visible vegetation was a sparse coat of a creeping liverwort, which occurred on some of the larger stones at stations from 4,600 to 4,200 feet. This condition appeared to be general in streams on the mountain, although in some streams at higher altitudes we found a little of the alga Lemanea, and some moss at very much higher altitudes. Fishes are scarce in the stream and are reported not to occur much above Bulucheke, up to which point the siluriid Amphilius jacksoni (Boulenger) ascends, and also a species of barbel. We ourselves collected A. jacksoni only at 4,200 feet, and we found a single small Clarias at 4,150 feet. As our collecting methods, particularly for crabs, were such as to allow us readily to catch and observe fishes, it is clear that they are an unimportant element of the fauna. 8o

The Dosage with DDT On December 2nd, 1960, in the course of testing a new preparation of DDT, Mr. M. A. Prentice, of the Department of Medical Entomology, Kampala, had applied insecticide to the stream st Bulucheke at a point just below our collecting-station at 4,400 feet. This was done in such a way as to maintain a concentration of one part per million of DDT in the river water for 30 minutes, which was completely effective in eliminating Simulium neavei from crabs for a considerable distance downstream. We should, however,

County boundary Road Track Forest 0 Collecting station • Dosing point

MAP of the section of the river Manafwa on the slopes of Mount Elgon in which the investigations were carried out.

emphasize here that this single treatment was designed as a test of a new insecticide prepara- tion, and not as a control measure for S. neavei. For the latter, repeated treatments during several weeks would have been carried out. On the same day, the river Sume, a tributary which joins the main stream at 4,300 feet, was treated at a high altitude with a soluble block containing DDT ; but, as this had little effect on the fauna of the tributary, our study was largely confined to samples taken from the river Manafwa alone. Data from the Sume are, however, given in the Appendix. 8 METHODS In order to study the fauna, a large general collection was made at each of eight stations distributed from the forest edge to the mouth of the river Sala. These were made on December 31st, 1960, and January znd, 1961, i.e., 29-31 days after the application of the DDT. The collections were made by kicking up the substratum and overturning stones upstream of a fine-meshed hand-net (6o meshes per inch), which was periodically emptied into a dish. The entire collection was then preserved unsorted in formalin. This method of collecting is to some extent selective, failing to take ancylid limpets and other closely adherent (Macan, 1958). At each station, therefore, a special search was made for such animals as Burnupia and Afropsephenoides, and their presence or absence was noted. Apart from these, however, the net samples can be expected to contain a fair representation of the fauna and to be comparable between stations. At the same time, extensive collections were made with a coarser-meshed net for crabs (Potamon spp.) and fishes, and any invertebrates taken in this way were noted if they had not been seen in the fine-mesh samples. In the laboratory in England, the fine-meshed samples were washed free of formalin, and the animals and light debris were floated off in a saturated solution of calcium chloride, by a technique modified from that of Beak (1938) and described by Hynes (1961). The debris was then hand-sorted, the animals being picked out, identified and counted. This operation was particularly tedious because of the large numbers and small size of many of the animals ; but, as there seem to be few other observations on the numerical composition of the fauna of African streams, the work seemed worth while.

RESULTS The Invertebrate Fauna Table I shows the animals recorded, but excludes those of which fewer than i o specimens were present amongst the 56,000 in the collections. These were enchytraeid worms, the Diceromyzon and Polymitarcys, the dragonflies Brachythemus, Paragomphus and an aeschnid, the caddis worms Goerodes, Chimarrha and an unidentified genus, the bug Macrocoris, caterpillars of the family Hydrocampidae, the water-penny Afropsephenoides, an adult hydrophilid resembling Hydraena, and maggots very similar to those of Limnophora. The state of the of African stream animals is such that, with the exception of Simidium and Potamon (see below), the only animals which we were able with certainty to identify to species were Allonais paraguayensis form aequatorialis (Spurber), Prosopistoma africana Gillies, and Neoperla spio (Newman) (Hynes, 1952). It would, however, probably be possible for an appropriate specialist to name the six (possibly seven) species of adult Helmidae and the hydrophilid. The Chironomidae from all stations were mostly Orthocladiinae, but they included a few Tanytarsini and Tanypodinae. The Potamon species, which were not retained in the samples and so do not appear in Table I, were the species referred to as P. berardi by Barnley and Prentice (1958), which we found at from 6,000 to 4,200 feet (this specific name is probably incorrect but we hope to pursue the matter in another publication), and P. niloticus, which we found at from 4,200 to 3,950 feet, as well as at lower altitudes. In addition, a single specimen of a third species was taken at 4,150 feet ; this species occurs elsewhere on Mount Elgon but in a very different type of habitat (Hynes, Williams and Kershaw, 1961). 82

