Communication among phlebotomine sandflies: a field study of domesticated Lutzomyia longipalpis populations in Amazonian Brazil

Anim. Behav., 1991,42, 183-192

Communication among phlebotomine sandflies: a field study of domesticated Lutzomyia iongipalpispopulations in Amazonian Brazil C H R I S T O P H E R D Y E * , C L I V E R. D A V I E S * & R A L P H L A I N S O N t *Department of Medical Parasitology, London School of Hygiene and Tropical Medicine, Keppel Street, London WCIE 7HT, U.K. ~fThe Wellcome Parasitology Unit, Instituto Evandro Chagas, C.P. 3, 66.000 Beldm, Pard, Brazil

(Received 1 June 1990; initial acceptance 20 September 1990; final acceptance 3 December 1990; MS. number: 3592)

Abstract. Stimulated by laboratory work which has shown that adult male Lutzomyia longipalpis sandflies (Diptera: Psychodidae) possess sex pheromones, this paper describes a field study of communication in domesticated populations in Amazonian Brazil. Comparative observations on adults caught in animal pens, together with a series of mark-release-recapture and aggregation experiments, lead to the following inferences about host and mate-finding by males and blood-sucking females. During early colonization of a new site, males attending a host increase the rate of recruitment of other flies, males as well as females, but especially females. The pool of female recruits is consequently exhausted before that of males, whereupon the proportion of males increases, culminating in a male-biased sex ratio (in light traps) at equilibrium. Natural site-to-site variation in the sex ratio depends on the number of hosts available; among sheds supporting populations of flies near equilibrium, males are more responsive to host abundance than females. The implication is that, as long-range attractants (and then possibly arrestants) of females, pheromones are of little value to domestic males. Besides pointing to novel selection pressures under domestication, the results have practical significance: putative pheromone traps would need to catch or disorientate sandflies already present in animal pens, because they could not markedly increase female recruitment to pens, or attract a large proportion of females to other sites.

The members of the Lutzomyia longipalpis (Lutz & Neiva) species complex are the primary vectors of Leishmania chagasi (Cunha & Chagas), agent of American visceral leishmaniasis. Adult females acquire parasites in blood meals taken from dogs and foxes, and transmit them to humans at a subsequent feed (Deane& Deane 1962; Forattini 1973; Lewis & Ward 1987; Lainson et al. 1990). The natural environment of the species group in the Amazon is probably 'terra firme' forest (not seasonally flooded), but nothing is known about their natural history in that habitat (Lainson et al. t990). Adult flies can be caught in greater abundance in domestic animal pens, particularly pig and chicken sheds, and in this environment they are far more amenable to study ( D e a n e & Deane 1957, 1962; Forattini 1973; Lainson et aI. 1983; Ryan et al. 1984). Observers of Lu. longipalpis and other sandflies in animal pens have frequently reported aggregations of males in which individuals are regularly spaced on the backs of the animals themselves. 0003-3472/91/080183+10 $03.00/0

Clearly, there are interactions between males in these two-dimensional mating swarms (Lane et al. 1990). There are physical interactions as males jostle for position, and wing-beating of both sexes produces 'songs' of characteristic burst repetition rate and intra-pulse frequency (Ward et al. 1988). For the females, these sites provide blood as well as a mate. Laboratory work has also shown that adult male Lu. longipalpis produce two kinds of semiochemicals, pheromones associated with two different members of the species complex (Lane et al. 1985; Ward et al. 1988, 1989; Morton & Ward 1989, 1990). As sexual signals, these can attract females over distances of at least 2.2 m and may act as arrestants on arrival (Morton & Ward 1989). Other functions of these chemicals, for example in interactions between males, have not yet been thoroughly explored. Apparently, Lu. longipalpis are members of that select group of species whose males release long-distance attractants from feeding sites (Thornhill & Alcock 1983), though it is not yet clear whether females can be attracted over

