Smith, A

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A  VEEANDAH-TEAP  HUT  FOE  STUDYING  THE  HOUSE-FREQUENTING 
HABITS OP MOSQUITOS AND FOE ASSESSING INSECTICIDES. 
I￿A  DESCEIPTION OF THE  VEEANDAH-TEAP HUT  AND OF 
STUDIES ON THE EGEESS OF ANOPHELES GAMBIAE 
GILES AND MAN80NIA  UNIFORMIS (THEO.) FEOM  AN 
UNTEEATED HUT 
By ALEC SMITH 
Tropical Pesticides Research Institute, Arusha, Tanzania 
(Plate IV) 
{Received llth November 1964) 
"  Experimental huts of the window-trap type ", as they are generally called, 
have  been  used  continually  in  East  Africa  for  15  years  for  studying  the 
house-frequenting habits of Anopheles gambiae Giles and  for assessing the excito- 
repeU&nt and  toxic effects of insecticides on  them. 
In recent years, the Tropical Pesticides Eesearch Institute has used window- 
trap huts extensively in East Africa for screening candidate insecticides, as part 
of the World Health Organization programme  for developing new  insecticides for 
use against insects resistant to those in common  use.  Analysis of a  vast amount 
of  entomological data accumulated from 20 window-trap huts each year,  with 
different  insecticides,  has more closely  defined the value and disadvantages of 
these huts as a research tool  for assessment  of insecticides.  These  findings, taken 
together with supplementary observations using specially constructed huts, have 
shown  that the window-trap hut  is quite satisfactory for assessment of mortalities 
inflicted, on A. gambiae by non-repellent insecticides because the trap is efficient, 
that is to say, only a few mosquitos leave through the eaves and escape assess- 
ment.  The window trap is,  however, less  efficient with A. gambiae when the 
insecticide is repellent, because when  this is so,  a greater proportion of the total 
numbers leaving from the hut does so through the eaves and  is unaccounted for. 
The explanation offered for the greater proportion leaving through the eaves is 
that mosquitos,  repelled by insecticides,  attempt to leave the hut earlier than 
they would  otherwise have  done, at night, and, since there  is no  directional source 
of light before dawn to guide them towards the window, they are less prone to 
leave by  it.  Another disadvantage of the window-trap hut  is that  it is unsuitable 
for assessing other domestic species with different habits; for example, Mansonia 
uniformis (Theo.) leaves even an  untreated window-trap hut largely by  the eaves. 
An  account  of the work  just described  is given, with  references, in a  recent general 
review of experimental hut techniques used in the study of insecticides in East 
Africa (Smith,  1964). 
There are good reasons for reconsidering the design of the experimental hut 
used in assessing the toxic and excito-repellent effects of insecticides on  domestic 
mosquitos.  First,  a more sophisticated trap hut is  required to supplement, if 
not replace,  the window-trap hut for assessing the effects of insecticides on A. 
gambiae.  Secondly, a trap hut  is required which can  be used to assess the effects 
of  insecticides  on any  species  of  mosquito  that  enters  it,  and  its  use may 
therefore have  world-wide  as well as  local application. 
(L 217&)  r 162  ALEC SMITH 
When considering  the  design anew,  it  was borne  in  mind  that  (a)  the 
unaccounted fraction was  that which  left by  the eaves, (b) the technique of inter- 
mittently surrounding a window-trap hut with a cage of mosquito-netting as a 
means of  assessing  the numbers of  mosquitos leaving through the eaves was 
found impracticable for long-terra routine work as it required,  collections to be 
made  at night,  (c) eave traps fitted to experimental huts may  be used, but  they 
are liable  to be contaminated easily by insecticides and they present problems 
in assessing mortalities due  to fumigation  in situ (by an  eave  trap  is meant  a  trap 
fitted under the eave, not a trap fitted in a hole near the top of a wall),  (d) the 
new  trap hut should be designed so that a direct evaluation can be made of the 
numbers of mosquitos leaving through the eaves from. the model of window-trap 
hut now  in current use and their survival.  In this way, the results would be of 
immediate value  in  testing  insecticides.  An experimental hut was therefore 
designed that consisted essentially of the existing type of window-trap hut fitted 
with additional verandah  traps. 
Description of the verandah-trap hut 
The  verandah-trap  hut  is basically  the  existing  type  of  window-trap  hut  (Bapley, 
1961) with verandahs constructed on  all four sides.  By  addition of mosquito-wire 
screens to one  of the verandahs, the  latter could be  converted into a  verandah  trap 
for mosquitos  that had escaped from the hut  by  the eaves on  that side. 
