Ai W O R L D H E A L T H ORGANIISATIOW MONDlAtE

, ' ; ' . . . * _ .;>. r

. . . . . . . . .. , 1. . : .

... . . . I

. . . . . .' .. . . . . . .

........ . . . . , . ) . . : ' . .:.; , 2 .

. .

. . . . . . . . . . . . . . . . . . . . . . .. :. - AT~ONS 'OF' ~NSE~TIGDE 'RES~STANCE

. . . . . . . . . . . . : . . . . . . . . . . . . " . . j ; . . . :,,:i. :': ; .

;

. . . . . . . . . .by ;., :. . . . . . , . . " . . ., .I . .~ 1

. - ?."klamon,.' 6. Palf .J. . Cozi 1. and .. 'J."Mouchet 1 ' , ' ! .. ,

. . . . . . . . . ..... . . . . . . . . . . t̂ . . ' ' \ . a .

. . . . . . ' . tl;{.; : , . le INTRODUCTION:' . ' . . . . . . . . . . . . . :

. . . . . . . . . . . . . . . . Th& de'finition of insecticide resistance, as mulated' by the WHO

- i t ' ' ' " . ' ' . . . . ,. . . , .

Committee on Insecticides at its s nth meeting, is the foliowing:' "Resistance ,,>:.. . . ; . . , ! ;,. . , :" . . . . . . , _

: ? ' . :. ;.' . . . . . ~ ........ . . . . . . . . . . . . . .

to insecticides is the development of an ability in a strain'of insects to tolerate . ! ::

y' OE' îndividuals in a

viohristic resistance ' . .

ich could'pr . . . . . . . ' . . . . . . . . . . . . I , : . . . . . . . . . . . . . . L . . ? , , .

, . .

The behaviouristic resistance supposes a change In behaviour of the insects . . . . . . . . . . . . . . . . . . ;..; ... : . . : . , ; . . . 1 :;. . . . . . . . - . . . . . i .... ~ . . . .L . . r: i _ . .;; . . . . - . , ;

_ _ . . . , . , . -

due to selection following' 'the 'use of insectfcides. The importance of behaviouristic . . . ... . . . . . . . . . ,.?,. ..".!'. : ."! ... . . . . -

I . . . . . . . . . . ., I . '. . , ,. r . ,

resistance ..... 'is difficult to assess in the absence of universally

pears that' at least 'one of methods of' me behaviour 'change. Besides, it n

the observati behasriourist ic changes was no intra&ecific seiection,

but by interspecific selectionk-:be$peen ... - . s;%b.&Zl?@;..,:s-peqiee .. _ _ - -.. J . : . - . L - . - . ~ .:of . . the . . .Anopheles. gamb&ae.

complex (Davidson, 196410). One wonde suspected cases of

behav.iour changes are in re

suggested. To avoid useless dis

. . . . . . . : . . I . . . . . . . . . . . . . . ~ .. : ; .. , . ..!'__ . . . , : ..;- : '

,. . . . . . . . , . . . . . . . . . . . . . . . . . . . . .- .:. ~ . 7 .:., . I . . : .! -. .

. . :. , ,. '_ . i ' ' :' .I . .

pecific as usually . . . . . . . .

. . . . . . . a . ' . . . . . . .

... . . . . . . . . :. . , : . , . . restricted t o , insecticide resis

. . . . . . . . . . . . ._ . , .

. . . . . . . . ... . . . . . . I . .

1 . . I Medical Entomologist, Office de la Recherche Scientifique et Technique

Outre Mer, Paris, France.

Health, y..

Scientist-Biologist, Vector Control, Division of Enviro 2

i World Health Organization, Geneva, Switzerland.

VC/ENT . SEM o /WP/2.65 pagë 2

..... .,. ..... ;.;, . . . . . . ..... <..."

. . . ,,," . . .>.? . -1 i . ;;.- i.? I' .... . . . . . . . , . .

IF is.generally ,admitted that two types of insecticide resdstance acapposq$de:: . . . .:. . . . . . . ,; ........ .r. ..

, ,: > , Ir physiological resistance" (= true resistance) and "vigour toleran6d!':'i . ; '.*. : . . . . . . . . . . . . . . . . . . . . .

> _ . . . . . .

Vigour tolerance is a term suggested by Hoskins & Gordon (1956) to describe a generalized lowering of susceptibility to one or many types of"'poTson, depending Ón-----

non-specific mechanisms. The consensus of opinion seems to be that such non-specific ,- I

mechanisms stem either from a high level of heterogeneity (heterosis vigour) or from

the accumulation of several minor genes improving the natural defences of the insect

against toxicants (increase of cuticle thickness, higher proportion of fat body, and

so on). Nearly all published observations on vigour tolerance deal with strains developed by artificial selection by insecticides on Laboratory colonies. .IQ s,uch s .

cases the level of resistance is not very high and is soon lost when the selection . . . .

Some similar observations have been carried out on field . . . .

' . I ' .; _ I '.> . . n treated areas; there also the tolerance disappears with the removai of

, . . _ . . . % , . i . , , . : . . . I. ... ;

e (A. gambiae In the Ruzizi valiey and in South-Weste

' ' appear to'be of serious pra6ticai im

cies which are already of a low natural susceptibility

... . . . . . . , , . , >!- I - ' i' '

. . . . . . .. ,

Coast). Vigour toierance does

(Busvine, 19'63) 'except for some to inseeticides and become almost immune if vigour tolerance appears.

, I

...

. I ' . . . . Physiological resistance, which is specific to a particular type of poison, often

. . . . . . .: :

gives a very high degree'' o Î protection;' approaching complete immunity. "lie 'resistance , :

ism responsible is 'inherited and it' is comparatively easy t o maintain resistant . . . . . . . . . .

. . . . . . . : ,.; . , ,

iods without any constant selection by insecticides. . . . .

... . . . . . . , . , . . 2 T€B* .._ SELECTION ... ....... .OF, -... PHYSIOLOGICALLY RESISTANT POPULATIONS

A S far as . . . . . . . . . . ....

. . W, physiologically resistant populations appear by selection,

, .. ... ... . : . t, ; :: j " . .

under idsectici.de pressure.

proportion of spontaneously resistant individuals;

specimens is killed by the insecticides, or gives rise to a less susceptible progeny,

and gradually the proportion of resistant individuals increases, until the appearance

Natural populations' generally contain a very low . . . . . . . . . . . .

'.. < . I

the big mass of susceptible .. ' .

of a resistant population. . . . . . . . . . . .

." :.. . . . . . . . . .