9 199l The Association for the Study of Animal Behaviour 183

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distances as long as the 10 m attributed to scorpion flies (Bornemissza 1964). The discovery of pheromones in male Lu. longipalpis has led Ward and colleagues to speculate, as have Rechav et al. (1977) in the similar case of the bont tick, Amblyomma hebraeum, about the use of pheromone traps for sandfly control. Pursuing this laboratory work on sandflies, we have carried out an experimental and observational field study of communication among Lu. longipalpis inhabiting rural settlements in the Amazon delta. Our aims in this paper are to demonstrate explicitly communication between Lu. longipalpis sandflies in the field, to investigate some consequences for host and mate finding, and to explore the limitations of pheromones in this unnatural domestic environment. METHODS

Study Area We studied domestic Lu. longipalpis among a group of rural communities (Pingo d'Agua, Cearfi, Chacara, Chiquita, Campinas) in Salvaterra District on Maraj6 Island, Parfi State, Brazil. The area is dominated by natural, seasonally flooded savanna ('campo'), supporting open woodland in parts, and varzea forest along rivers (Lainson et al. 1983). Much 'terra firme' forest and woodland on higher ground which is not flooded during the wet season has been cleared for cultivation (most commonly, pineapples). The housing density varies from 5/kin z to isolated houses 1 km from their nearest neighbour. Houses are most commonly mud on a wooden frame, with a palm-thatched roof. Homesteads have domestic dogs, pigs and fowl. Many have animal pens, principally chicken sheds with chickens, ducks and occasionally turkeys. Their walls are a closely bound pallisade of vertical wooden poles, and the roofs are normally thatched with naja palm. Their function is to protect fowl from predators, mainly foxes. Where there is no shed, chickens generally sleep in the fruit trees found in most domestic compounds. Pig sheds of various sturdy wooden designs are plentiful, though not as abundant as chicken sheds. In what follows, 'shed' means a chicken shed unless otherwise qualified.

Identification of Sandflies Female Lu. longipalpis were routinely identified on external morphology, though doubtful females were confirmed by examining spermathecae (Ryan 1986). Males were identified by the presence of a single pale spot on abdominal segment IV (Ward et al. 1988); apparently only one member of the species complex is to be found on this part of Maraj6 Island.

Reproductive Age of Females Female Lu. longipalpis are gonotrophically concordant, that is, they produce one batch of eggs from each full blood meal taken. Unfed females with ovaries in stage IIa can be identified as nulliparous or parous, parous flies having expanded ovaries with follicular sacs and relics (Detinova t962; Gillies & Wilkes 1965; Ready et al. 1984; Dye et al. 1987). The remainder were graded in two simple categories l i b / I l l (engorged) and IV/V (semi-gravid or gravid). Other species were identified by genital (males) and spermathecal (females) morphology.

Trapping Biases Most catches of sandflies were made with CDC miniature light/suction traps installed just before darkness (1830 hours) and collected just after dawn (0630 hours). The principal sites were in chicken sheds. To interpret the trapping data correctly we need to know what fraction of the sandfly population is caught in light traps. Three ad hoc experiments investigated sampling biases. The first compared the sex ratio and abundance of flies caught in paired traps set with and without light bulbs in the same shed. The second compared the sex ratio and abundance of flies caught in the first (1830-0030 hours) and second (0030~)630 hours) halves of the night in two sheds. In the third, we released a known number of males and unfed females into a shed which had been sheathed in sandfly-proof netting, and recorded numbers of males and females recaptured on five successive nights.

Timing of the Study

Mark-Release-Recapture Experiments

All fieldwork was carried out between June and September 1988 and 1989, that is, during the dry season.

Allexperiments were carried out with wild-caught adult males and females. Females were allowed to engorge on an anaesthetized hamster, Mesocricetus

Dye et al.: Communication among sandflies auratus, before we marked them with fluorescent powders of five different cotours (Industrial Colours, London). The technique marked 100% of flies, and marked males and females caged in the laboratory for several days did not lose their colours. We released about 3600 males and 1200 females in three pairs of sheds in Cearfi (C2/C3) and Pingo d'Agua (Ch/Be and Mn/Jo). Flies were released in five rounds at Cearfi and four rounds at Pingo d'Agua, in the first round, feeding took place late in the afternoon, releases were made at dusk and recaptures began on a subsequent night. On all other rounds, flies were fed late in the evening and released at dawn the following morning so that recaptures could begin on the day of release. On all rounds, recaptures continued until no further marked flies were recovered, about seven consecutive nights.

