Smith, Alec

English

Digital Library of the Tanzania Health Community

028615 
028615 275 
xﬁc^ ^ 
8615 
A  VERANDAH-TRAP  HUT  FOB  STUDYING  THE  HOUSE-FREQUENTING 
HABITS OF MOSQUITOS AND  FOB  ASSESSING INSECTICIDES. 
II.￿THE  EFFECT OF DIOHLOEVOS (DDVP) ON EGRESS AND 
MORTALITY OF ANOPHELES GAMBIAE GILES 
AND MANSONIA  UNIFORMI8 (THEO.) 
I  ENTERING  NATURALLY 
; 
^ 
By ALEC SMITH 
Tropical Pesticides Research Institute,  Arusha, Tanzania. 
(Received llth November 1964} 
A  verandah-trap hut, designed to enable assessment  to  be  made  of  the numbers 
of mosquitos  that  leave an  experimental  hut  through  the open  eaves, together with 
studies on  the egress from  it of Anopheles gambiae Giles and Mansonia  uniformis 
(Theo.) have  been. described by Smith (1965).  It was  found  that, in the Umbugwe 
area of Tanzania, A. gambiae  rarely  left an  untreated hut  by  the open eaves when 
window traps were available for egress.  M. uniformis, on the other hand, left 
largely by  the  eaves and  was  caught  in the  verandah  traps. 
Methods 
A dichlorvos  (DDVP) dispenser was suspended in  the middle of one hut, 
half-way between the ridge and the eaves, in order to study the effects  of the 
insecticide both on  the..egress and  on  the  mortality  of A. gambiae and M.  uniformis 
entering naturally.  A  second (untreated) hut was used as a  control.  The same 
methods of  counting  and collection  were used as  described  in  Part I,  with 
additional observations to determine mortalities.  Dead  mosquitos were collected 
on the floor indoors and in the verandah and window  traps.  Delayed mortalities 
over a period of 24 hours were determined for mosquitos caught alive in the 
window and verandah traps.  Experimental-hut assessments of kill  inflicted by 
dichlorvos  were complicated by the mode of  action  of the  insecticide;  it  is 
brought  into contact  with  the  mosquito  as a  vapour  that  diffuses from  a  dispenser, 
and thus mosquitos confined in window traps  are  subjected to the fumigant 
action of the insecticide  diffusing through the funnels of cotton netting.  Ad- 
ditional studies were therefore made to assess the fumigant effect of dichlorvos 
in the window traps and in the verandah traps 
The results  are based on daily  counts and collections,  from. 2nd March to 
30th April 1964,  from. the hut treated with dichlorvos and from the untreated 
control hut.  The  dichlorvos was  dispensed from a Ciba XI  type dispenser opened 
on 2nd March. 
Results 
The egress and  mortality of mosquitos entering treated and  untreated huts 
The numbers of living and dead mosquitos counted or  collected  daily from 
2nd March to 30th April 1964,  and their distribution within the verandah-trap 
huts,  are shown in Table I,  and the egress from the huts, calculated from the 
data in Table I,  is shown in Table II.  The numbers of mosquitos caught in 