TABLE I SHOWING THE PERCENTAGE COMPOSITION OF THE INVERTEBRATE FAUNA COLLECTED AT EIGHT POINTS ON THE RIVER MANAFWA, EXCLUDING POTAMON SPP. FOR BURNUPIA ONLY PRESENCE (P) OR ABSENCE (0) IS INDICATED. (ANIMALS OF WHICH LESS THAN IO SPECIMENS WERE COLLECTED ARE NOT INCLUDED)

ALTITUDE, IN FEET ...... 6,000 5,200 4,600 4,400 4,300 4,200 4,150 3,950 IN METRES ...... „. 1,830 1,590 1,400 1 ,340 1,310 1,280 1,265 1,205

DISTANCE ABOVE DOSING-POINT, IN MILES ... ••• 4 3 1 1 100 YDS.

BELOW L., ,, I, ... 1 I Z I 31

INVERTEBRATE ANIMALS GROUP GENUS OR FAMILY

TRICLADIDA ...... DUGESIA ...... 0.8 + 02 O.5 0-9 0.5 1-3 CRI NEMATODA ...... MERMITHIDAE ...... + 0.2 03 0.3 OLIGOCHAETA...... ALLONAIS ...... + 0' I 02 07 1.1 OSTRACODA ...... CYPRIDAE ...... O-I + 0.1 ± 0.1

OLIGONEURIELLA ... I ø 09 0-3 ± ± O-I TRICORYTHUS ...... 0.4 32 2.0 42 5.9 2' I EPHEMERYTHUS ... + 0' 7 0.2 0' I 0'4 1 4 PROSOPISTOMA... ..• -1- 02 + ORI ± + 0' I + EPHEMEROPTERA ... CAENIS ...... 0-9 0.5 74 93 0. 5 O.6 5.3 4.2 EUTHRAULUS ...... 0-2 02 04 05 O.I O.3 O.6 0.7 ACENTRELLA ... ..• .. 102 233 23.8 7.9 9.3 22 03 0.2 CENTROPTILOIDES ...... 03 0-2 BAETIS AND CENTROPTILUM .. 375 433 375 37 8 477 58 9 336 695 AFRONURUS ...... • 1-2 2' 3 1O9 15-9 47 43 I8.5 2.2

PLECOPTERA ...... NEOPERLA ...... O.6 O.7 13 I8 0.2 0.5 15 1.8

HYDROPSYCHE ...... ± O-1 1-I 0-8 0- I 0.1 + TRICHOPTERA ... CHEUMATOPSYCHE ...... 6.1 2.5 55 45 1-2 I6 3.3 0.9 HYDROPTILIDAE (?STACTOBIA) ... 0-4 + 0-2 + + ±

GYRINID LARVAE ... ..• + ± 0' I 02 02 CO1B0PIERA ... HELODID LARVAE ...... 0- I 01 0-3 01 HELMID LARVAE ...... CRI 0.7 23 4.1 03 03 08 '5 HELMID ADULTS ••• .•. 0-3 0.9 0.9 0•8 ± 02 0' I 03 B PEDICIINE LARVAE ...... + ± - 0_I LIMONIINE LARVAE ...... 0' I 0' 2 BSID ± + ± . 0 SINT/ILL/MT LARVAE ... ••• 341 21 5 4' 282 20' I 132 6.9 B

DIPTERA ...... SIMULLUM PUPAE ...... 06 02 0' I ± CHIRONOMID LARVAE ...... 6-3 2-5 2-6 4B++ 3- 2 4-4 8-6 5.6 CHIRONOMID PUPAE ...... 0-2 ± O.I -I- 02 I3 0-2 CERATOPOGONID LARVAE ... ± ± O - I ± ± 01 ±

? EMPID LARVAE ...... -I- ± ±

ACARINA ...... HYDRACARINA ...... O. I + + O - 1 0. I 0.4 0.9 0.3 ANCYLIDAE ...... BURNUPIA ...... 0 O O? P P P P 0

TOTAL NO. OF ANIMALS ... ..• ... 6,381 9,015 7,234 5,751 8,259 10,437 4,300 4,757

= O.O5 PER CENT.

