Jpn. J. Infect. Dis., 53, ll‑14, 2000

Original Article

Insecticide SusceptibilityStatusof Culex tritaeniorhynchus Giles,
Ⅵctor of J叩aneSe Encephalitis in Delhi

Thekkevilayil George Thomas*, Sushil KumarSharma, Anand PrakashI ,Jotna Sokheyand Brij Raj Shama2
National Institute of Communicable Diseases, 22‑Shamnath Marg, Delhi‑ ll 0054,

1DA V (P G) College, MuzaHarnagar‑251001, Uttar Pradesh and
2National Zoological Park, Mathura road, Delhi, India(Received October 19, 1999. Accepted January 3 1, 2000)

StJMMARY: Laboratory studies were camied out to ascertainthe current susceptibility status ofadultand laJVal
stages of the Culex tritaeniorhynchus mosquito, vector of Japanese encephalitis, to variousinsecticides used
under public healthprogramSinhdia･ The present study revealedthat exposure of adult mosquitoes to diagnostic
concentrations of DDT ‑ 4･0%, malathion ‑ 5･0%, fenitrothion ‑ l･0%,and propoxur ‑ 0･1% couldinduce only
50･0, 10･0, 1 5･0,and 5･0% mortality, respectively,indicatingthatthe species was resistant toall oftheseinsecdcides･
The LT50and LT,5 Values calculated using diagnostic concentrations of DDT, mahthion,fenitrothion,and propoxur

were found to be 56･4and 136, 138and 272, 1 85and 258,and I 87and 249 min, respectively･ However, when adultmosquitoes were exposed tothe diagnostic concentration of synthetic pyrethroids, viz･, deltamethrin‑0･025%,

permethrin‑ 0･25%,and lambdacyhalothrin ‑ 0. 1%, loo.0% mortalitywas observed,indicatingthatthe species
was highly susceptible tothese adulticides･ Larval susceptibility tests carried out using diagnostic dosages of
DDT‑ 0･00i, temephos‑ 0･02, fenthion‑ 0･008, fenitrothion‑ 0･ 125,and malathion‑ 0･005 mg/I failed toinduce

any mortality, indicating that laJVae Were resistant tothese larvicides. The LC,.and LC,. values calculated forcommonly used lamicides, viz･, temephosand fenthion, were O･151 land l･9098,and O･6151and 2･395 mgn,
respectively･ Increaseintolerance level were estimated at 95･5‑and 299･4‑fold whenthese LC,. values were･compared withdiagnostic dosages of temephosand fenthion, respectively･

Zoological Park, Delhi丘om various channels and moats

INTRODUC TION

aroundanimal enclosures, where stagnant water collects withfree waterflOating vegetation favorable for the breeding ofCula tritaeniorJwnchzLS Giles is one of the vector mosquito
species largelyinvolved in the transmission of Japanese

Cx. tritaenior句/nchus mosquitoes. Mosquitoes at immature

encephalitisinSoutheastAsia, notablyinIndia. It has beenreported to be a widely distributed speciesinthe IndianSub‑

stages were collected from different breeding sitesand
transported tothe laboratory, where samples were separated

continent･ This vector mosquito species has not only been

by genus. Late thirdinstar larvae of Cx. tritaenior勿/nchus

exposed to the larvicides/adulticides usedunder public healthwere used for larval susceptibilitytests. Fourthinstarlarvae

programs but to theinsecticides used for the control ofagn‑

were keptuntil emergence, for purposes of adult susceptibility

cultural pests, particularly in the case of the paddy crop, astests.

the mosquitoes have outdoor restingand feeding habits･Therefore, it is essential, before contemplating use of any

Adult Bioassay: The Cx. tritaeniorb/nchus adults emerged
from the field‑collected materialina mosquito cage (12 byinsecticide either as an indoor residual spray, application

12 by 12in.)and were allowed to feed on 10%glucose solution

through ULV (ultra low value) Spray or thermal fogging, or

soakedinto a cotton pad. Therea鮎r, three day‑Old female

implementation as a laJVicide, that the susceptibilitystatus

adult mosquitoes were exposed to a diagnostic concentrationof adultand larval stages of the vector mosquito species be(concentrationthat has a highprobabilityofkilling all suscep‑

ascertainedinorder to ensure the effectiveness of vectortible populations ll]) of DDT‑ 4.0%, malathion ‑5.0%,fenitrothion ‑ I.0%, propoxur ‑ 0. 1%, deltamethrin‑ 0.025%,

control measures.