The Bapley type of window-trap hut is,  briefly,  a semi-permanent structure 
with walls of burnt brick 5 ft.  6 in.  high,  internally plastered with a layer of 
mud 2 in.  thick,  and there  is  a thatched hipped roof supported on a frame- 
work of  sisal  poles,  and.  culminating in  a short ridge  2 ft.  long.  The hut, 
which  is  8  ft.  square,  is  supported  on short  concrete  pillars  each  with  its 
Pig. 1.￿Diagram  to show  the  basic  design  of  the  verandah-trap  hut.  1, verandah  trap; 3, removable 
screen of copper gauze;  3, window traps in walls to right and left;  4, window (closed) in wall 
facing the observer;  5, beam supporting wall of hut;  ,6,  beam supporting verandah;  7, concrete 
piliar; 8, roof of  hut; 9, roof of verandah; 10, eave; 11, partition of thatch  cutting off upper  part of 
verandah  roof-space.  It should be noted that not all the parts shown  lie in the same  vertical plane. VERANDAH-TRAP  HUT  FOB  MOSQUITOS  163 
own water  channel  to  prevent  ants  entering  the  hut  (see  PI.  IV,  fig.  2). 
The roof  and mud linings  are renewed at the end of  each  insecticide  trial. 
The verandah traps are added as indicated in the basic plan outlined in fig.  1. 
A  hut  is illustrated in PI. IV, figs. 1 and  2.  Each verandah  is 4 ft. wide and  is 
made by extending the wall-plates,  at the top of each wall,  and by additional 
beams at the bottom as shown in PI- IV, fig.  2.  Each beam  for the verandah 
trap extends five feet from the wall.  The separate roof of each verandah, which 
is supported by  4  in.  x 4  in. uprights and a 4  in. x  2  in.  cross-piece at the  front, 
joins that  of  the  hut  half-way  between  the  ridge  and  the eaves.  A  thatched  partition 
is placed between  the bottom edge of the  roof of the hut and  the underside of the 
roof of the verandah to cut off the upper part of the roof space of the-  verandah 
and so aid collection of mosquitos in the verandah trap.  The screens of copper 
mosquito-wire can be fitted  to the framework, by means of thumb-screws,  to 
enclose all sides of the verandah.  On  the  side of each  verandah, near to the wall 
of the hut, is fitted a screen with split hinges that can be married to counterpart 
hinges on the side of the hut  to form a doorway  to the verandah  trap.  The  door 
to the  hut  opens from  the  verandah  facing  north.  Removable  white-painted sheets 
of hardboard are placed on the floor of the hut and on the floors of the verandahs 
to aid detection of dead mosquitos.  An  indication of the quantities of materials 
used  and  their  cost (in Bast  African  shillings)  is listed below  together  with  estimates 
of the cost of labour and  transport involved in building a hut 90  miles away from 
the Institute headquarters. 
4^ cwt. cement 
.  .  .  .  .  .  . 
15  cu. ft. of  ^-in. crushed stone  .  .  .  . 
4  pieces of softwood (9x4  in.) 8  ft. 4  in. long1 
8  pieces of softwood. (9x4  in.) 5 ft. 4  in. longj 
245 ft.  of softwood (4x2  in.)  .  .  .  . 
876 ft.  of softwood (2 x 1 in.)  . 
48 ft.  of softwood (4x4  in.)  .  .  .  . 
48 ft.  of softwood (2x2  in.)  .  .  .  . 
70 ft.  of softwood (2 x ^ 
in.)  . 
350 ft.  of 1-in. tongue and groove board 
6 sheets of hardboard (8x4  ft.) 
1000 bricks  .  .  .  .  .  .  .  . 
120 sisal poles  .  .  .  .  .  .  . 
800 bundles of grass  .  .  .  .  .  . 
42 pounds of sisal twine  .  .  .  .  . 
40 handles for screens  .  .  .  .  . 
80  ft.  of copper mosquito-gauze 6  ft. wide  . 
48 yd. of sheep wire (4 ft. wide, 6-in. mesh) 
Nails, hinges and bolts for doors 
Labour  . 
.  .  .  .  .  .  . 
Transport  .  .  .  .  .  .  .  . 
Shs. 
49 
15 
200 
110 
45 
22 
11 
5 
175 
72 
185 
18 
75 
48 
82 
125 
65 
50 
1000 
500 
Total  2752  (£137 12s.) 