SOME ITEMS IN TABLE I REQUIRE FURTHER EXPLANATION. THE MERMITHID NEMATODES WERE PARASITES OF SIMI/HUM LARVAE, FROM WHICH MANY HAD APPARENTLY BROKEN OUT IN THE PRE- SERVATIVE. AT LEAST TWO SPECIES OF TRICORYTHUS AND EPHEMERYTHUS AND FOUR OF ACENTRELLA WERE PRESENT. OWING TO LOSS OF GILLS IN THE PRESERVED MATERIAL, NYMPHS OF BAETIS AND CENTROPTILUM, OF WHICH THERE WERE SEVERAL SPECIES OF EACH, COULD NOT ALL BE DISTINGUISHED 83 and so were counted together, and this category probably also includes small specimens of Centroptiloides. Otherwise the various genera of nymphs were readily dis- tinguishable down to the smallest caught, which were below i mm. in length. There were certainly two, possibly more, species of Cheumatopsyche, one of which was bright- green when alive, and the Hydroptilidae (?Stactobia) contained both specimens with cases and specimens without cases and with fully armoured abdomens. Some indication is given in Table I of a marked altitudinal zonation, even though the table is necessarily somewhat condensed and taking into account the fact that the DDT entered the river immediately below the station at 4,400 feet. Thus Tricorythus, Ephemerythus and Centroptiloides were not found above 4,600 feet, and an altitudinal limitation was also shown by Allonais, Gyrinidae and Burnupia, as well as by the crabs (see above). When the uncondensed data are examined, further evidence of altitudinal zonation can be seen. Chimarrha, in this and other streams on Mount Elgon, was found only at high altitudes, and Polymitarcys and Diceromyzon were found only at low altitudes. Among the Helmidae, at least three of the species were found only below 4,300 feet, and two were found only above Bulucheke, although this may have been due to the DDT. Within the genus Acentrella, the four species showed a clear zonation. Species C ', which has pointed spines on the abdominal terga, was found only at 6,000 feet ; species A', with similar but blunt spines, occurred at from 6,000 feet to 4,150 feet, although it was rare at the last point ; species B ', a small narrow spineless insect, occurred at all altitudes, but was rare at the lowest stations, where, at and below 4,200 feet, it was joined by the broader-bodied species D A similar altitudinal zonation was shown by the species of Simu/ium occurring in the net samples, and, although the extent to which the natural succession was disturbed by the DDT treatment cannot be gauged, the effect was probably rather to alter the size of the Simu/ium population than to change the species composition. Two hundred Simu/ium larvae were selected at random from each sample and were identified, the remaining larvae being examined to determine whether other species were present which had not occurred in the sub-sample. These results are shown in Table II, which also gives the species of Potamon occurring at the sampling-station and indicates whether or not the crabs were found to be carrying the immature stages of S. neavei ; the numbers and identities of Simi/Hum pupae taken in the net samples are also given. Crosskey (1960) has described the general facies of the larvae of the phoretic species of Simu/ium which live attached to crabs or mayfly nymphs. These larvae are characterized by the extreme reduction of their eye-spots and by the position of the posterior circlet of hooks, which is ventral rather than terminal as in free-living species. Crabs carrying the immature stages of S. neavei were found at between 4,200 and 4,600 feet in the river Manafwa, both P. berardi and P. niloticus being involved ; but S. neavei must also have occurred further down, as a single larva was found in the net sample taken at 3,950 feet. (Crabs taken in the 6o-mesh net were usually removed from the sample, but several small crabs were inadvertently included in the material collected at 3,950 feet.) The absence of S. neavei larvae from other samples can be attributed to the removal of crabs from the net collections. Larvae and pupae of S. neavei were never found above 4,900 feet or below 3,800 feet in any of the rivers which were visited, and there is some indication that this 84 distribution is to some degree determined by temperature. Two further phoretic species, S. copleyi and S. lumbwanus, occurred in association with mayfly nymphs. Larvae of these species had become separated from their nymphs in the net samples, but S. copleyi was elsewhere found to be associated with nymphs of Baetis and Afronurus, while S. lumbwanus was taken only in association with Afronurus. Examination of Table II indicates that the distribution of both of these species is restricted in a similar manner to that of S. neavei, although the distribution of S. lumbwanus in particular has been obscured by the effect of the DDT. The occurrence of S. copleyi below the dosing-point can be attributed to the presence of mature nymphs of Baetis at all stations, but S. lumbwanus did not reappear until 3,950 feet, where well-grown nymphs of Afronurus were again present.