The present report deals with the results of laboratory

permethrin‑ 0.25%,and lambdacyhalothrin‑ 0. 1%, respec‑

bioassay tests camied out during 1 999 to ascertainthe current

tively. Control experiments were runinparallel. For eachrun

susceptibilitystatus of larvaeand adults of Cx.かitaeniwb7nChw

of insecticide, four replicates wererun, eachrun containing

mosquito to variousinsecticides commonly usedunder public

20female glucose‑fed mosquitoes. A鮎r the recommended

healthprograms in India for purposes of controlling vector

exposure period ( 1 ‑3 h) toinsecticide‑impregnated papers ( 12mosquito species.

by 1 5 cm), mosquitoes were transferred to recovery chambers

provided withcotton pads soakedin1 0%glucose solution as
food. Mortalitycounts were made after a 24‑h recovery

A4ATERIALS AND METHODS

period (2). The experiments were conducted at anambient

The biological material was collected at the Nationaltemperature of27土loCand relative humidityof75 to 80%under laboratory conditions (3,4).

*Corresponding author: Mailing address: Dy.Asstt. Director,

To determine the lethal time (LT) inducing 50and 95%

Medical Entomology Division, National Institute of Communi‑
cable Diseases, 22‑Shamnath Marg, Delhi 1 10054, INDIA(LT50and LT95) mortality, respectively, adult mosquitoes wereexposed atintervals of 15 min(ranging 80m 15 minto 4 h 45llmin) tothe diagnostic concentration ofthoseinsecticides towhich they had shown resistance; viz., DDT, malathion,

RE SUIJS

Adult Bioassay: The results of the susceptibilitytests

propoxur,and fenitrothion.

Larval Bioassay: Field‑collectedthirdinstar laJVae Of Cx.
tritaeniorhynchus were identified, separated, washedintap

carried out using the adult Cx. tritaeniorJwnchus mosquito

are glVeninThble 1. In adult mosquitoes exposed to diagnostic
concentrations of DDT, malathion, fenitrothion,and propoxur,

water to remove debris,and kept for observation for 2 h to

weed outthe deadand moribund larvae. Healthy laJVae Werethen exposed to the diagnostic dosages of DDT ‑ 0.008,50%, 10%, 15%,and 5% mortalityrates, respectively, wereinduced,indicatingthatthe species has developed resistanceto all four oftheseinSecticides. However,inthe mosquitoes

temephos ‑ 0.02, fenthion ‑ 0.008,fenitrothion ‑ 0.125,andmalathion ‑ 0･005 ppm, respectively. Tests were camied out
by placing 25 laJVaeina 5001ml capacitybeaker containing

exposed to diagnostic concentrations of deltamethrin,pemethrin, and lambdacyhalothrin, 100% mortality was

250 ml of water and the required dosage of larvicide. For

induced, which showedthat the species is highly susceptible

dosage of each larvicide, four replicates were runand control

to synthetic pyrethroids.
In order to determine the LT50and LT95 Values, the adultexperiments were runinparallel. Mortalitycounts were madeafter a 24‑h exposure period. To determine LC5.and LC,.

mosquitoes were exposed to diagnostic concentrations of(lethal concentration) values,the teclmical form of temephos

DDT, malathion, fenitrothion,and propoxur, respectively, fora range of expostweperiods. The data obtained were subjected
to probitanalysis to calculatethe regression equation, LT50,
LT,5 Values, and heterogeneity(5)･ The resultsare given i.n(50% emulsion concentrate)作C)andthat offenthion (82.6%EC), obtained from Cyanamid Ltd., Mumbai, hdiaand BayerLtd･, Mumbai, India, respectively, wereused.
Bioassay tests showing more than20% control mortality

were discardedand repeated, although,inthe control case

Table 2. The LT5.and LT95 Values calculatedusing diagnostlC
concentrations of DDT, malathion,fenitrothion,and propoxurmortalityranged between 5 to 20%,the corrected mortality

were foundto be 56.4and 136, 138and272, 185and258,

was calculated using Abbot's formula (2).

and 1 87and 249 min, respectively. The heterogeneibT (刑.05)

was found to be signiflCant, implyingthatthe adult popula‑

Table 1 ･ Results of susceptibilitytests exposing Culex LritqenjorJynchus adult mosquitoes to variotwinsecticidcs
at diagnostic conccntraltions

ConcenInsecti ci deExpostJTe No. of adults No. dead Mortality Susceptibility/tration

time

(%)
DDT

4 5 L o o o o 0 0 0 1 0 つ▲ 1(h)exposedLL 2 2 L 3 L0 0 0 0 0 0 01111111(aRer24h)(%)

0 1 つ一 ′0 ′b 6丘V ′0 ′hV ′8 6 ′〇 人V

0000000

Resistant

511 000055000

0 ′b 4 0 0 0 0status+

良良良良sss0000000

Mat adlion
Femitrothion
Propoxur

Deltamethrin

日‖一 = =2 55

1 1 1Permethrin

Lambdacyhalothrin+S: susceptible (> 9名% mortality), VR: veriGcahon required (80‑9i% mortality), R: resistant (< 80% mortality) (2).