the two  operational verandah  traps, as shown  in Table  I, are doubled in Table H ^3 
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to  give  the  estimated  total  number’  leaving by the  eaves.  The window-trap 
catches are not doubled because the windows not leading to the two operational 
traps were kept closed. 
The egress  of  A.  gambiae  and M. uniformis  into  the  window traps,  and 
through the eaves of the  hut treated with dichlorvos,  is  shown in  Table  II, 
with results from the untreated control hut for comparison. 
The results from the untreated hut show that 48 per cent.  of A. gcwnhiae 
in  all  gonotrophic  stages  left  the  hut  each night,  with 91  per  cent.  of  the 
egress by the window traps and 9 per cent. by the eaves.  With M. uniformis, 
on the other hand, 94 per cent.  left the hut each night, with 39 per cent.  of 
the egress through the window  traps and 61 per cent. through the eaves.  When 
recently fed mosquitos only were considered,  it  was found that 19 per cent.  of 
A. gambiae left the same night after feeding, with 82 per cent.  of the egress 
by the window traps and 18 per cent. by the eaves; with M. uniformis, 94 per 
cent. left the same night after feeding, with 44 per cent. of the egress occurring 
by the window traps and 56 per cent. by the eaves. 
The  results from the dichlorvos-treated hut, however, show that 27 per cent. 
of  A. gambiae in  all  gonotrophic stages  left  the hut’each night,  with 62 per 
cent. of the egress by the window traps and 38 per cent. by the eaves.  With 
M. uniformis,  on the other hand,  88 per cent.  left  the hut each night,  with 
41 per cent.  of the egress through the window traps and 59 per cent.  through 
the eaves.  When recently fed mosquitos only were considered,  it  was found 
that 14 per cent. of A. gamhiae left the same night after feeding, with 50 per 
cent.  of the egress by the window traps and 50 per cent.  by the eaves; with 
M. uniformis, 84 per cent. left the same night after feeding,  with 49 per cent. 
of the egress by the window traps and 51 per cent.  by the eaves. 
A  smaller proportion (27%) of the number of A.  gambiae present left  the 
treated hufc as compared with the untreated one (48%); but of those that did 
so a higher proportion (38%) left  the treated hut by the eaves than left  the 
untreated hut by this route (9%).  Although a smaller proportion (88%) of the 
number of M. uniformis  present also  left  the  treated hut  as  compared with 
the untreated one (94%), about the same proportion (59%) left the treated hut 
by the eaves as left the -untreated hut by this route (61%). 
Mortalities were calculated from the basic data in Table  I.  The equation 
for assessing ’  over-all mortality  ’  in a window-trap hut is as follows: 
,,  ..  , 
-
  100 (0 +  F)  Over-all mortality == ￿￿￿￿p￿’-, 
where D  =  Number dead in the window trap, 
.
 