TABLE II Showing (A) the percentage composition, by species, of zoo Simullum larvae randomly selected from samples collected at eight points on the river Manafwa ; (B) the specific identity of all pupae and empty pupal skins found in the samples ; and (C) the distribution of the crab species and associated S. neavei

Altitude, in feet ...... 6,000 5,200 4,600 4,400 4,300 4,200 4,150 3,950 A. Larvae S. akocki Pomeroy ...... o-5 aureosimile Pomeroy 4 dentulosum Roubaud ... 97 89-5 II , 2.orax Pomeroy ... P 24 25 0.5 hirsutum Pomeroy ...... 25 2 1 5 3 6 3 8 medusaeforme form hargreavesi Gibbins ...... 475 85 965 89-5 69 21 cervicornutum Pomeroy ...... 1-5 P 4.5 28 695 alcocki group ...... P copleyi Gibbins ...... P 3-5 15-5 1.5 P P neavei Roubaud ...... 0%5 lumbwanus De Meillon ... 8 1

B. Pupae S. vorax form touffeum Gibbins ... 1 7nedusaeforme form hargreavesi Gibbins ...... z 46 16 4 cervicornutum Pomeroy ...... 1 1 hirsutum Pomeroy ...... 1 C. Crabs and associated S. neavei Potamon ' berardi ' ...... P P P P P P niloticus Milne-Edwards ... P P P S. neavei taken in association with crabs P P P P

P =Present.

Altitudinal zonation would appear to be due to temperature, and, although no measure- ments were made, it was clear that there was a very marked temperature rise during the descent of 2,050 feet covered by this study. Shaw (1959) investigated the distribution of Potamonnilotkus in the river Manafwa, and found that the temperature gradient of the water 0 ranged from close to o° C. at the source to zo-30 C. in the lower reaches. The distribution of P. niloticus appeared to be limited by temperature, since the crab occurred only in the lower reaches, where the midday temperature of the water was above i3° C. A steep temperature gradient is not, perhaps, unexpected on a tropical mountain, but it raises a point of fundamental interest, namely, that in a tropical region there should be species, such as Acentrella C' and Chimarrha, which are apparently confined to cold water. It is 85 possible that they have evolved in situ, but this seems unlikely, as the mountain is a young one and species of Chimarrha occur also on Mount Kenya. It seems more likely that they are relicts of a period of colder climate, which allowed interchange of faunas, and of such floristic elements as the giant groundsels, between individual African mountains, and also perhaps between these and temperate regions.

