Light intensity measurement and the biting behaviour
of some sylvatic moqsuitoes of the Amazon basin (Diptera: Culicidae)
J. Derek Charlwood (') José Lopes (J)
Peter C . Whalley (3)
Abstract
The design of a lightmeter suitable for use at low light levels is described. Catches of mosquitoes biting man at sunset near Manaus Amazonas indicated that light intensity controls biting activity in crepuscular species to a very fine degree.
INTRODUCTION
It is w e l l known that the f l i g h t periodicities of many species of mosquito is governed by an endogenous 'circadian c l o c k ' (Jones et al., 1967, 1974; Nayar & Sauerman, 1971; Saunders, 1976). A c t i v i t y in such species may be affected by the changing light intensities at s u n s e t . The d i f f i c u l t i e s associated w i t h measuring the very l o w light intensities prevailing at these t i m e s mean that, in general, the degree of control of behaviour by light intensity is u n k n o w n .
This paper describes a very simple light meter suitable f o r use at l o w light i n t e n s i t i e s and ojves the results of a preliminary investi
gation of the biting behaviour of some Amazo
nian mosquitoes at s u n s e t .
METHODS
LIGHT MEASUREMENTS
Accurate measurement of l o w levels of illumination has f o r m e r l y required complicated e l e c t i o n i c equipment (Davies, 1975; Callahan, 1964; Lumsden, 1966). The expanding research in electronics has led t o the development of
more e f f i c i e n t phototransistors than have previously been available- A c c o r d i n g l y an instrument, suitable for the measurement of light ievels below O . I Lux, w a s designed and constructed using a photodarlington t r a n s i s t o r as the light sensitive e l e m e n t . Incident light
was detected by a 2N577 ( G . E . ) t r a n s i s t o r mounted in a diecast a l u m i n i u m box. The output f r o m t h i s w a s monitored by a d i g i t a l , battery operated, v o l t m e t e r . Twelve 1.5 V battries, stepped d o w n by a 15 V regulator, supplied t h e power to the t r a n s i s t o r . This en
sured that the supply to the t r a n s i s t o r remained constant during t h e t i m e w h e n the batteries began t o run d o w n . In practise the meter uses very little current so that the life of one set of battel ies is e f f e c t i v e l y equivalent to their shelf l i f e . No suitable light source w a s availa
ble to standardize the m e t e r . A p p r o x i m a t e comparisons w e r e made w i t h a ' M e t r a w a t t Lux meter, the most s e n s i t i v e light meter that could be o b t a i n e d . This gave c o n s i s t e n t results down to 2 . 5 Lux. The calibration curve of the phototransistor down t o t h i s is shown in f i g u r e I. There w a s a good c o r r e l a t i o n b e t w e e n volts and Lux ( r = 0.960, P = 7 0 . 0 0 1 ) . Output belov 1.0 Lux w a s checked using the principle of the inverse square l a w of light t r a n s m i s s i o n ( i e .
light intensity drops decreases by a square
••oot as the distance f r o m t h e source doubles) w i t h a standard light s o u r c e . Since the output of the t r a n s i s t o r is e f f e c t i v e l y linear w i t h l i g h t intensity it is possible to light readings f r o m the digital v o l t m e t e r to values o f O . I Lux-
Values
below 0.005 v o l t s w e r e considered to equate t o zero as this level approached the l i m i t s of accuracy of t h e v o l t m e t e r .( 1 ) — Sunnybank Farm, Bolsterstone, Sheffield S305ZL. Inglaterra.
( 2 ) — Instituto Nacional de Pesquisas da Amazônia, Manaus.
( 3 ) — I.E.T., Open University, Milton Keynes, Bucks, Inglaterra.
ACTA AMAZÔNICA 1 2 ( 1 ) : 61.64. 1982 — 61
BITING CATCHES
Using the lightmeter the biting behaviour of mosquitoes in secondary f o r e s t near the city of Manaus w a s studied at s u n s e t . Mos
quitoes w e r e collected as they came t o feed on the exposed i o w e r legs of human bait at the campus of the University of Amazonas. A f t e r synchronizing watches one c o l l e c t o r stationed himself approximately 30 m inside the f o r e s t and another c o l l e c t o r remained in the open ten
metres f r o m the f o r e s t edge. Insects w e r e caught in numbered t e s t tubes as they started to probe and the t i m e that they w e r e caught was noted. On alternate days t h e collectors changed places.