. . . . . . . . , . . . . . . . . : I .

. . . . . . . . 1- ,. ~. . . . . . . I . . . ~ .... . .

VC/ENT SEM. /hP/2.65 page 3

The speed of selection varies according to several factors,.namely: .. . . , ' , .. . I .

. - the ,,intensity ,of the ,insecticide pressure, . . . . . . . . . .

- the .natural frequency of resistant ..i.ndiyiduals, and the density 'of the vector,

- the genetic basis of resistance, dominant qr recessive, monofactorial or .

'

. . . polyfactorial, and . the .. nature of . : characters linked with resistance, '

. . .

- the amount of advantages co,nferred.to the resistant individuals in treated ' '

, ' .

_, . . .. . . . and in untreated environments.

The ..speed of selection increases with insecticide. pressure which, in tmh;' depends . ..",, .I ._ .-.- - ....._. I. 1 4 - 1 . 1 . - . .

on t.he nature of the. %toxicant, of the application ,rate and methods, and of the surface

of the treated areas. Large-scale larviciding operations with residual inskctlcides,

to which : . immature stages,-cannot escape, exert a very strong'insecticide pressure and

are very favourable for,.selecting .. resistant populations. Such 1arviciding";operations

are very of$.en a. side-effect of crops prptection by agricultural inseetiaides..

s o i l , contamination .. .. stands . , . for years. after. the last applications and ensupes favo&rable ' "

environmenta1.condition.s for the long-term.maiiitenance of resistant populations. I

. .

.' The .' ' .

. .

. . .. . . , . ,

. . The speed of selection increases slightly with $he natural frequeiicy ' of resistant

., .. .. . , ! , .. individuals, but this 'is'not an important 'factor for' the majority of insects wi'ch "an

abundant progeny and a very shor t 'life-cycle, at least in tropical codtries.

if the.,.vgctor density is low-resistant, individuals have few chances to mate wZ-th"'each

. : .. . h

. . , . ,

However,

. z . . .

. . . .., . other, which delays the. appearance :.o$ resistant homozygotes. . . .

PYiobabXy more important are the genetic. bases' of resistance. . A monofactorial charactcW{-will be selected more qu2ekly -&&'a polyfact"oria1 one. Besides, if the

resistance "character is recessive the hybri'is' will n o i possess any advantage on the

susceptible, and' the" selection 'wili be 'very slow;

more or less. domin.int,- 'the. hybrids will survive 'to 'insecticide exposure in higher

proportion than the' susceptible, and the character will spread out' very quickly in the

wholé'; populat2on: ' Sometimes the character of resistance, even when monofactorial,

seems to be linked to unfavourable .charaéters which are usually transmitted wi'th it

. , ..

~.

. / . . but if the resistance character is

' r . .. t.

. . . . . . . I . . . ., . , . . . - ..< . 1. ' 2

and delay its spreading in treated areas (Busvine, 1964); the first phase of

selection is then very slow and resistant populations can appear Ónly after a long

period of insecticide pressure;

selection for resistance speeds up and later on the resistant populations are stable

even after removal of the insecticide pressure.

gradually new combinations of characters appears the

c - VChNT. SEM. /WP/2.65 page 4

The exact origin of resistant characters has been widely discussed, as it is

very difficult to prove their existence before any use of the corresponding insecticides.

However, it is well known that insecticide pressure exerted on laboratory colonies of

insects, even during very long periods, generally failed to develop any resistance.

On the contrary, some authors have succeeded in developing resistant populations from

laboratory colonies containing the character for resistance, without any contact

between the selected insects and the toxicant.

.

Bennett (1958) has worked on a heterogeneous stock of Drosophila melanogaster produced by combining 24 laboratory and wild strains.

was divided into sub-colonies, and the DDT susceptibility of each sub-coloAy was

assessed. Only the more resistant and the more susceptible sub-colonies were used to

raise F1 generations.

resistant sub-strain 625 times less susceptible than the corresponding susceptible

sub-strain. Since in such a selection the direct ancestors of the resistant sub-strain

have no more been in contact with DDT than those of the susceptible sub-strain, the insecticide could not itself have been the direct cause of the resistant variants.

That a reslstant strain could be produced by this procedure shows that the pre-

adaptation hypothesis is sufficient to explain the results.

After establishment the colony

The same process was repeated for 15 generations resulting in a

However in laboratory conditions strains of resistant insects have been selected

from apparently susceptible colonies by exposure of adults to sub-lethal doses of

insecticides, without any noticeable mortality of the treated adults. But, as shown

by Derbeneva-Ukhova (1962 and 1963) and by Beard(196-4). the sub-lethal doses cause in

females pathological ovogenesis irregularities. So the prageny of the susceptible

females is far less numerous than the progeny of the resistant ones, and after some

generations the laboratory population is almost only composed of resistant individuals,

the selection being done indirectly through the differential reproduction rate of

susceptible and resistant females, exposed to sub-lethal doses of insecticide. In such conditions the selection can be complicated by the occurrence of resistance-linked

unfavourable characters which are more liable to be maintained in laboratory colonies

than in wild populations. . .

, . .'i . .. .. ; -... . ..

; i . - . ... ,

, *. ' . . .. i - . . . . .

3 . VARIOUS TYPES OF PHYSIOLOGICAL INSECTICIDE RESISTANCE I - * , I . . . . . .I . . . . . .

, _ . _ . . _ . Each type of physiological resistance .appears to b.e specific ta a particular $ype

of toxicant, typified usually not .by one colnpound but by, a group of re,lal;ed comgounds. . . . . . . . . . .

... . . .. I . . . . . . . . , i .

.The maj:or types of physiologica1,insectictde resistance correspond respectiveXy to': DDT and related compounds; phosphate compounds; and carbamate compounds. Howeyer, resistance to organophosphates

can be limited to only malathion and malaoxon, .or to dimethyl-organophosphates, ,or to.

diethyl-organophosphates.

the correspoiiding insecticides and several types.can coexist in the same species (Brown,

BHC, dieldrin and related compounds; ' pyrethrfns;. organo-'

: . . . .

. . . . . . . .

Each type, corresponds to specific ways of detsxification of . . . . . . , I'

. . . ... . . . . 1964; Busvine, 1964; Georghiou, 1964).

, .. . . . . . . . ::. . 1

:'Besides there is some relationship betvxeen 'organophosphate-resis%ance and, _.,.,. "_ . . . . . . . . . . . .

carbamate-resistance, and both can induce, in some circumstances, DDT-resis%ance.