The Ecological Organization of the Fauna Hynes (1961) and earlier workers have shown that to some extent it is possible to determine the ecological organization of stream faunas by the consideration of seasonal cycles and feeding habits. In the tropics it seems less likely that seasonal cycles are important, and support is given to this suggestion by the work of van Someren (1952) and by the fact that, in the present study, above the zone affected by DDT specimens of all sizes, from newly hatched to fully grown, of most species were represented in the collections (see Table V). It seems likely, therefore, that the structure of a community does not undergo marked seasonal changes, as it does in temperate climates, although spates must selectively reduce the fauna. van Someren (1952) observed seasonal changes in the total number of invertebrates of the Sagana river and was able to relate these changes to the water level. Table I shows that most of the fauna of the river Manafwa is composed of mayflies and SimuHum, a fact which has been noted of streams on Mount Kenya by van Someren (1952) and which is true also of other streams on Mount Elgon. These animals are herbivores or detritus feeders, and, in the absence of marked algal growth, they probably depend chiefly on vegetable debris of terrestrial origin. Examination of the list of other organisms shows that most of them are also herbivores or scavengers. Indeed, the only ones which are or may be carnivores of any great importance in the community are Neoperla, Hydropsyche, Cheumatopsyche, Potamon, gyrinid larvae, Pediciine larvae, Tanypodinae and Hydracarina, and, among the rarer animals listed on page 81, the dragonfly nymphs and Macrocoris. It is possible also that Dugesia is to some extent a carnivore, although it could clearly tackle only very small animals ; Prosopistoma has been stated to have carnivorous tendencies (Marlier 1954a). Of these animals, clearly only the first three are sufficiently numerous to play an important role in the ecology of the animal community, and their food was accordingly investigated. From Table III it can be seen that Neoperla is entirely carnivorous, as has previously been shown by Hynes (1953), and that both Hydropsychidae are omnivorous, eating both animals and plant debris which may contain fungal hyphae. All three insects eat Ephemeroptera (mostly , but some Tricorythidae also) and all eat some other insects, including Simu/ium larvae, of which up to three had been eaten by individual specimens of Neoperla and two by individual hydropsychids. They must therefore exercise some control over the numbers of mayflies and Simu/ium, which might be expected to increase if they were removed. The less-abundant invertebrate carnivores were not investigated, as they were relatively scarce or small. It seems likely that Tanypodinae and Hydracarina may have some influence, but certainly much less than Neoperla or the Hydropsychidae. Fishes are, as we have seen, fairly rare, but on the basis of two stomachs we can state that Amphilius jacksoni feeds on invertebrates, including Simu/ium. Corbet (1961) states that the food of this species is entirely composed of lithophilic insects. The only other 86 large animals are the crabs, which are certainly common and, in captivity, feed readily on plant material, meat and small stream animals (Williams, 1961). Further gut examinations (Williams, unpublished) have shown that over 6o per cent. of crabs collected on Mount Elgon contained animal remains. These were mostly mayfly nymphs, but included some Simu/ium. As no attempt was made to determine the size of the crab population in the Manafwa, the effect of crab predation cannot be adequately assessed. It is unlikely, however, to be of the same order as predation by Neoperla and the hydropsychid caddises. We may conclude, therefore, that in rivers similar to the Manafwa such control of the numbers of mayflies and Simu/ium as is not carried out by physical factors, such as floods, is exerted mostly by Neoperla and Hydropsychidae, and to a much lesser extent by other invertebrate predators and fishes. We may note here that fishes and many of the other

TABLE III Showing the food of the three principal invertebrate carnivores of the river Manafwa, based on specimens above the zone treated with DDT. In the Hydropsychidae, insect cuticle is recorded as such only when none of it could be certainly identified, i.e., 20 specimens of Hydropsyche had eaten insects, all of which were probably Ephemeroptera, but in only eight specimens was the identity certainly established

Genus ... Neoperla Hydropsyche Cheumatopsyche

No. of specimens examined ... 50 34 51 No. and percentage with empty fore guts ... 13 (z6%) 4 ( 12%) No. and percentage* containing only animal food ... 37 (I00%) 15 (SO%) 7 (14%) No. and percentage* containing only vegetable food 8 (27%) 14 (27%) No. containing each food item, and percentage com- position of the diet : Ephemeroptera 29 (52 %) 8 (i7%) 21 (24%) Insect cuticle (probably Ephemeroptera) 12 (26%) 12 ( 14%) Hydroptilid larva . (2%) Helmid larva (2%) Hydropsychid larvae (Cheumatopsyche) 5 (9 %) (2%) I (I %) Simulium larvae ... 8 (I4%) 3 (6%) 2 (2%) Chironomid larvae ... 12 (21 %) 2 (4%) Higher plant material (leaves and wood) ... 1.4 (30 %) 43 (50%) Fungal hyphae (always with higher plant material) 2 (4%) 3 (3%) Filamentous green algae ... I (2%) 2 (2%) Bluegreen alga (?Oscillatoria) (1%) Detritus (in the absence of higher plant material) 3 (6%) (1 %) Sand grains ... I (2%)