Aggregation Experiments The above mark-recapture experiments were also used to investigate aggregation by comparing the numbers of unmarked males and females caught on the nights of and preceding release. These experiments were followed by a further series of seven releases at C2, each of 200 unmarked males, carried out over 14 days. In a third experinaent, flies were attracted to CDC light traps placed in a pair of newly constructed sheds sited 4 m apart, about 10 m and 13 m from the two nearest resident sheds at Pingo d'Agua. Below each trap a single chicken was caged, inside sandfly-proof netting, with or without 120 male or female flies. Experiments were carried out every second night, and each shed was alternately used as 'control' (C) or 'experiment' (E).

ColonizationExperiments Four new sheds were constructed at Ra, R1, Ma and Sa, sited about 3 m from the resident 'old' sheds. Each new shed permanently housed three or four chickens. At Ma and Sa, new sheds were introduced 4 weeks after the old sheds had been sprayed with D D T (75% wettable powder administered with a Hudson sprayer). At Ra and RI, the old sheds were not treated. Using a single light trap in each shed, catches were made on nights 1, 3, 5, 8, 11 and t4.

ComparativeSurvey In a comparative survey of 16 sheds, we recorded numbers of males and females, and their parous

185

rates, taken in five catches made every 3 or 4 days. Seven of the sheds had been treated with DDT. Where paired comparisons between sheds are made, the paired sheds were nearest neighbours, excepting Mr/El (separated by a DDT-treated shed containing no sandflies) and C2/C3 (C2 was flanked by a pig shed).

RESULTS

TrappingEfficiency Four examples illustrate the interactions between light trap efficiency and sandfly behaviour. The first compares catches made by pairs of CDC traps installed in the same shed, one with and the other without a light bulb. Over 13 nights in 11 sheds, the ratio of males:females caught with and without light were 348:254 and 107:43, respectively. Not surprisingly, the traps with a bulb caught more flies, but the light also changed the sex ratio by attracting relatively more females ()~2=9'15, P<0.01). Presumably, males are caught relatively easily without light because they are more active in the shed. The second example shows that males are accessible to light traps for a greater part of the night. Catching in two sheds between 1830-0030 hours and 00304)630 hours, we took a significantly greater fraction of females (94%) than males (72%) in the first half of the night (;(2= 19.99, N=369, P<0.001). Third, in three cases, more than 71% of males, but fewer than 22% of females, were recaptured during the first night after release in the sandfly-proofed shed (Table 1). Finally, data from the comparative survey revealed that much of the apparent variation in feeding success was explained by how far light traps are placed from hosts, which is often constrained by the shape and size of the shed (least squares linear regression of proportion of females caught blood-fed against trap-host distance had a significant negative slope, r2=0'63, t = 5-62, df=7, P < 0-001).

Mark-Release-Recapture Experiments In mark recapture experiments, 15.7% of males were recaptured, 2.7% in a foreign shed ('away'). Figure 1 shows the cumulative proportion of males recaptured away on different days after release. The line continues to increase in height so long as the rate of discovery of foreign sheds exceeds the cumulative average rate of return to the site of release.

Animal Behaviour, 42, 2

186

Table 1. Recaptures of males and females in an enclosed chicken shed Number recaptured (%) Number released Males

Females

Colour

200

100

Unmarked

200

100

Unmarked

162

92

203

104

Red Green

Day of recapture

Males

Females

1 25 1 2-5 1 2-5 1 2-3

143 (71.5) 6 157 (78.5) 5 l 18 (72.8) 1 55 (27.1) 1

20 (20.0) 4 7 (7.0) 6 20 (21.7) 3 5 (4.8) 0

0'2

0'15

3

t~

u

o

-g, 0.1

o

a.