Pyrethrin-resistance may also induce DDT-resistance.,

I . . . . . . . . . .! . . . . . - . . . ; :. i. .:;,

In field conditions the situation is further complicated by.the frequent selection of populations with multiple resistance but generally each resistance mechanism, acts

separately. . , . . -

._ . . . . . . . . .--_. .. , .

However, Davidson (1964a) has recently stressed that the DIE-susceptibility . . . . . - .

of Culex p. fatigans adults varies according to their dieldrin-suscgptibility, the . . . . . . -..._ .I.. .

DDT-susceptible DL-resistan$ . . . . adults being Îar more DDT-tolerant than the DDT/DL- susceptible ones;

. . . . . . . .................. - .- ..... -, .. . . : . _

. . . . . . . . , . ' . . . ... . . . . , . . for the time such a phenomenon is unexplained.

.. .- ........

. . . . . . . DDT-susceptible DL-resistan$ . . . . adults being Îar more DDT-tolerant than the DDT/DL- susceptible ones;

. . . . . . . .................. ..... -, .. . . : . _

. . . . . . . . , . ' . . . ... . . . . , . . for the time such a phenomenon is unexplained.

.. .- ........ . . . . .

4. THE GENERAL PREVALENCE OF INSECTICIDE.-.REX~~TAN~E.~:IN.,.ARTHROPODS' OF' MEDICAL AND VETERINARY IMPORTANCE ......... " . . . . . . . .._. .. " ....

. . . f . , , , " . . .

. . . . . . . . . .' . ,

Before the second World War resistance t o I insecticid& was'very uncommon. ' The

first instance occirre& 4 s long.:ago as' 1908, when. the. Sari .José scale .WES i'ound t b be

resk&an;t..to ,Eine .sulfur .in-.Washington Stat&, . United :.States .of America, : but by '1946 only one arthmpod:.bf 3veterinary:or medical importance was res2stant (Brown, 1958). . . . .

Howeve.P, the development of modern. synthetic .insectlhides and their largezscale. :use'. I ' .

al.X.ijver %he:.world has favoured the .selection .of many resistant populatibns.

arthropods. 8 ':

. .

. . . . :.. . . . . , ' <'. .

VC/ENT , SEM I . _ /WP/2.65 page G

In spite of the standardization of susceptibility-%esting methods by WHO, all

field and laboratory workers did not use the same assessment techniques and it was

not always clear whether or not they were dealing with resistant populations. So

the number of resistant species varies slightly from one author to another, according

to-the criterion used and according to the group of insects included under the label

"of medical and veterinary importance".

The number of insecticide-resistant arthropod species has increased from 1 in 1945

t o at least 18 in 1955 and 60 in 1965. If we include some semi-domestic pest diptera ., . and some mosquito species for which we do not have the full set of data, the progression

of insecticide resistance is 1 species in 1945, 37 in 1955 and 75 now. Almost all

rnajor groups are involved: house-flies, blowflies, mosquitos, fleas, lice, bed-bugs,

ticks, stable-flies; biting midges (WHO, 1963) and since two years Simuliidae (Suzuki et al., 1963).

The first steps in the discovery of vector resistance to insecticides can be very

easily illustráted by the following events:

1946 : 1947 : 1947 :

1948.: 1949 : 1951 : 1951 : 1963 :

In the insect5cide

populations

. . . I

. . . . . . . . . . . . . . . . DDT-resistance of Musca domestica in Denmark and Sweden.

DDT-resistance of Culex pipiens in' Italy.

DDT-resistance of Cimex lectularius in Hawai.

. . . .

. . . - . .

"

, . . . . ............. ........ ." .. . .

Dieldrin-resistance of Boophilus decoloratus in South Africa.

DDT-resistance of Pulex irritans in Peru.

DDT-resistance of Anopheles sacharovi in Greece.

DDT-resistance of Pediculus humanus in Korea.

DDT-resistance of Simulium aokii in Japan.

beginning, almost all resistant populations were DDT-resistant because this

was the most commonly used in'vector control operations, and the resistant

were restricted 'Co small areas.. Now the most common resistance is the

BHC-dieldrin-resistance and the resistant populations are much more widespread.

present status of resistance of each of the major groups of vectors and pests of medical

and veterinary importance is discussed below.

' The

. " ~. . -, 4.1. Synanthropic f l i e s . . ' . . . . I . . . _:,*_.:_I' , . . . . . . . <'. : I ' ' .

! ._ , . , ' : , " / " ': : ' , 1 - ? . , . , . . . . . . . . . . . . , 4.1.1 House-flies' . . .

. . I . . . I

. . " . . , .I . .

House-flies have n submitted t o very strong insecticide

rogrammes and much more by millions of domestic applications' of . . . . . . . . . ...... -.

3

. . . . . . . . . . . :* I ? ~. . . ::: ?: I ( , % >

scale house-fly contro

insecticides against household pests.

has been' encoktered almost everywhere and toivards' almost a l l . insecticide groups.

. . . . . . . . . . . . . So it is not surprising that house-fly resistance

. . . ' . . 1

, DDT-resistance ... developed f i rs t i n 1946 and' is present now a l i over, the -. world, . the . . . . . . . . , . . . . . . . .

most recent records coming . . . . :rpm , I ,Eastern Europe, the USSR, China

(Kovchasov, 1963; Suzuki, 1963; Lupasco e t al . , 1,963; Tsukam.

Ukliova, 1963). becoming ubiquitous.

'stïlx' effectively contro¡ house-flies a t higher dosag"es than usual', kiÜt-.witc:dieldrin

the control fails.

. . i.', .., . . :.

.... . . . . . I .. : ' ~. . ,

' .%i. ' ]

Resistance t o BHC and dieldrin, which appeared i n 1948,. is a l s . ' . . , ..: 1 f :>,? ' ; y , . . . .

I j j areas of developing resistance, DDT o r BHC can' sometimes . , . I. . ,

. . . . . . . ,i <. :.: ' , . . ' . : ' . . , . . , '. -, ..

. . . . , . ., ., :... . . . .

Organophosphate-resistance ( to parathion and diazinon) deveioped

malathion-resistance (which i s s l igh t ly different from the general O.P.-resistance)

appeared i n 1956. States of America and in Japan. :, Carbamate-resi.s%ance, which is not a s5.ngle. enti$y

(Hoskins & Nagasawa, 1961; resi.stm,ce, ,first developed i n

Europe,, and t.he, United States -o

. ,. They are now present in . .&stem 'and ... N o ~ t ~ e ~ ~ . . euro^, ~.. :in .. .-thel. United.