* Percentages only of specimens which contained food. invertebrate predators occur only at the lower altitudes, and also that Neoperla, Hydropsyche and Cheumatopsyche belong to orders of insects which are particularly susceptible to poison- ing by DDT (Hoffmann and Merkel, 1948; Hoffmann and Drooz, 1953; Corbet, 1958).

The Effects of the Insecticide Although it was still possible to gain much information as to the action of the insecticide from samples taken after an interval of a month, it would clearly have been preferable to have had samples taken from below the dosing-point shortly before and after the DDT had been applied. Mr. Gutaka, who was present during the dosing, informed us that, in addition to the larvae of S. neavei, large numbers of mayfly nymphs died. Table I shows, however, that a month later only a few organisms which occurred above the site of dosing were absent from some stations below it. These include Cypridae, Oligoneuriella, Centrop- tiloides, helodid larvae, Pediciinae, Limoniinae and Empidae. All these were, however, 87 relatively rare animals, and their absence from the sample taken at 4,300 feet may well have been due to chance. The table does, nevertheless, show some abrupt changes in abundance : thus Caenis, Afronurus, Neoperla, Hydropsyche, Cheumatopsyche and Helmidae were distinctly less important at 4,300 feet than they were at 4,400 feet, and, conversely, Acentrella Baetis and Centroptilum, and Simi/hum were markedly more important. It is, however, when one considers the sizes of the organisms occurring above and below the dosing-point that the effect of the insecticide becomes apparent. In many, no differences were found : these were Dugesia, helmid larvae of at least three species, Cera- topogonidae, Sinudium and Chironomidae. Pupae of the last two were also present below the dosing-point, indicating that development was continuing normally. Similarly, fishes, crabs and S. neavei occurred in normal abundance and size distribution. For these organisms it must be assumed, therefore, either that they had been completely unaffected by the insecticide or that their eggs had survived, and that 29-30 days had been sufficient time for at least some of them to grow to full size. It has been suggested (Hoffmann and Merkel, 1948) that insect eggs are not killed by DDT in streams, and it is now well established (Hynes, 1961 and references therein) that egg diapause is almost universal among stream-dwelling insects. It is quite possible, there- fore, for eggs to survive a single dose of DDT and to continue hatching over a period, so that once the first-hatched specimens have reached full size the population-size structure is fully restored. This must have happened with S. neavei, which we know to have been eliminated ; and it is therefore reasonable to assume that it had also occurred with other species of Si/nu/him and possibly with the Chironomidae and Ceratopogonidae. Similarly, it seems to have been the case with the Helmidae, the adults having been eliminated, apart from a single specimen which may have escaped or flown or drifted downstream. It is also certain that crabs and fishes were unaffected, as dead specimens would not have escaped notice on December znd ; and Dugesia is also likely to have been unaffected, as Tricladida have frequently been reported as not being susceptible to DDT. In contrast, many of the Ephemeroptera, the Neoperla, and the Hydropsychidae were noticeably smaller below the dosing-point than above it (Table IV), and in many genera this smallness persisted for a considerable distance. Here, then, we must assume that these animals had been killed and had re-established themselves from eggs. Some, such as the caddis worms, Neoperla, Afronurus and Oligoneuriella, had, apart from one specimen each of Cheumatopsyche and Afronurus, been killed off for a long distance, while others, such as Acentrella ' A' , Euthraulus, Ephemerythus and Tricorythus, had been eliminated for a shorter distance. Caenis, Acentrella B Baetis and Centroptilum showed, like Simullum, no change in maximum size, and it is therefore possible to assume either that they had been unaffected or that they had had time in 30 days to grow to full size and ripeness. The second alternative is the more likely to be true, in view of the proved susceptibility of other mayflies, and even of another species of Acentrella. It is particularly noticeable that among the most severely affected animals are the three predators Neoperla, Hydropsyche and Cheumatopsyche. Not only were they reduced in numerical importance (Table I) but they were also reduced in size (Table IV). In Table V this last effect is shown in detail for Neoperla, which is the most satisfactory of the three to measure, as, unlike the Hydropsychidae, it does not die in a tight coil. From this table it is clear that for a considerable distance below the dosing-point all the Neoperla were very small. 88