Methods 
Two verandah-trap huts, a simple hut used as a temporary laboratory,  and 
another hut for  storing  the  screens of  the verandah traps,  were built in the 
Umbugwe  area of Tanzania,  fences of sheep wire enclosed each verandah-trap 
hut to prevent cattle, sheep and goats damaging the huts and drinking the water 
in the ant-moats.  The basis  of the technique is  that two verandah traps,  on 164  ALEC  SMITH 
opposite  sides,  are put into use for trapping nominally half of the number of 
mosquitos leaving the hut by the eaves, while the other two sides are left open 
to allow mosquitos to enter the hut.  In the studies described in this paper, a 
window trap was also fitted in each of the two operational verandah traps (the 
other two windows being closed) in order to compare egress by the two different 
routes.  In  practice, the pair of verandah  traps in use  is changed  weekly, i.e., the 
east and west traps alternate with the north and south traps.  The  screens of the 
verandah traps remain  in position throughout the week, and  the traps are entered 
for  searching through their detachable doors.  The window traps  are fitted  at 
4  p.m. and  removed  at 7.30  a.m.. the  following day. 
The work was done by a resident assistant  field  officer and four mosquito 
collectors.  Apart from a preliminary study in which hand-catches were made  of 
mosquitos  resting indoors, the daily routine comprised (1) a  ’ resting count’ made 
of  mosquitos indoors,  (2)  a collection,  by sucking tubes,  of mosquitos in the 
verandah traps,  (3) removal of window traps and collection of the mosquitos in 
them.  All mosquitos were classified  as unfed,  fed or gravid,  those resting in 
the hut  being counted and  classified m  situ. 
Results 
It was possible that the addition of the verandahs to the basic design of the 
window-trap hut might  affect  mosquito behaviour.  Preliminary  studies  were 
therefore made, in which hand-catches and collections from a single window  trap 
placed in the east wall were taken in an  unscreened verandah-trap hut and  in an 
ordinary window-trap hut.  The  results of 35 collections between 16th December 
1963  and 28th January 1964  are  shown m  Table  I.  These show that  the 
additional verandahs had apparently no effect on the resting habits and egress 
into -the window  trap of A. gambiae and M.  uniformis. 
TABLE I.  Day-time resting and egress  into a window trap  fitted to  a 
simple window-trap hut and to an unscreened verandah-trap hut 
Verandah-trap hut  Window-trap hut 
Abdominal  Nature of  ,￿￿￿￿￿￿￿f￿￿￿￿￿￿￿\  ,￿￿￿￿￿￿￿A￿￿￿￿￿￿￿^ 
condition  observation  A. gambiae  M.  uniformis  A. gambiae  M.  uniformis 
Unfed, fed and  Percentage resting  95 (2279)  64 (22)  69 (7475)  60  (97) 
gravid  on roof 
Fed  only  Percentage egress  8 (2559)  76 (88)  11 (7882)  81 (469) 
Unfed  only  Percentage egress  91 (501)  98  (51)  95 (4005)  96 (186) 
The  numbers  of mosquitos collected are shown  in parentheses.  The  percentages 
resting on  the roof are of  mosquitos caught  in  the  hut. 
The  record of 95 per cent. of A. gambiae resting on  the grass undersurface of the 
roof of the hut with a verandah was more typical of the proportion normally 
observed in the same  position in simple window-trap huts (Smith, 1962) than  the 
69 per cent. recorded in this preliminary trial.  It was  therefore concluded, from 
earlier observations and  from. the results of this verandah-trap hut  (Table I), that 
the two types of hut gave similar results in respect of percentages of mosquitos 
resting and making egress into window traps.  Therefore, when a window trap 
was also  in  place,  mosquitos  caught in the verandah traps  would essentially 
represent the unassessed  fraction leaving through  the eaves.  Unpublished  results 
obtained from the other verandah-trap hut (unscreened) were similar to  those 
obtained from  its facsimile and  recorded  in Table  I. VERANDAH-TRAP  ETTI FOR  MOSQUIT05 
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P?  S  H  El 166  ALEC  SMITH 
Counts or collections  were then made with, the verandah traps fully opera- 
tional, i.e., with screens fitted on two verandahs on  opposite sides of the hut  and 
with a window trap placed to extend into each of the two verandah traps (the 
other two windows being closed).  The total number leaving by the eaves was 
assumed to be twice the number caught in the two  verandah  traps.  The  results 
of collections of A.  gambiae and M.  uniformis from 29th January  to 29th February 
1964 are shown in Table II.  The results show that 51 per cent. of females of 
A. gambiae in all gonotrophio stages left the hut each  .night, with 85 per cent. of 
the egress occurring by the window  traps and 15 per  cent.. by  the eaves.  In  the 
case of M.  uniformis, on  the other  hand, 90  per  cent. left the hut  each  night, with 
31 per cent. of the egress occurring by the window  traps and 69  per cent. by the 
eaves.  When  recently fed mosquitos only were considered, it was found that 19 
per cent.  of A. gambiae left on the night of feeding, with 70 per cent. of the 
egress occurring by  the window  traps and 30  per cent. by  the eaves.  In the case 
of  M. uniformis,  91 per cent.  of recently, fed mosquitos left  on the night of 
feeding, with 34 per cent. of the egress occurring by  the window  traps and 66  per 
cent. by  the eaves. 