Georghiou, 1964), mcurs often i n the same. areas. . . . . . . -. 2yeethrin-

en, is present now i n several areas of Western . . . . . . . . . ' : A . . . . . . . . _ . , . -. 11. ......................... - .... . , .. , . - , . . ._ .

.. , , . . . . . . . , i..

.I.

.:. . . 1 . ' . .

I n laboratory coQditions, s,ecticide-r.esLs$ance . . can be very s table , i n house-flies . . . . .

(Mallis & Miller, 196$),. .but in. f i e l d populat,i:ons- .it, .i.s not always s p s table a s only. . . :.,

peridomestic..,populations, of house-flies are af

the withdrawal of the insecticide, res is tank '3

ted by the control operations; a f t e r

VGlUalK "are'"slow1y' diluted into the , .. . . . . . . . . ., . . . .

. . . . , . . . . . . . . ' . , . I . . " . . . . . . . . . . . . .

. . . ... wild liouse-fly populatioiis wlaffected by ,the t reat iEnts (Yasutomi, 1961; Peffly & . . . . . . . . . . . . . .

Shawarby, 1956; Ozburn & Morrisson, 1963; Keiding, 1963). However, due t o the

i-esidual effect of most of the chlorinated insecticides, the seiection pressure can

stand f o r months, and even years, a f t e r the l a s t application.

serious with organophosphates and carbamates, which are not s o s table , especially if

they have been used i n bd$ti.--:or o n treated. cords.

compounds seem t o select for.Dlk-resistance, and sometimes f o r dieldrin-resisemce,

. . . I . . . . .

. . I , . .

The s i tua t ion is . . l e s s . . . . . ' , ,

A s organophosphates, ,and carbamates

. . . . . . . . :<

. , , . . . . . . . :

VC/ENT SEM./wP/2.65 page 8

as well as for homologous resistance, chlorinated insecticides become. -1,

efficient for house-fly control (Keiding, 1963; situation is not the same in all countries because DDT- and carbamate-resistance do

noi have the same'geneticái bjsis' everywhere (Milani, 1963; Rdman & Khan, 1964;'

- Winteringham & Hewlett,,1964,),. . . The

. . . .. .. .

. . , . . . . I ,', ; . . . . . ..

Tsukamoto 6 Xuzuki, 1964; . - . . . .. 1 . :.:. .

Georgiiiou, 1964). . .

In spite of some cross-resistance between carbmate and O.P. compounds, these , . ,

insecticides offer some promise even now, as a great variety; of toxicants are avail- able *

DDT-resistant , A. aegypti, offer few hopes for controlling DDT-resistant house-flies (Spiller, 1963; Pillai B Brown, 1963; Fine, 1964) ; malathion-synergists are probably

WARF-antiresistant DDT compounds and deutero-DDT, which seem efficient against . .

more promising (Plapp et al., 1963).

The best way to control house-flies is probably the combination of source reduction

with the successive use of several unrelated insecticides, using each of them only for

a short period before the reappearance of homogeneous resistant populations (Keiding,

1963)

4.1.2, Blowflies and other synanthropic flies

The situation is not so serious with blowflies because resistance is much mope

restricted to some compounds and to some areas.

Sheep blowflies (Phaenicia cuprina and Ph. sericata) have developed BHC/dieldrin-

resis"cce in South ATrica, Australia and New Zealand, but csun be controll

conventional insecticides and, if necessary, could probably be controlled as'we11 by

systemic compounds (Bushland et al., 19631, like ronnel and Co-ral.

The African blowfly (Chrysomyia putoria) has developed resistance to DDT, BHC/

dieldrin and to various O.P. compounds i n the Leopoldville area where it has been

submitted to very strong insecticide pressure during several years;

tant to DDT and BHC/diel&in only have been recorded from Madagascar;

populations resis-

in Leopoldville, ' <

where the situation was most serious, a good control. has been obtained in replacing

insecticides by environmental sanitation, including water-sealed latrines. .._.- ~. : , a . . .

. , ....,.. .... 2 . , . . . . . ..

. . . , . . . ! ( .

More recently, one sarcopliagide fly, Xarcophaga peregrina, has developed a high '. .

dieldrin-resistance after exposure of only a few generations to the toxicar$,,in i J

Nagasaki, Japan (Matsuo, 1963). This fly is susceptible to DDT and to O.P. compounds. .

4.1.3 Stable-f l i e s r *

DbT-resistant Stomoxys calci t rans has been reported i n 1948 i n Sweden, but without

comparative studies of susceptible populations (Kjellander, 1949). investigations have been cak-ried out i n Norway (Sömme, 1958) on natural farm populations

as well as on laboratory colonies of St. calcitrans and strongly suggest t h a t the stable-

f l y could be res i s tan t t o DDT and tolerant t o BHC.

More recent

More investigations are needed.

4.2 Anopheline mosquitos

The.development of insecticide resistance i n anophelines and its pract ical

implications for the world-wide malaria eradication . . programme have been swnarized.and

reviipwed recently by Hamon & Garrett-Jones. (1963 , Davidspn (1964), Coz e t .al.,. (1964),

Ungureanu. :(l$k) . . . . . . and by. several WHO contributors (WHQ., 1965) e

vectors are. involved and t ae s i tua t ion ,is ;as follows.

.. . . .

........ -. . . . . . . . . _._... . . I ~~

Almost a l l , _. major , . , . . . I : . :. . . . . .

. . ' . . ; '.' ' ' - , . . . . . .I . ' , 1 .: ,~ ;. , : , . . .

Only two types of resistance a re encounteFed i n anopheliriEs, DDT arid BHC/dieldrin

resistances. DDT . . . resistance is of a low amplitud-e, does not preclude always.,.the ._ . I use

of ,DDT,...and .is. very . . . slow t o appear i f house-spraying only. is ,involved. . BHC/dieldrin

resistance. ..pas. a very great amplitude, precludes any use of .dieldrin, while BHC efficacy

is redyced . . . . . . . . . t o .;Some weeks only;

house-spcaying. \, .. , . .alone,. .

t h i s type,of resis'caace is very rapidly selected . _ . by . . . .

. : , ; : . . . . . . . . . . . . . I :: . . ; . . , ' . I

" .I._I_ . . . . . . . . ......

i ' .'%!t''f'i&Td c'ondit'ioixGthè select ion of resistan% anophelhe 'populations has'"been

spegded 'Üp by d i rec t :and"indirect larviciding, e i ther during- vecrtor .c'antrol a c t i v i t i e s

o r much more commonly through agricul tural "-pest control.

seeins t o have been due t o cotton, coffee and r i c e spraying and dusting.

conditions, DDT-resistant populations can be selected very rapidly because the whole

anopheline population is submitted t o a strow' insecticide pressure, which occurs rarely

with DDT house-spraying.

i n Egypt and in'West Africa.