TABLE IV Showing the maximum sizes, to the nearest 1 mm., of selected insects in the samples collected from the river Manafwa. Measurements based on less than io specimens are bracketed

Altitude, in feet ...... 4,600 4,400 4,300 4,200 4,150 3,950

Maximum size, in mm. : Oligoneuriella ...... 27 (194) (I) (2) Tricorythus ...... 3 R 5f R if 4 R 3 31 R; 6 R Ephemerythus ...... (1) 24 R i ‘3 6 R 41 R 51 R Prosopistoma ...... (zi) (24) (II) (ii) (4) (2) caenis . • . ... • • • • . • 4 R 4 R 31 R 41 R 31 R 4 R Euthraulus ... - 5 R 3i •... • • •- - 54 R 4 31 51- Acentrella 'A ' ... 5 R 41 z 1 (3) (4) Acentrella ' B' • •• --- 4 R 31 It 4 R 41 R (21) (31-) R Baetis and Centroptilum ... 61 R 6 R 6 R 6 R 51 R 6 R 1* Afronurus ...... so R 81 12 z 2 2 31 81 Neoperla ...... 14 R 14 R 4 4 3 12 Hydropsyche ...... 13 13 - 1 (31) (4) (3) Cheumatopsyche 8 81 41 41t 5 5

R =Ripe, i.e., with black wing-pads and nearly ready to emerge ; these were always the largest specimens, except in Tricorythus at 3,950 feet. * Apart from a single larger specimen measuring 61 mm. 1- Apart from a single larger specimen measuring 7 mm.

Only at 3,950 feet was the normal size range present, and the same applies to the two caddis worms (Table IV).

Here, then, we have a likely explanation of the increased abundance of Baetidae and Sinudium below the dosing-point. These were both either unaffected by the insecticide, or—as seems much more likely—were eliminated but grew up very fast from surviving eggs. In either event they were left in an environment in which their principal predators were relatively rare and all of small size, so that a greater proportion of the eggs in the river bed were able to develop into larvae or nymphs.

DISCUSSION This study has shown that the effect of a single dose of DDT on an African stream was almost certainly to eliminate the majority of insect species for varying distances. It seems

TABLE V Showing the size-distribution of the nymphs of Neoperla spio in samples collected from the river Manafwa

Length, in mm. Altitude, in feet 0-I 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 11-12 12-13 13-14 14-15 15-16 6,000 4 6 6 4 6 6 5,200 17 9 3 3 3 4,600 21 24 19 r3 4 4 2 2 3 4,400 26 38 io 8 4,300 12 5 4,200 II 30 II s 4,150 8 39 17 3,950 5 39 24 7 2 3 2 2 89 fairly certain, however, that almost all the spec]. s survived as eggs, from which the popula- tion was replenished. The three major predato s were, however, among the most severely affected, and this led to an increase in the principal prey organisms, Baetidae and Simu/ium. This is an effect which has been observed in streams in Europe and North America (Hynes, 1961 and references therein), and indicates that DDT treatments could lead to severe outbreaks of the Simullum species which they were intended to control. Of course, in this instance a single dose would not be regarded as a control measure, and in any event the Simu/ium which increased was not the one which it would be wished to control. Although we know nothing as yet about the enemies of S. neavei, it seems not improbable that it could be similarly affected by control measures. A single dose clearly produces only a transient effect because of the presence of diapausing eggs, but it seems possible that continued treatment over a long period might entirely eliminate the natural controlling factors of S. neavei and lead to severe outbreaks. In any event, our findings as outlined above are relevant to the control of S. damnosum, which is a free-living species. We must reiterate the warnings drawn from agricultural experience that effective control measures can only be successfully applied within a framework of knowledge of the whole community in which the pest species lives. Such knowledge about the African streams which harbour the vectors of Onchocerca vokulus is badly needed. Finally, we should like to draw attention to the fact that in this instance DDT has shown itself to be a useful ecological tool. Because of the lack of a seasonal cycle, it is very difficult to determine the growth rates and lengths of life of tropical stream-dwelling insects. Table IV, however, shows how large some nymphs and larvae can become in a single month, and there are fairly clear indications that Caenis, Acentrella ' B', and at least some Baetis and Simullum can reach maturity within 30 days. A more extended and lengthy study of this type could very readily determine the growth rates and voltinism of all the principal insects in a tropical stream.