F  == Number dead on the floor, 
T =  Total in the window trap. 
When egress from the eaves is  taken into  account the  equation becomes: 
￿
 
￿  , ,.,  100 (P  4- 2V + F}  Over-all mortality == ￿￿￿:￿;,-,-?￿￿p,￿￿-,  -  T + 2M  +  -P 
where D  =  Number dead in the two window traps, 
F  == Number dead on the floor, 
T =  Total in the two window traps, 
V  =  Number dead in the two verandah traps, 
M  =  Total in the two verandah traps. 
Calculation of the over-all mortality of A. gambiae in the treated verandah- 
trap hut, for example, is  as follows: ALEC SMITH 
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P VERANDAH-TRAP  HUT  FOE  MOSQUXTOS  279 
n  n  ,  T,  100(26 + (3 x 15) + 182)  Over-all mortality =  ￿￿￿￿￿^￿￿,-, 
￿
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_
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~ 427 
== 56 per cent. 
It  should  be noted  that  the  number of  mosquitos resting  indoors  is  not 
brought into the equation.  This is  because of  the  cyclical  pattern  of  entry, 
resting and egress; that is,  mosquitos resting in a hut one day are the dead or 
surviving mosquitos of the nest day.  The equation assesses .kill  in a practical 
way by -taking into account mosquitos that have died indoors,’ or died within 
24 hours,  after leaving the treated hut, and by counting as survivors only those 
mosquitos that have left the treated hut and survived a further 24 hours. 
The over-all mortalities including and excluding the fraction leaving by the 
eaves are shown in Table III.  The results show that over-all mortalities in the 
treated hut were lower when the fraction leaving the eaves was included,  and 
also that the over-all mortalities  of A. gamUae were higher than those of M. 
umformis. 
TABLE III.  Over-all mo-riaHties  (%) of A. gambiae and M. uniformis in 
a dichlorvos-treaied verandah-trap ’hut 
A. gambiae  M. uniformis 
Treated hut  Untreated hut  Treated hut  Untreated hut 
Including  fraction  leaving  by  the  eaves  56  3  34  2 
Excluding  fraction leaving  by  the  eaves  63  3  48  5 
The fumigant effect of  dichlorvos on mosquitos confined w  the traps 
.Experimental-hut assessments of kill,  where the insecticide has a fumigant 
as well as a.  contact action,  are complicated by the mortalities inflicted by the 
fumigant action  of  the insecticide on mosquitos confined in the window fcrap- 
While a polythene  funnel  fitted  to  a window trap greatly  reduced mortality 
attributable to the fumigant action of dichlorvos on mosquitos confined in the 
traps,  it  seems likely  that there may be a higher concentration  of  dichlorvos 
in  the atmosphere in  huts in which ventilation through the window trap has 
been  greatly reduced by the use of a  plastic funnel, and that this might account 
for the higher over-all mortalities in such huts (Smith,  1963a). 
The fumigant effect of dichlorvos in different parts of a verandah-trap hut 
was studied by observing the mortalities of caged mosquitos placed in the hut, 
in  the  window trap  and in  the  verandah  trap.  Mortalities  24  hours  after 
exposure were observed in wild-caught blood-fed individuals of A. gamhiae that 
were confined  in  cages  of  cotton mosquito  netting  measuring 6x3x  3  in. 
Twenty mosquitos were placed in a cage,  and the results,  based on 40 to 480 
mosquitos for  each  test,  are  summarised in  Table IV;  it  can be  seen that 
mortalities were highest indoors,  lower in the window traps and lowest in the 
verandah traps.  There were no  mortalities during  this period in caged mogquitos 
placed in similar positions in the untreated hut.  The results from the treated 
hut are of value for comparative purposes, but the mortalities  are  excessively 
high compared  with. those of mosquitos entering naturally.  Although close com- 
parisons are not possible,  due to wide differences between the two techniques, 
it  seems likely that the higher mortalities  in  treated huts were partly due to 
the restricted ventilation in -the  cages and also to the dichlorvos vapour being 
absorbed by the cotton netting. 
(L 2263)  0 ALEC SMITH 
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a VERANDAH-TRAP HUT  FOB  MOSQUITOS  281 
The concentrations of dichlorvos vapour in different parts of the verandah- 
trap  hut.  were  compared  by determining,  by  an  anticholinesterase  method 
(Webley & McKone,. 1968),  the amount of dichlorvos absorbed by niter papers 
suspended there for a known  length of time.  The  results of a  total of 59 chemical 
analyses are summarised in Table V, and those of esperiments 1 and 2 show 
that,  on  average,  the  verandah  traps  contained  less  than  a  quarter  of  the 
concentration of dichlorvos in the window traps. 
Discussion 
.
 
.
  The results show that,  over a period of two months,  a Ciba XI dispenser 
gave average  over-all .mortalities  of .56  per  cent.  for  A.  gambiae  and 34 per 
cent.  for  M. uniformis.  Over a period  of 24 hours,  a greater  proportion  of 
A. gambiae that entered the dichlorvos-treated hut were killed than left,  with 
a greater proportion (38%) of the egress from the treated hut compared with 
the  untreated  hut  (9%)  occurring  through  the  eaves.  .This  finding may be 
interpreted’ as indicating that there was an earlier egress from the treated than 
from the untreated .hut.  The 
’
 