, . :.

Sesistant individuals decreases slowly a f t e r the.remova1 of . ' DDT.sprays \ . .if no agricul tural

treatment is involved,, but .the dieldr in resistance is usually . . very s table .,for, long ,

periods and, seems, a t l ea s t i n West Africa, 'LO expand . . . . . . . .sl,owly:. " . . I _ . . _ i n . . . un5reat

(Hamon, e t al.:, 1958 and recent personal observations;. Service,, % . 1964; Service 8c

Davidson, 1964). Ti?e s i tua t ion can be different under laboratory conditions

The' most important selection

I n such . _- . . . . . . . . . .

. . . . . .

. : .

With agricul tural treatments, resistance has a l so appeared . .

: . . . ::, ? . . - . . . .

, . , , ! . ' ~ . , , . .......................

. I n the f ie ld , DDT-resistance . i s not . always . very stable, and the.prop.oTt.ï,o . . .- . . . . . . . . . . . . . . . . . . . . . .

. . . .

'VC/ENT. SEM. /wp/2.65 page 10

(Keppler et al., 1964). selected for resistance in the past is very soon followed by the reappearance of

highly: resistant popuAations ..

In? any case, the reintroduction of the .insectic.ide having

. . .. . . . .

. . . . . . . . ' . . . . .

In nearly every case the zone 6f':resistance forms only a small part of the area - '; . of distribution of the. species.

populations of the sanie species do not overlap and one of these powarf.ul inseckicides

can be used. As far as resistance alone is involved, its operational malaria eradication is restricted to some zones where the vector presents double

DDT/dieldrin resistance and where malaria is n o t eradicated as yet, like" Persian Gulf

coastal areas (A. stephensi), Central America (A. albimanus), Egyptian delta"'

(A. pharoensis) , Indonesia (A. sundaicus and A. aconitus). However, in' many more

areas insecticide resistance, either to DDT or to dieldrin, is a complicating factor augmenting very seriously orner technical difficulties, such as mosquito behaviour,

. Veg~often DDT-resistant and dieldrin-resistant

ce for .. , .. .. .

. . . . . . , . . . ' .

, . ' i ,:

envirohental conditions, nomadic habits of inhabitants, and so on. . . i:

. . . .

DDT- and dieldrin-resistant anophelines can be controlled by O. P. compounds , .

' . . _ I :,.

spraying,' . . but these toxicants, up to now, must be used'a't short intervals, which is often very difficüit "to do in tropical countries and increases the costs of the

programmes. In some conditions the only O.P. compound assessed, malathion, has not

given satisfactory results, e.g. against A. pharoensis in Egypt (Shawarby et al., 1965). However, many. -other promising O.P. and carbamate compounds are under field assessment

and will be available for mass spraying, which could improve the picture very soon

. . .

. . ' ' (Bar-Zee+ .Be'Bracha, 1965; Coz e t al., 1965). . .

. . . . ._

4,,3 Culicine mosquitos . . . , S I

. . . The . development of resistance in culicine mosquitos has followed about the same

course as in anophelines.and has been partly a side-result of malaria control and,.

eradication programmes.

against Aedes aegypti in the Americas and in some areas elsewhere, and against members

of the Culex pipiens complex in almost all urban areas. Large insecticide operations

have been carried out also against pest mosquitos in touristic or irrigated or arctic

areas in developed countries.

situation have been presented by several WHO contributors (WHO, 1963;

. . . . . . . . . . : .. . .

Specifically anti-culicine programmes have been carried out . . . . :. , . ' .

. , . .

Recent general and regional surveys of the present . .

Pal, 1964; Gratz, 1964; Anonymous, 1965), and by many national specialists (Lewallen, 1960;

VC /ENT SEM.. /WP/2-.'65 page 11

. . Yasutoomi; ,1961; . Haman::&' Moucbet, 1961 and 1965; " Piga t t i & MeLo,' 1961; Brown e t a l . , 1963; Suzuki, 1963; Yu e t al:-.,: :1963; Geor'&Zbuj. 1964; Liu e t a l , ,

Bar r , 1962;

. . . . . . . . . . I ; y r , . . . . . . . . * .i: .. 1954; Wu e t a l , I 1964). a . .. ? . . - .

. . ,. . . . . . . . . c .. 1.l.3.1 Aedes aegypti and related species

. . . . . I -7 -- 8 '. ._

, , i ' .! . . . . .Aedes aegypti is normally f a i r l y suscept ible . to DDT and dieldr in and.house-

spraying with these insecticides has v i r tua l ly eliminated the specles from a number' of

amas.. An Aedes aegyp%,i eradicati.on.campaign w a s s ta r ted i n the Americas in 1947 an& success has been achieved in Mexicp,, a l l of3Centpal,America and a l l o f South America. .

except the Caribbean area, where A. aegypt ihas developed a high resis tanc

i n some places also t o dieldrin, and a low and indiscrimlna%e' resistance"%

coh$ound:s. (Bi-oh,' ,1964) o

have hk'en reinfeste& by a res i s tan t s t ra in , e.g..':'French Guiam (Flrock, 1964). pyogFamrtie. f o r - . % ~ e 'eradication of A. ae.gypti hes%Jso been started- i n the United States

of America and the recent outbreaks of haemorrhagic fever in South-Easi Asia w i i i ' probals17 induce the organization.of .A. aegyptj, .and.A..albopicixs control programmes i n

severaX .countries of South-East' A s i a .

. . _. ........ . .

I-

'. , . ;.. ,--:..:=

, . . _ . . . . . Several ' f ierr i tor ies from which A. 'Bas been eradicated

A j y . . . .

. . '

. . . . . .

, : . , - - Aedes aeg$ptl.-l?esistance -- t o DDT and. d i e is.'now recorded from the majority of

Car?lb&an Islands, the three Guianas, Colombi'a; Venezuela, as well as from South TJiet-

N&.,. i PópGl-&tibns' res is tant t o DDT alone a re much more widespread and occur i n many

. .

. r . 1 ..

. . . . '-, : ' couh-b&.es of"South-.East and FaruEast,.Asia,,, as .,well as i n the United' Stakes of America.