5 ,~ 0"05

-I I

0

I

I

I

2 3 4 Time after release (days]

I

I

5

Figure 1. The cumulative proportion of males captured in a foreign shed ('away') with time since release.

Apparently, an equilibrium is reached after only 2 days. The rate of discovery of foreign sheds is probably less than the rate at which males leave the site of release. Thus, Fig. 1 can be interpreted as giving an upper limit to the time spent in sheds before leaving. Given that males are confronted by a light trap (installed at dusk) as soon as they become active on the day of release, and assuming that the majority of marked flies behave normally, we infer that the majority of males spend no longer than two nights at a time in a shed. The flies caught away had travelled distances of 20, 85, 200 and 700m, generally to the nearest

I

0

5

I

I

I

I

I0 15 20 25 Square root distance (m)

I

30

35

Figure 2. The percentage of males captured in a foreign shed (ln scale) versus the distance to that shed (square root scale). The triangle is set on the Y-axis at in 50%, the result expected when two light traps are placed side by side in the same shed. alternative shed. The relationship between the percentage of recaptures made away from the point of release (In scale) and distance moved (square root scale) appears roughly linear (Fig. 2). The regression line (slope -0"16) has an intercept on the Y-axis (4.1) which is very close to the expected In 50% (3.91). The ovarian condition of females recaptured in sheds on different days after release in shown in Fig. 3. Most interestingly, 50% of all females recaptured parous and unengorged (presumably seeking blood

Dye et al.: Communication among sandflies

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~iTI~"

PQrous TTO

I.O

187

the ratio of females caught on release:control nights and the characteristic abundance of flies (males plus females) at sites (linear regression of ln[(E + 1)/ ( C + 1)] females on total flies; F4,12 3-63, P < 0-05). In other words, in the animal pens that have a larger resident fly population, it is harder for those extra flies released to attract more females. The equivalent regression for males was not significant (F4,12 = 0.9, P>0"05). The results of releasing unmarked males over seven nights in the second experiment were as follows: 390 males were recaptured on the nights of release, but only t 59 on control nights; freed males did not therefore leave the shed immediately. The females' response to the presence of extra males was slight and statistically insignificant in this experiment too (138 in total on nights of release, 109 on control nights; t = 1-36, df= 6, P > 0"05). By releasing males into these sheds we increased the male: female ratio from 1.46 to 2.83. The third experiment, carried out in newly constructed sheds at Pingo d'Agua, showed that males can attract both females and other males (Table IV). Though all four treatment totals are greater than controls, the only statistically significant results are for attraction by males (paired t-test, one-tailed with the H o that males do not increase the recruitment rate of other flies; females: t = 6.81, df= 3, P < 0.005; males t =2-69, df= 3, P<0"05). From Table IV, we estimate that males attracted 1-53 times as many females as males, but this ratio is not significantly greater than one (Z2 = 1.89, P > 0.05). The controls in this experiment may be taken to represent the first night's recruitment to a new shed; as in the colonization experiments described below, males caught on the first night were somewhat more abundant than females in traps (79:50), but not significantly ~-

O'8

0'6

g 0.4 o

0.2

0-1

I [-2

I 2-3

[ 3-4

I 4--5

I 5-6

Time (doys)

Figure 3. The cumulative proportions of marked females recaptured in three ovarian states on various days after release: engorged or partially fed ([Ib/III), gravid or semigravid (IV/V), and unfed parous (IIa). 'Days after release' overlap on the abscissa because data from morning and

evening releases have been combined. Thus, ' ~ 3 ' days includes flies recaptured in the interval 4840 h after release. having laid eggs) were collected by day 3. Thus the median interval between blood meals (and hence ovipositions) was about 3 days. Notwithstanding the high rate of recovery in the mark-recapture experiments, marking with fluorescent powders increased the mortality rate of flies (Table II). The death rates of marked males and females in traps were about twice as high as those of unmarked flies (females: Zz=25-7; males: Z2= 129.9; P<0.001). However, the death rate of marked, recaptured males did not change with the age of the marked fly: for flies that had been marked for 1,2, 3 and 4 + days the death rates in traps were 56.8, 58.7, 55.9 and 52.9%, respectively.