Discussion 
The results from the verandah-trap huts are in line with those obtained from 
the same area by the more cumbersome methods of surrounding a window-trap 
hut by a cage of mosquito-netting.  For example, it was  found that 12 per cent. 
of recently fed females of A. gambiae entered the window trap compared with 
10  per cent. leaving by  the eaves, and  that 10 per cent. of recently fed females  of 
M. uniformis  entered the window trap compared with 72 per cent.  leaving by 
the eaves (de Zulueta &  Cullen,  1963; Smith, 1963).  The techniques used with 
the present design of verandah-trap hut are practicable and would appear to be 
suitable for  studying the egress of any species of mosquito that enters houses. 
It is concluded that, owing to the high proportion of individuals of M. uniformis 
that leaves by the eaves, the verandah-trap hut  is of a more suitable design for 
studying the house-frequenting habits of this species, and of other species with 
similar  habits, than  the simple  window-trap  hut. 
Summary 
A  description is given of a  verandah-trap hut  designed to assess the egress and 
survival of mosquitos escaping through the eaves of the type of window-trap hut 
used  in  insecticide testing  in East  Africa. 
Fifty-one por cent. of females of Anopheles gambiae Giles in all gonotrophic 
stages and 19 per cent. of those recently fed left the hut each night, with 15 and 
30  per  cent., respectively, of the egress occurring through  the eaves. 
Ninety per cent. of females of Mansonia. uniformis (Theo.) in all gonotrophic 
stages and 91 per cent. of those recently fed left the hut each night, with 69  and 
66  per  cent., respectively, of the egress occurring  through  the eaves. 
It was concluded  that,  owing to the high proportion of  individuals  of  M. 
uniformis that leaves by  the eaves, the verandah-trap hut was  of a more  suitable 
design for  studying the house-frequenting habits of this  species,  and of other 
species with  similar habits, than  the simple  window-trap  hut. 
Acknowledgements 
I owe thanks to Mr.  K.  S,  Hocking,  Director  of  the Tropical  Pesticides 
^Research Institute, and to Laboratory Technician Mr. E. T. Mesmer, for help in 
the design and construction of the verandah-trap hut.  G-rateful acknowledgement 
is  made to  the World Health Organization for financial  assistance in building 
the huts. VERAKDAH-TRAP  HUT  SOR  MOSQTHTOS  167 
References 
EAPLEY, B. E. (1961).  Notes on the construction of experimental huts.￿Bull. 
World HUh  Org. 2$  pp. 659-663. 
SMITH,  A.  (1962).  Studies  on domestic habits  of  A.  gambiae  that.  affect  its 
vulnerability to insecticides.  Part I.  Besting places in huts.￿B.  Afr. 
med. J. 39  pp. 15-24. 
SMITH, A. (1963).  Deterrent effect of insecticides on malaria vectors.￿Nature, 
Lond. 200  no. 4909  pp. 861-862. 
SMITH, A. (1964).  A  review of the origin and development of experimental hut 
techniques used in the study of  insecticides in East Africa.￿E.  Afr. 
med. 3. 41 pp. 361-374. 
DE ZULTJETA,  J.  & CULLEN,  J,  B.  (1963).  Deterrent,  effect  of  insecticides  on 
malaria  vectors.￿Nature,  Lond. 200  no. 4909  pp. 860-861. 
' 
Commonwealth Agricultural Bureaux, 1965 PLATE IV  BULL. ENT. RES. VOL. 56 
’[’he  verandah-trap hut. 
Vio. .1..  One verandah trap with window  trap in position.  FIG. ". The hut in 
irnp.rfltiftn. 
window 
operation. 

A Verandah-Trap Hut for Studying the House-Frequenting Habits of Mosquitos and for Assessing 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.

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A Verandah-Trap Hut for Studying the House-Frequenting Habits of Mosquitos and for Assessing 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.

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