APPENDIX THE EFFECTS OF DDT ON THE RIVER SUME As mentioned above, the river Sume, a tributary of the Manafwa, was treated with DDT on the same day as the main river was dosed. In the Sume, however, the DDT was incorporated into a soluble block composed of vermiculite grains and plaster. Three sets of samples were taken from the Sume, one at 4,400 feet on December 3 ist, 1960, and two at 5,000 and 5,300 feet on January 3rd, 1961, the two lower stations being below the dosing-point. This method of applying the insecticide had apparently had no effect in controlling S. neavei in the Sume, and at the time of our visit there were no indications that the fauna was in any way different above or below the dosing-point. This contrasted very strongly with the effect of the treatment applied to the Manafwa. The samples taken from the Sume were therefore not dealt with in detail, as were the Manafwa samples, but only a few selected animals were studied. Table VI shows the size-distribution of nymphs of N. spio occurring in the river Sume samples, and of Jo° randomly selected Afronurus nymphs from each sample. The data referring to Neoperla may be compared with those in Table V, from which it is evident that in the Sume the DDT treatment had been without effect upon the species. Similarly, there were no differences between the maximum sizes reached by larvae of Cheumatopsyche above and below the dosing- point. In the Manafwa, no nymphs of Afronurus larger than 31 mm. occurred for several miles below the dosing-point (Table IV), but this effect was not seen in the Sume. Table VI shows, however, 90 that at the station above the dosing-point (5,300 feet) 34 per cent. of Afronurus nymphs were less than 2 mm. in length, while at the two stations below the dosing-point 55 per cent. and 62 per cent, of the nymphs were below that size. This could be accounted for by the DDT treatments having killed only a part—i.e., the most susceptible nymphs—of the Afronurus population.

TABLE VI Showing the percentage size-distribution of the nymphs of Neoperla spio and Afronurus in samples collected from the river Stwne

Altitude, Length, in mm, No. in feet counted 0-I 1-2 I 2-31 3-4 14-5 5-6 16-7 17-8 8-9 9-xo xo-x 11-12 12-13 13-14

Neoperla spio 5,300 8 26 2 13 13 8 38 4 3 5 5,000 Is 18 17 20 5 5 5 5 2 3 3 3 66 4)400 5 30 32 16 5 5 5 5 44

Afronurus 5,300 4 30 16 14 10 5 2 100 5,000 II I2 100 44 7 4 3 4,400 7 55 18 9 2 3 2 3 100

ACKNOWLEDGEMENTS.—We should like to thank all those who made this study possible. Professor Kershaw, of the Liverpool School of Tropical Medicine, was the moving spirit who inaugurated our expedition, the Colonial Development and Welfare Fund financed it, the Colonial Office organized it, and the Ministry of Works of the Government of Uganda lent us a car and a driver. Mr. G. R. Barnley, of the Department of Medical Entomology, Kampala, gave us much assistance and local information. He also seconded to us from his department Mr. Gutaka, whose local knowledge and assistance were of great help in the field. Miss Joan Venn, of the Department of Zoology, University of Liverpool, did all the tedious work of sorting the collections. To all these persons and organizations we tender our grateful thanks. We are also grateful to Mr. D. E. Kimmins, of the British Museum, who identified specimens of the mayflies and caddis worms ; to Dr. R. 0. Brinkhurst, of the University of Liverpool, who identified the Oligochaeta ; and to Dr. T. B. Reynoldson, of University College, Bangor, who confirmed our identification of Dugesia.

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