Aggregation Experiments In the first experiment with releases of marked flies, we chose a priori to use as controls the numbers of unmarked males and females recaptured at Cear/t and Pingo d'Agua on the nights preceding release (Table III). Although more males and females were caught on the nights following releases, differences overall were not significant (paired t-test: females: t = 1.68, df= 17, P>0.05; males: t = 1.12, df= 17, P > 0.05). There was an inverse relationship between

SO.

Colonization Experiments Figure 4 shows the rate of colonization of the new sheds by males and females. Clearly, populations at the DDT-treated sites had been diminished outside as well as inside the old sheds: flies had been killed, or had left the area. Recruitment to the treated sites was such that flies became equally abundant at both pairs of sites within 2 weeks. In examining the temporal change in sex ratio, we make two comparisons using two different controls (Fig. 5). During days 3-8 of colonization, the proportions of females are larger than those

Animal Behaviour, 42, 2

188

Table IL Death rates of marked and unmarked flies in traps Females Dead

Live

7 7 10 11

%Dead

Dead

7 3 4 2

50 70 71 85

60 72 76 45

35

16

69

253

190

57

511

1022

33

650

1593

29

Pink Green Red Yellow ~marked ~unmarked

Males Live

%Dead

32 52 74 31

65 58 51 59

Table IlL Unmarked males and females captured on nights of and preceding release of marked flies No. of males caught

No. of females caught Site

Released

Night of release

Preceding night

Nights with no releases

Released

Night of release

Preceding night

Nights with no releases

C2 C3 Ch Be Jo Mn

203 158 154 108 168 203

32 57 130 193 35 52

13 14 38 148 30 112

9.1 5.9 9.0 39,9 ll.l 19.8

251 361 386 309 466 682

65 71 112 218 107 47

31 10 39 210 62 90

15.4 7.8 5.8 55.3 23.3 31.4

994

499

355

2455

620

491

Each integer (except totals in the bottom row) is the sum of three replicates. Table IV. Captures of flies attracted by males and females caged with a single chicken Males Experiment

Females Control

120 Females attracting E 57 E 7 E 8 C 6 C 2 C 7 87 120Males attracting E 60 E 19 C 55 C 9 143

Experiment

Control

11 0 ll 5 3 3

14 8 13 2 9 35

2 2 8 5 6 3

33

81

26

11 3 30 2

24 36 36 18

8 10 6 0

46

114

24

E and C indicate whether the left-hand shed was used as the experiment or the control.

observed in controls, and these p r o p o r t i o n s decline through time towards those o f the controls (least squares linear regression weighted by sample size: combined data, t = 2.66, df= 5, P < 0-05). A l t h o u g h sex ratios o n day 1 were male-biased, and the proportion o f females appears to have a m a x i m u m between days 3 and 5, these curves have no significant quadratic c o m p o n e n t . A t a minimum, these results provide evidence that females colonize sites faster t h a n males during the first few days o f recruitment.

Comparative Survey In c o m p a r a b l e light trap catches made at six pairs o f chicken sheds, the p r o p o r t i o n o f females/ total was significantly lower in sheds that had more hosts (paired t-test: t = 2.68, dr= 6, P < 0.025; Table V). Evidently, males were m o r e responsive to changes in host abundance t h a n females (paired comparisons o f numbers o f males (one-tailed): t = 1-77, d f = l l , P-~0-05; females: t = 0 . 7 7 , d f = l l , P>0-05).

Dye et al.: Communication among sandflies 6

0.8

4-

0

189

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2

4

"",',.d,/

6

!