S t - r a b s with a 'high tolerance ~ o r _. a low level , -of resistance, ' to malathion and other O. P. ,; .;. :. ~- .-: : , .-LI .. ,-;:

: t i I ,

compounds, have been obtained from Puerto Rico &d 'French Guiana, the resistance being

mahly? i f not only, 'nothGable in ' , larvae (Flynn ' e t a l , 1964). ." _ . . . -,:.

> . . ' . . . In Southern Viet-Nim, Aedes albopictus, which is almost as domestic as A. aegypti.,

and 5s also a very good experimental vector of several arboviruses, seems t o be DDT I . . L aid dj.eldrin-resistai?t e : . -

. . . . . . , . In F i j i the local vector o f . Wuchereria bancrofti Aedes pseudoscutellaris, has

. .- I -> . :

developed, on laboratory selection, a high resistance t o DDT, showing t h a t the resistance

cliarzcter is already present i n w i l d populations. ,

.:, , . . . . , . .

~. . . . ,8.

j h A. aegypti the level of resistance t o DDT and dceldrin precludes t h e i r use as ---- La:cv3.cic?.es a s well ES imagocides and i n some of the Caribbean Islands as well as i n

Fren.ch Guiana the erad-ication programme is a t a standsts-11, in sp i te of severe outbreaks

of dengue in Jarxaica, The s i tua t ion is probably worse i n South-East Asia, where

VC/"T. SEM./WP/2.65 page 12

..-

A.

contrslled < > . . around and in-1 the near v ic in i ty of a i rpo r t s a s required under the Inter-

national Sanitary Regulations. However, i n most areas O.P. insecticides can be used

t o give an effective control and, theoretically, deutero-DDT consti tutes a subst i tute

f o r DDT, being as ef f ic ien t on DDT-resistant s t ra ins as DDT is against susceptible

ones ( P i l l a i e t a l . , - lg@b and 1.963~).

t o A. aegypti and related species control, with the a i m of finding out insecticides

and formulations which could be used i n domestic environments without toxic hazards i f

introduced i n drinking-water ( P a l , 1964; Gratz, 1964).

aegypti has never been considered in the past as a dangerous mosquito and was only - .

'WHO has sponsored a research programme devoted

4 .j. 2 Culex pipiens complex , 2.

. .., .

,Members of the Culex pipiens complex are pest mosquitos almost a l l o v e r t h e world, .. . I ,. . .

.. . .. . and are good Wuchereria f i l a r i a s i s vectors i n most of the urban areas of t ropical

countries. ,_ The larvae of $he C. pipiens complex are normally very susceptible t o

insecticides, but adults have always been naturally DDT-tolerant. . _-

DDT.-resistance appears : i n , the C. pipiens coppfex as early as 1947 i n Ita3y and

w a s followed i n 1950 by the development of dieldrin-resistance i n the same country...

Since ..,. . then, DDT and dieldr in resistance.have been observed i n the C. pipiens c?mplex

almost everywhere.

group, in.Southern Cameroon an6 i n Sierra Leone, but the resistance disappeared rapidly

a f t e r the withdrawal of O.P. spraying.

and the frequency of res i s tan t individuals decreases only slowly a f t e r . . the remova1,of. .- .

the. , insecticide pressure (Yu e t a l . , 1963; both DDT and dieldr in cannot be used any more, either . . against adults o r very often ....

against larvae.

Malathion and diazinon resistance were recorded twice i n tha?

Dieldrin, and.DDT resistance,.are more s table . - .

Wu & Djou, 1964; ..Liu e t al., ,1964) . , and . .

Carbamate-resistance has been observed i n laboratory-selected s t r a ins . . . .

' . , . . . .. . . . ! . 1 . .., .,. .. .. ' . .

(G'iorghioÚ, 1964)'. . .

A special screening of toxicants which could-be used safely as larvicides f o r the

C. pipiens complex i n urban areas is being carried out by the WO F i l a r i a s i s Research

Unit' o? Rangbon, with very promisïq resul ts , arid i t ' w i l l be possible very soon t o

con&oi'"larval stages' lof t h i s mosquito.

* ,~

, . .. . . " . , . But' the control of adu1i;"mosquitos of the

C. pipiens complex ra i ses the same d i f f i c u l t i e s as the control of anophelines w i t h

double resistance. - -

L

. - 4.3.3 Pest mosquitos . . .

The salt-marsh mdsquitos Aedes so l l ic i tans and Aedes taeniorhynclius have developed I

DDT-resistance i n 19-47 i n South-Eastern United States of America, then dieldrin-

resistance i n 1951, but can be controlled by O.P. compounds.

Both Aedes melanimon (previously &own as A. dorsalis) and Aedes nigromaculis

from i r r iga ted areas became DDT- 'and dieldrin-resistant i n 1949-51 i n California,

United States of America, followed by parathion-resistance i n 1958-1962 and the

resistance i n the second species w a s a lso present f o r malathion and other O.P. compounds

(Brown e t al., 1.963). stable and decreases w i t h t l d number of insect ic idal treatments and w i t h a rotat ion of

the O. P. toxicants used (Lewa'llen, 1960) .

The resistance of A. nigromaculis t o O.P. coripounds is not very

Culex tarsalis lias been found t o be DDT- and dieldr in-resis tant i n 1951 i n

California and has developed a specif ic and high malathion-resistance i n 1956. " The

malathion-resistm-t populations a r6 susceptibye t o other O.P. compounds and the first

discovered malathion-resistant; popglations, An. the Fresno County, were- found t o have

en t i re ly reverted t o malathion-susceptibility by 1960 a f t e r four years of coctrol by

other organophosphates.

Half a dozen of other culicine mosquitos have developed resistance t o chlorinated

insecticides but do not r a i se specific control problem up t o now (WHO, 1963; Pal , 1964).

4.4 Bed-bugs . . . . . . . . . . __.- .

Only two species of man-biting bed-bugs are widespread; both have developed DDT-

and dieldrin-resistance - Cimex lectularius, 1-947 and- 1954, and Cimex hemipterus 1952

and 1956.

i n I s r ae l (Barkai, 1963) , and C. hemipterus can develop a pyrethrin-resistance i n

laboratory conditions (Fine, 1963;

. . ' i . .

Bed-bug populations res i s tan t t o mala%iokt and fen-thìon , " have been observed . . I . . . . . . . . * ,..

-. .,i .'

Busvine, 1958).

DDT-resistant populations of Cimex lectularius, first observed i n areas i n Hawaii : .,

and i n the United States of America, now e x i s t . i n t rop ica l America, i n Europe, i n the

Mediterranean region, i n India, i n the Western Pacific and i n some areas of Africa.