explanation, offered, for  the greater  use  of the 
eaves for egress from the treated hut  is that A: gambiae, exposed to a  repellent 
insecticide,  prematurely leaves  the  hut at night  and is-  thus  more’ prone  to 
leave by the open eaves than by the window trap since there is no directional 
source of light before dawn to guide it towards the window (Smith,  19686). 
The effect of dichlorvos’ on M.  ’uniformis was somewhat different,  as might 
be expected from  its  habits  (Smith,  1965).  The lower  over-all  mortality of 
34 per cent. may have been due to its short exposure to the insecticide,  since 
it normally left a hut during the same night as  it  took a blood-meal,  unlike 
A. gambiae, most of which normally rest indoors for  at least 24 hours  after 
feeding.  Any differences in intrinsic susceptibility to the insecticide could also 
have  affected  the  kill.  There was no effect  of  dichlorvos  on the  egress  of 
M. uniformis  through the  eaves,  which may in  some way be related  to  its 
normally greater use of the eaves for egress than A. gambiae (Table II). 
Summary 
Assessment of the  effects  of diohlorvos (DDVP), released from a Ciba XJ 
dispenser,  on  females  of  Anopheles  gambiae  Giles  and  Mansonia uniformis 
(Theo.) entering  a verandah-trap hut in the Umbugwe area of Tanzania was 
made  over a  period of two months in 1964.  Of the numbers of A. gambiae that 
entered one treated and one untreated hut, 27 per cent.  of those entering the 
hut treated with diehlorvos and 48  per cent. of those entering the untreated hut 
left  again.  Of the numbers leaving  each hut,  38 per  cent.  left  through the 
eaves of the treated hut as compared with 9 per cent.  in the untreated one. 
In the case of M. uniformis, 88 per cent. of those entering the treated hut and 
94 per cent.  of those entering the untreated hut left again.  Of the numbers 
leaving each hut,  59  per  cent,  left  through the  eaves  of  the treated  hut as 
compared with 61 per cent. in the untreated one. 
Over-all mortalities were 56 per cent.  for A. gamhiae and 34 per cent.  for 
M. un-iformis when the eave-egress fraction from the treated hut was taken into 
account, compared with 62 per cent.  for A. gambiae and 43 per cent.  for M, 
uniformis when the  eave-egress  fraction was ignored. 
The results of bioassays and of chemical analyses showed that the problem 
of mortality from fumigation in  situ was considerably less in verandah traps 
than indoors or in window traps fitted with funnels of cotton netting. 
Acknowledgements 
I owe thanks to  Mr.  S.  Esozed for  the conscientious  supervision of mos- 
quito  collections  and to Mr.  C.  E.  McKone for  kindly making the chemical 282  ALBC SMITE 
determinations of dichlorvos in the atmosphere of the treated hut.  Thanks are 
also due to Mr. K.  S.  Hocking,  Director of the Tropical Pesticides Research 
Institute,  Arusha,  for  helpful comments and criticisms of the manuscript. 
References 
SMITH,  A.  (1963o).  Effects  of  dichlorvos  (DDVP) on the  behaviour  of  A. 
gambiae.￿Indian  J.  Malanol. 17 pp. 131-140. 
SMITE; A.  (19636).  Principles  in  assessment ot insecticides  by experimental 
huts.￿.Nature,  Lond. 198 no. 4876 pp. 171-173. 
SMITH, A. (1965).  A  verandah-trap hut  for studying the house-frequenting habits 
of  naosquitos  and  for  assessing  the  tosicity  of  insecticides.  I.￿A 
description of the verandah-trap hut and of studies on the  egress  of 
Anopheles  gambiae  Giles  and  Mansonia,  uniformis  (Theo.)  from  an 
untreated hut.￿Ml.  ent. iJ?es.  56 pp. 161-167. 
WEELEY,  D.  J". &  McKoNB, C. E- (1963).  The  estimation of dichlorvos vapour.￿ 
Trop. Pestic.  Res.  Inst.  Misc. Rep. No. 424,  7 pp., mulfcigraph. 
' 
Commonwealth Agricultural Bureaus, 1965 

A Verandah-Trap Hut for Studying the House-Frequenting Habits of Mosquitos and for Assessing Insecticides:The Effect of Dichlorvos(DDVP) on Egress and Mortality of Anopheles Gambiae Giles and Mansonia Uniformis (THEO.) Entering Naturaly

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A Verandah-Trap Hut for Studying the House-Frequenting Habits of Mosquitos and for Assessing Insecticides:The Effect of Dichlorvos(DDVP) on Egress and Mortality of Anopheles Gambiae Giles and Mansonia Uniformis (THEO.) Entering Naturaly

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