8

I

I0

I_

12

I

14

0

I

5

I0

Time (doys)

Figure 4. The number of flies colonizing new sheds erected at homesteads where the old sheds had (solid lines, sites Ma and Sa) or had not (dotted lines, sites Ra and R1) been treated with DDT. [~, 9 females; A, (>: males. DISCUSSION

Animal Pens as Foci of Activity T h e simplest a s s u m p t i o n a b o u t dispersal is that each fly moves r a n d o m l y in one d i m e n s i o n (backwards a n d forwards between a n i m a l pens) at the same rate. In this case, log recaptures would be linearly related to the square o f distance (Taylor t978). In our r e a r , r e c a p t u r e experiments, the decline in log p r o p o r t i o n o f males c a p t u r e d 'away' was linear with the square root of distance between sheds (Fig. 2). Put simply, this implies that, when male Lu. longipalpis have to move further to get to the nearest alternative shed, their capabilities are rather more t h a n those of the B r o w n i a n fly. Rate of dispersal is thus a variable, n o t a constant. T h a t dispersing males do increase distance m o v e d per unit time is evidence that, a m o n g o t h e r possible loci o f male activity, chicken sheds are relatively important. This inference is not altogether trivial: it c a n n o t be m a d e simply from light trap records which show t h a t m a n y more flies can be c a p t u r e d inside sheds t h a n outside. A light trap h a n g i n g in a tree will certainly n o t catch flies with the same efficiency as one placed in a n animal pen. The four observations o n t r a p p i n g efficiency constitute ample warning

15

Time (days)

Figure 5. The changing sex ratio of flies during colonization of the four new sheds. Solid lines compare the proportion of females in all new sheds with the mean (horizontal line=518/1928) obtained in the preceding month's catches in old sheds. Broken lines compare, for sites Ra and R1, the proportion of females during colonization of new sheds with the mean proportion of females collected at the same time in old sheds (horizontal line = 159/430). Vertical bars indicate binomial standard errors (one arm only for clarity).

TaMe V. Sex ratio and host abundance (number of fowl) in a comparative survey of animal pens

Site

Number of hosts

Males

Females

Males/ females

Ma 1988 Ra

17 9

898 584

616 413

1.46 1-41

Ma 1989 Ra

18 12

669 127

242 66

2-76 1-92

Jo Mn

7 2

737 959

335 625

2.20 1.53

Be Ch

16 1

1821 424

1349 615

1.35 0"69

C2 C3

12 4

572 336

331 256

1.73 1'31

RI Ri

II 3

346 148

118 130

2.93 1'14

Mr E1

25 16

266 54

100 18

2-66 3.00

Animal Behaviour, 42, 2

190 against making between samples.

unstandardized

comparisons

Aggregation at New Host Sites

Collecting together the evidence from comparative observations on animal pens and field experiments, we propose the following scheme for the dynamics of host discovery by Lu. longipalpis. Catches in a new shed are made by arresting passing flies; it seems unlikely that males and females in a neighbouring shed, already in the vicinity of numerous hosts and other flies, could detect the same kind of signals produced at least several metres away. The first males colonizing a site increase the rate at which both females and other males are recruited, and the number of flies increases faster than linearly with time (Fig. 4). The aggregation of males (Table IV) is a phenomenon that has not previously been demonstrated, even in laboratory experiments. Males, however, are exceptionally attractive to females. Two separate experiments (Fig. 5, Table IV) provided evidence (neither alone being statistically significant) that the sex ratio changes over the first few nights of recruitment, favouring first males then females. Two processes would help to create early male-biased sex ratios. First, males are easier to catch than females, possibly because they are more active in sheds; this is the inference to be drawn from paired catches in CDC traps with and without light. Second, on any one night there are likely to be more males than females on the wing. Mark-recapture experiments indicate that males find new sheds (Fig. 1) faster than females (Fig. 3). Given that females have to digest a blood meal and find oviposition sites, whereas males do not, this is a predictable result. From a change in sex ratio during early colonization arises the hypothesis, yet to be properly tested, that males attract females most successfully (per caput) at some intermediate group size. Presumably, the attraction of females, and other males, is mediated wholly or in part by male pheromones produced in swarms (Ward et al. 1988; Morton & Ward 1989, 1990; Lane et al. 1990). The structure of these swarms is reflected in catches of flies aspirated off the backs of animals. For example, catching for 1 h after dusk off two pigs and six fowl at site R1 we obtained 81 males and five females on one night, 129 males and three females on a second night and 27 males and one female on a third.