Dieldrin-resistance i n -the sanie species has been observed first i n the Mediterranean

zone, then i n Africa and i n Soutliern and Eastera Asia.

- <

I n the t ropical bed-bug

Cimex hemipterus, DDT- and dieldrin-resistance have developed i n t rop ica l A s i a and

i n Africa (Busvine, 1928).

VC/ENT SFN./WP/2.65 page 14

Bed-bug resistance t o chlorinated insecticides is probably more widespread, but

Ls sometimes unrecorded i n the absence of proper surveys.

range, bed-bugs are O.P.-susceptible and can be easi ly controlled by malathion.

I n most part of t h e i r ._ I

Bed-bug-resistance in i t s e l f is not a major public health problem, but it in te r -

fe res very often with main vector contirol programmes. ' When the insecticides used

against vectors are no longer e f f ic ien t against bed-bugs, the inhabitants of t reated .

premises complain about insecticide and do not co-operate any more with the spraymen;

sometimes they refuse t o have t h e i r houses sprayed i f bugs cannot be ki l led. It could

be possible t o k i l l the bugs by.adding O.P. colilpouncls t o conventional iiisecticides, but

t h i s adds. to the . . cost of the programme.

avoid such misunderstandings, but w i l l never give a smoother development of the disease

control ac t iv i t i e s than- a good bed-bug control.

4.5 Lice

. . . . . , . .. .

.. . Y .

Better basic health education can help t o . , , .

. , . . . .. , . . .. ,

.. . . . . . .. - During the end of the Second World War and thereaf ter u n t i l 1951, DDT was used with

f u l l success t o control the body-lice Pediculus limanus. -I_

DDT-resistance i n body-lice w a s observed simultaneously i n Korea and Japan i n

1951 and then i n the Middle East i n 1952. recorded i n many areas of the world but, even now, i s not present everywhere. Sub-

sequently body-lice have developed resistance t o BHC and d ie ldr in i n Europe, Africa

and Asia, and t o pyrethrin mainly i n Japan (Wright & Brown, 1957). r a i se big concern, as body-lice are the vector of endemic typhus caused by Rickettsia

Since then, DDT-resistance has been

These resistances . .

prowazeki.

The resistance is moderately s table and is only conspicuous i n regularly-treated

populations (Yasutomi, 1961; Shawarby e t al., 1962). Wright & Pal (1965) have

recently compiled the resu l t s of a seoond survey 02 insecticide resistance i n l i c e

carried out i n 22 countries from 1958-1963.

Laboratory and large-scale experiments have been carried out t o assess the

efficacy of subst i tute O.P. and carbamate insecticides on DDT/dieldrin-resistai-r.t;

poimlations of body-lice.

powder, which is innocuous for human beings, is f u l l y e f f ic ien t f o r a t l e a s t one t o

Shawayby e t al. (1962) have shown i n Egypt t h a t l$-iilalathion

. .

c,

d . . three weeks when dusted on infested carriers.

carbaryl powder has been used with .aery goodr r e su l t s t o stop a typhus epidemic,

t h i s last formulation having %he big, advantaGe of being almost odourless (de Veld, 1963)-

I n an area of South Africa,. ?.1’5$

.. . , . . . ”-, .__.I ._ ....

. .

So, for the present . . . time, body-lice-resistance is i10 longer a serious ,problem.

4.6 Ticks - . .

Insecticide-resistance developed i n t icks following tick-control operations for

c a t t l e protection by dipping or area sprays*

Boophilus decoloratus has been arsenic-resis tani since 1538 i n South Africa. I ” --

It developed BHC/dieldriii-resistance i n 1948 and DDT-resistance i n 1956 i n the same I - 8

country, and also appeared t o be pyrethrin-tolerant (Wliitnall e t al., 1952; Whitehead,

1956; Fine, 1963) Boopllilus I microplus has developed arsenic-resistance i n the past e . . .”

and became BHC/dieldrin-resist&-t since 1950 i” Australia, Brazil and Madagascar

, 1963), and DDT-riesistant since 1954 i n Australia and z i l . BHC/dieldrin-

resistance has spread very rapidly i n B. decoloratus and B. microplus populations and

decreases only slowly when the insecticide pressure is removed (Whitnall e t a l . , 1952;

Stone, 1962; Roulston, 1964).

Dermacentor var iabi l is , which is a potential vector of R o c b Mountain spotted fever,

has sliotvn resistance i n N.assacliu.setts, United States- o f Afi1e:-ica, +;o both ~ i - o W s of

cldorinated insecticides applie s area sprays since 1959.

The other insecticide-resistant t i c k species which are only res i s tan t t o Lhe

t

BHC/dieldrin g o u p of toxicants are Amblyomma americamyn i n the United States of

America since 1954, Rhipicephalus ever t s i i n South Africa since 1960 (Whitehead &

Baker, 1961), and RI?. sanguineus i n Central America and i n the United States of America

since 1954 (Hansens, 1956).

Several organo-phosphorus insecticides- can be safely applied t o c a t t l e for t i c k

control by dipping. Diazinon has been used i n South Africa and i n Australia (Morel,

1963), and sevin has given promising resu l t s i n f4adagascar (Uilenberg, 1963), as w e l l

as malathion i n the United State’s of America (Hansens, 1956).

resistance i n t i cks t o O.P. compounds could be serious because the r e s i s t an t species

include some major vectors or” c a t t l e diseases.

Extension of insecticide-

VCIENT SEM./WP/2.65 page 16

4.7 Fleas - DIE-resistance lias evidently developed i n Pulex i r r i t a n s about 15 years ago i n

Greece and i n the Near East, but d i rec t suscept ibi l i ty t e s t s were not performed.

Dieldrin-resistance has been confirmed by laboratory t e s t s i n P. i r r i t a n s i n

Tanganyika following dieldr in house-spraying operations, and DDT-resistance has

been strongly suspected i n Senegal as P. i r r i t a n s reappears as a pest i n sprayed

vi l lages (Hamon & Mouchet, 1961).

Ctenocephalides f e l i s and C t . canis have probably developed DDT-resistance i n the

very ear ly days of DDT application i n Southern and Northern America. C t . f e l i s has

been investigated and proved DDT-resistant in the Southern United States of America,

as ear ly as 1352, and could not be controlled e i ther by DDT or by chlordane (of BHC/

dieldr in group) i n 1956 i n Florida. DI%ì'-resistant C t . f e l i s is now a very trouble-

some problem i n the United States (Brown, 1960). C t . f e l i s has been also strongly

suspected t o be DDT-resistant i n Southern Cameroon a f t e r some years of spraying f o r

malaria eradication (Hamon tk Mouchet, 1961). DDT and BHC/dieldrin has forced the substi tution of organo-phosphorus compounds i n the

Southern United States of America and these toxicants liave been fu l ly effective.