Td)le 2. Dosage mortality response of adult CuLex Lrilaeniorbmchus mosquitoes, used todetermine LT5｡ and lJ95 Values and heterogeneity
Insecticide REeqEIst諾onn LT;omvi:)ueS LT;Lvhd)ues Het;:o(gdeP;iサ∠U 3 8 8′0 2 00 9

20.3 12(6)DDT4.0%‑7.4997土1. 8596x

Malathion15. 0%

1. i883土2.4 1 65x

29. 1912(5)Fenitrothion‑ 1.0%

25.93 89土4.965x

1 7.0607(4)Propoxur･0. 1 %3. 8087土3. 8682X

1 2.2598(4)5111.4 8 5 7

1 つ一 2 つ一 3 7 5 4+df: degrees of Beedom.

Table 3･ Results of susceptibility tests carried out using Culex LritaeniorbJnchus larvae
agalnst different larvicides at diagnostic dosages

L‑icides doDsILg蒜,蓋sifd (nOe･,d2C.add, "o(T%.r RSeSs"itti:禁書醐mmmm

0 0 0 0 0 〇 0 0 1 0 2 0 つ一〇0 各55

NNNNN2 つ】 2 2 2 0 0 0 0 0 0 0 0 0●l ●l ■l ● ‑ 一l■l 一 l 1 1 100000 00000

n'S: suspeptible (> 98% mortality), VR: veriGcation required (80‑98% mortality),
R: resIStant (<80% mortality) (2).

12

氏良良氏良

Table 4. Dosage‑mortality response of Culex Lrilaeniorhynchus larvae to various
dosages oftemephos and fenthion, used to deterTnine the LCS.and LC,. values,and heterogeneity

Insecticide

Tcmephos琵慧15.Sion LC(5Avgnd)ues LC("mvgnd)ues "eT(gS;iサ

0.9547土0.5052x

0.

151Fenthion 0.4582土0.9427x 0.6 1 5 11

1.909825.5986(5)2.395

26.93 87(4)+df:degrees of Beedom.

has been reported Bom Japanand Korea (13). Susceptibility
tests carried out earlierinDelhi againstthe larvae of the Cx.

tion tested was heterogeneous.

Larval Bioassay: The results of the larval susceptibility
tests camied out using respective diagnostic dosages of DDT,tritaeniorJwnchus mosquito recorded LC5｡and LC9｡ Valuestemephos, fenthion, fenitrothion,and malathion revealedthat

for temephosandfenthion ofO.0383 land 0.2214,and 0.02939

these larvicides failed toinduce any larval mortalityaRer aand 0.1614, respectively (14). In the present study, LC50and

24‑h exposure period,indicating that larvae were resistant tothese laJVicides (Table 3).

LC90 Values recorded for temephosand fenthion were foundto be 0.1511and 1.9098,andO.6151and2.395, respectively;

The results of larvae exposed to various dilutions of themuch higher thapthose previously reported･ Comparatively,

teclmical formof larvicides revealed that the LC50and LC,.

LC90 Values found in the present study indicate a 95.5‑andvalues estimated for temephosandfenthion were 0. 15 I land

299.4‑fold increase against the diagnostic dosages of

1 ･9098and O･615 land 2.395 mg/1, respectively (Table 4). The

temephosandfenthion, respectively.
The development of DDT and malathion resistance inadults,and temephosandfenthion resistancein1arvae, of Cx.

heterogeneity(P=0･05) was found to be significant, implyingthat the larval population tested was heterogeneous･tritaeniorhynchus mosquitoes warrants strict monitoring of

the susceptibilitystatus of this species to variousinsecticides
inJapanese encephalitis proneareas before contemplating
the use of these substances for ULV spray/thermal fogglng,

DISCtJS S ION

During 1958,the flrSt report Onthe development of DDT
resistanceinCx･ tritaenL'orJwnchus mosquito appeared fromas a residual spray, or as a laJIVicide.

Okinawa Island, Japan (6). In India, DDT and dieldrin
resistance were reported from different parts of the country
by severalresearchers (7,8). DDT resistance in the Cx.
tritaeniorkynchus mosquito has also been reported fromadditional countries, including Bangladesh, Benin, China,Nigeria, Korea, Sri Lanka, and Thailand (4). During theACKNOWLEDGMENT
The audlOrS Wish tothank Shri. Rai馳ddinKhan, Techmician,for providing teclmical assistance.

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13

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14