For light measurements the lightmeter w a s placed on the ground w i t h the light sensitive element facing the s k y . Readings w e r e taken at the start of every m i n u t e . A digital w a t c h was used to monitor the t i m e . Light measure
ments w e r e taken inside and outside of the forest on alternate days- Temperature and humidity w e r e measured using a w e t and dry
bulb t h e r m o m e t e r . Catches w e r e made on ten moonless occasions in October 1978 and on three occasions in May 1979.
RESULTS
In all collections the temperature w a s 2 5 ± 2°C and the relative h u m i d i t y was 75-90%.
The predominante species collected w a s Culex coronator D.&K. This species comprised
I -
- 2 -
- 3 , , , r
- 1 0 1 2
Log Lux
Fig. 1 — Calibration curve of the lightmeter derived with a Metrawatt Luxmeter. The dotted line gives the estimated Lux value obtained from a standard light
source and the inverse square law.
17.30 11.00 10.30 1000
H o u r s
Fig. 2 — Light intensity and numbers of mosquito biting man at the dge of and inside secondary forest near the city of Manaus. -".Numbers collected inside the forest. Light intensity inside the forest. Bars
indicate range of values from the ten colletctions.
approximately 7 5 % of all insects caught biting inside and outside the f o r e s t . Mansonia ama- zonensis (Theo) and M. titillans ( W l k ) together made up the bulk of the remainder of the insects caught. Small numbers of Psorophora albipes (Theo), P. f'erox (Humboltd) Anopheles mediopunctatus (Theo) A. oswoldoi Peryassu and three species of u n i d e n t i f i e d Culex w e r e also c o l l e c t e d - The t o t a l number of mosquitoes collected inside and outside the forest at sunset is s h o w n in Figure 2 w i t h t h e mean
light intensity obtained at these t i m e s . The peak of biting occurs approximately ten m i nutes earlier inside t h e f o r e s t than o u t s i d e . This is related t o the drop in l i g h t intensity w h i c h also occurs ten minutes earlier inside the f o r e s t .
A similar relationship b e t w e e n light in
tensity and biling activity of C. coronator collected on succesive days in the f o r e s t . The change in light level w a s f i v e minutes earlier on the f i r s t day than on the succeding d a y .
Figure 3 and 3a d e m o n s t r a t e that the peak of
biting also d i f f e r e d by five minutes in t h e t w o days. Peak biting occured at approximately 0.1 Lux on both days.
DISCUSSION
Flight activity of Culex quinquefasciatus Say, in the laboratory, is initiated at a set light intensity irrespective of the speed of a r t i f i c i a l 'dusk' to w h i c h the insects are sub
jected (Charlwood, 1976). Swarms of mos
quito also appear to be inaugurated at particu
lar threshold light intensities (Corbet, 1964) • The results of biting catches reported here also indicate that light intensity, rather than t i m e , acts as a cue f o r initiating activity in some
species of Neotropical m o s q u i t o . Undoubtedly the response to these cues is under t h e overall control of the circadian c l o c k .
The fact that peak biting activity took place at levels of i l l u m i n a t i o n b e l o w that of f u l l
moonlight (calculated as 0.21 Lux, B i d l n g - mayer. 1964) means that lunar phase is likely to affect biting behaviour in these species- This aspect of t h e i r behaviour remains t o be i n v e s t i g a t e d .
ACKNOWLEDGEMENTS
We w i s h t o acknowledge t h e f o l l o w i n g persons f o r t h e i r assistance in the f i s l d : A . M . Soares and A . V . P . Nápoles.
19
— o
O
O
f-10 O
o
17.30 17 45 U 00 18 15 18 30
H o u r s
I
V \
2 3 . V . H 7 ?
18 00 181S
H o u r s
Ftg. 3 — Light intensity and number of Culex coronator collected biting man in secondary forest near the city of Manaus on two succesive days: Light intensity derived from the "Metrawatt" Luxmeter; m m m m Light in
tensity derived from the lightmeter described.
Light — 63
RESUMO
É descrito um luximetro apropriado para uso em baixo nível de luz. Mosquitos picando o homem captu- rados ao entardecer, em uma área próxima a Manaus, Amazonas, indicaram que a intensidade de luz contro- la a atividade de picada em um grau muito sensível, nas espécies crepusculares.
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279p.
(Aceito para publicação em 10/06/80)