Resistance i n dog and dat f l ea s t o

The first proven record of DDT-resistance i n the primary plague vector

Xenopsylla cheopis came from Western India i n 1959, and similar populatioiis were

observed i n the following years i n Northern and Southern India -(KrishnaIïmthy &

Joshi, 1962). simultaneous DDT- and dieldrin-resistance i n some par ts of Madras and Mysore States,

India (Mohan, 1962; Choudhuri, 1963). In India the insecticide pressure selecting

for resistance i n f l e a s was in i t i a t ed by the malaria eradication programme sprayings.

The malaria eradication programme had largely put plague under control during past

years, but the appearance of DDT/dieldrin-resistance could have serious developments

i n favouring new plague epidemics. Resistant f l ea s can be controlled by O.P.

insecticide, such as malathion, which should be used- i n specific ant i - f lea campaigns,

whereas i n the past the chlorï-mted insecticides employed fo r malaria eradication

were suff ic ient t o control Cleas and t o prevent plague epidemics.

Soon X. clieopis and the other plague vector Xenopsylla astia developed

4

4.8, - Ceratoppgonide midges " *-- ,- - P

. - - &o species of midges, hAch ab- not carby any

. , ., .. some''.hwlkiï&' areks,. kave''&eveLoped insectfcide resistance 'in America, Lepioeonops

. . ., ...~ ~ - . . Fiorida and' P&t.ma. .S'iriZe\' 1958; following specific pest control programmes.

When resistant to chlorinated insecticides, brackish marshes midges can probably

be effectively conirolied' by application of fenthion pellets which have been very

' : ' "effective for controlling other 'midges 'I (Patterson & Von Wbdeguth, 196$), bük "permanent

cont$o¡ 'can be better established by. impound5ïïg and filling salt marshes (Rogers, :.1$62).

4.9 Blackflies

There is only one record of an insecticide-resistant blackfly. Simulium aokii

developed moderate DDT_resistance, as well as some BHC-tolerance, after six years of

control by DDT and BHC larviciding near Tokyo, Japan (Suzuki et al., 1963). species does not have any medical importance.

I .

r a

This

5. * DISCUSSION AND CONCLUSIONS

Insecticides have probably inducea vector control agencies to aban&Ón mahy oXd

It methods of pest control by general "sanitatibn and- env bnmentai manifiula%.ions

must be remembered that' even with po&rful

always th6 best'way to reach a' state 'ÖP' $erm&ent vectór con'croi.. : : So~roe-4?educ

can' a1so.iprevent,"'6r delay f o r . a consid

. .. .. :. insecticides sourcd.-r&uc%ion is

. . .

e 'tim'e; the ap:pe$rhc:e. 'Of in.sed;yc$ae- , i

. . - ' i ' .

<CJd.'?:Lt resistant pOpulLtiòii could be wo~t~whï~le . t b ' assd¿,lst wh&+ep fD6'$3-5bk&',

environmental sanitation procedures and insecticide applications as a first measure

to decrease the practical implications of insecticide resistance.

Insecticide-resistance lias developed in many species of arthropods of medical

and veterinary importance, and. is present sometimes in the whole area of distribution

of the species involved, but usually resistant populations occur only in restricted

areas. Multiple resistafice is widespread, bui; resistance to only one or two groups

of insecticides is much more frequent.

%/ENT. SEM. /W/2 65 page 1%

. . . . . WHO and national programmes for the development of new ; i&ec.&ckies.

formula%ions ,haye.resulted _ . in..the discovery of new efficient insecticides,,' Investi-

gatiqns, aiming to . , discover. . . . . anti-resistant aqd. synerg&st ehemicals have .been:.les.$

successful .@u$ hpe.. suppl unds which could,be used in emergency-si$uatio

Associations of npn-Telatsd . . insecticides can help to delay the appearance of purely;

............

._. . ..- .- .......... . . . - ,. ..

resistant populations. . . . , , .

A better knowledge of dynamics of resistant populations in field conditions, and of cross-resistance phenomenon between chemically unrelated insecticides, could further

help to employ more efficiently available compounds for controlling rwlti-resistant

vectors. " . .

The combination of insecticides or chemosterilants with selective and powerful . . . . 7 .: ......... . " . . . . . I . ' . .

attfac%"W, '-and the ' development of biological and genetical' methods . . . of vector control

offer other interesting opportunities to cokteract "insecticide-resistance. . . . . . 'But 'a . . . . . . . ., .I . . . . . . . . . . . . .

. . . . . . . . - considerable amount of work is necessary before any of these methods are available for

operat ional use.

The situation is serious but not hopeless. However, it must be stressed that 'in

the first years of vector control with residual insecticides one or two compounds and

less than 10 formulations were sufficient to control almost any vector. Now dozens of insecticldes and hundreds of formulations have. to be used, increasing the, costs *and

the management difficulties of all vector control operations. As many developing -

countries lack funds-aqd

resist.anc,e slows down

ot have extensive chemical industries,, insecticide-

ealth and welfare improvements wheye.they are most needed. .*- '

/ _ I

* " - .

- "a. . . . . . . . L . . mFERENCES'. ' . . . .. ....... . .. .L. . . . . . . . . . ~ . <

Barb.Zj..::A . (1963) A preliminary. survey of bed-bug resistance t o or&anophospl?oru& ' .. compounds i n I s r ae l (Document WHO/Vector ~ontrol /58) , Geneva ! "-- (( . . j . 2 . l . ... .." . I

. . _-_-._...- "'... .-..

Barr, A. R. (1962) Resistance of mosquftos to-insecticides i n California: a review.,,,,,, .,? . . . . . . . ; . . . . . . ;. , .

. .

In: '€%OCÖ .Pap. 30th Ann.. Conf. C a l . Mosq. Contri ASS., 88-~Oj . , . . , .

.......... .....-- ~ , . ~ - . . - ...... -- "... , . . , .. ,,".(., Y . , . . . . , .

h . ,

... ............................. :. ... ...-. -.~. ...... .,..,..,. ...... 2". ...... .-

Beard, R. L. (1964) Experiments on physiologic induc%iion of insecticide resistance. ... . .Proceeding o f. . the ?!re &fj;h..:Xn$ erhaV2~haL.k alagres s . .on...EQtomology,

. . , , . . .-.-

(In. pres-s) .'-i.

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