As the early recruitment rate of females is higher than that of males, the supply of females is exhausted first; thereafter the sex ratio becomes less femalebiased, reaching an equilibrium which usually yields more males than females in animal pens (Fig. 5, Table V). Males arriving at a shed supporting a population near to equilibrium cannot expect a proportional increase in female recruitment rate (Table III). In sum, our current, parsimonious view of male and female recruitment to new host sites (as perceived in light traps) could be represented as two logistic curves, with that for females having a higher rate of increase but a lower carrying capacity. Natural Sex Ratio Variation

The paired comparisons made in Table V restrict spatial scale (20-200 m) and suggest a behavioural explanation (rather than an ecological one) for the association between sex ratio and the number of hosts. If the ratio of male:female survival rates is invariable between sites (we have no evidence of insecticide use at these sites), the data imply that males and females are attracted to sites at different rates: natural site-to-site variation in the sex ratio is evidently generated by males who are more responsive than females to variation in the number of available hosts. This is fully expected in animal pens which support fly populations in the vicinity of equilibrium; female recruitment rate approaches a maximum in the presence of relatively few males. Sylvatic Versus Domestic Environments

Thornhill & Alcock (1983, Chapter 6) posed the general question of why males that use a sexual signal are found in groups. Our experiments suggest that males emitting pheromones will be more successful in attracting mates (per caput) when they are members of small rather than large groups. Such small groups of males and females on hosts may be more frequent in sylvatic than domestic environments: little is known about the ecology ofLu. longipalpis in forest, but it is improbable that wild vertebrate hosts, whatever they are, live in such large, sedentary and accessible groups as domestic animals. We suspect that finding hosts and mates is a greater problem in the forest, and the equilibria characteristic of domestic animal pens are a rarity in that habitat.

Dye et al.: Communication among sandflies The problem facing domestic male Lu. longipalpis visiting an animal pen at dusk, after other males have assembled, is an increase in the number of males per female rather than the reverse (Tables III, V). When female recruitment rate is somewhere near its maximum, emission of a sex/aggregation pheromone will be of doubtful value. One possibility is that arriving males, attracted partly by other males (Table V), have the option of remaining silent, taking advantage of residents that are already signalling (see ThornhiU & Alcock 1983 for examples among other insect species). Another option for these late males is to leave the animal pen and go elsewhere. R. D. W a r d and colleagues (unpublished data) have recently found that high concentrations of pheromones can repel males. However, exactly how Lu. longipalpis males use pheromones under these conditions, whether they take the best possible options for mate finding, and how in general they respond to the selective pressures presented by domestication, remain to be investigated. The idea that males courting in chicken sheds inhabit a world of diminishing returns has a practical implication. We suggest that pheromone traps ( M o r t o n & Ward 1989, 1990), whose aim would be to trap-out the female population, could not work by markedly increasing the recruitment rate of females to a shed. The efficient catching and killing (cf. impact of D D T , Fig. 4) or disorientation of flies already present in sheds would be the key to success in sandfly control. ACKNOWLEDGMENTS The work was generously funded by The Wellcome Trust. We thank Roberto Baia, Iorlando Barata, Enricke Bouma and Paulo Cruz for excellent technical help. REFERENCES Bornemissza, G. F. 1964. Specificity of male sex attractants in some Australian scorpion flies. Nature, Lond., 203, 786~787. Deane, L. M. &Deane, M. P. 1957. Observagoes sobre abrigos e criadouros de fleb6tomos no noroeste do Estado do Cearfi. Revta Bras. Mal. Doen9. trop., 9, 225-246. Deane, L. M. &Deane, M. P. 1962. Visceral leishmaniasis in Brazil. Rev. Inst. Med. trop. S. Paulo, 4, 198512. Detinova, T. S. 1962. Age-grouping methods in Diptera of medical importance with special reference to some vectors of malaria. Monograph Series, WHO, No. 47, 1-216.

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